SCOPE 56 - Global Change: Effects on Coniferous Forests and Grasslands

6.1 INTRODUCTION

Photosynthesis provides the driving step to the biogeochemical global carbon cycle. Whilst much is known of the direct effects of temperature, water vapor pressure deficit and nutrient supply on this process (reviewed: Long 1985; Long and Woodward 1988), until recently little has been known about the long-term effects of growth in elevated CO₂ concentrations. Because CO₂ is commonly a limiting substrate for photosynthesis, the current and projected increases in atmospheric CO₂ concentration (c_a) are of direct significance to this step in the carbon cycle. The biophysics and biochemistry of photosynthetic carbon assimilation appear highly conserved across terrestrial vegetation, regardless of taxonomic affinity. This has allowed the construction of mechanistic models of photosynthetic response to increase in c_a which apply to the vast diversity of terrestrial plants, and provides a rare opportunity for scaling from biochemical to landscape levels. The dominant primary producers of all ecosystems except grasslands are C₃ species. These are species in which the first product of photosynthetic CO₂ assimilation is a 3-carbon compound, phosphoglycerate (PGA). CO₂-fixation is catalyzed by the enzyme ribulose-1, 5-bisphosphate carboxylase / oxygenase (Rubisco). PGA is metabolized through the Calvin cycle to resynthesize ribulose-1, 5-bisphosphate (RubP), the primary acceptor of CO₂. Carbohydrate for storage and post-photosynthetic metabolism is gained by the transfer of intermediates of the Calvin cycle into synthetic pathways, particularly that leading to sucrose, the major carbohydrate exported from photosynthetic cells. Virtually all grass species of tropical grasslands and a significant proportion of the grasses of the steppe/prairie biome are C₄ species, i.e. plants in which the first product of photosynthetic carbon metabolism is a 4-carbon compound. These species utilize an additional carboxylase, phosphoenol-pyruvate carboxylase (PEPc) in the C₄-phosynthetic cycle, to concentrate CO₂ at the site of Rubisco within the bundle sheath cells of the leaf. Essentially, these additional steps provide a light-driven CO₂ pump. The result is that photosynthesis in C₄ species is saturated at a c_a below the current ambient concentration whilst in C₃ species saturation is only approached at a c_a of three times current atmospheric levels (Edwards and Walker 1983). This review is therefore primarily concerned with C₃ species, since no direct effect of elevated c_a on photosynthesis is anticipated for C₄ species. Whilst the primary concern of this book is with coniferous forest and grasslands, it is appropriate to consider the wider knowledge of the effects of elevated c_a on terrestrial plants, since the basic mechanisms are the same in all C₃ species regardless of biome. Atmospheric concentrations of CO₂ are predicted to rise from the current concentration of 355 µmol mol^-1 of CO₂ in air to about 500 µmol mol^-1 in 2050 and c_a. 700 µmol mol^-1 by 2100, following the Intergovernmental Panel of Climate Change (IPCC) IS92a scenario (Leggett et al. 1992). This chapter examines the implications of these changes for photosynthesis, by examining field and laboratory experiments, and model predictions. There is a good mechanistic understanding of the instantaneous effects of variation in temperature and c_a on C₃ photosynthesis. This, coupled with the conservative nature of the mechanism of C₃ photosynthesis, has allowed the development of generic models of cell, leaf and canopy photosynthesis in which the effects of both external changes, e.g. temperature increase, and internal changes, e.g. decrease in Rubisco activity, may be explored. Such a model, WIMOVAC  (Humphries and Long 1995) is used here to examine the implications of changes in both climate and atmospheric composition on the process of photosynthesis and on carbon uptake, and how these may be modified by acclimation. 

6.1.1 Photosynthesis as the primary site of response

Carbon dioxide is a substrate of photosynthesis and is limiting to the rate of photosynthesis in C₃ species at all light levels. In the short term, increase in C_a will, in C₃ plants, lead to increased net leaf photosynthetic CO₂ uptake (A) both by increasing the velocity of carboxylation of RubP and suppressing photorespiration. Because increase in c_a increases the net efficiency of photosynthesis, an increase in photosynthesis must therefore be expected regardless of whether other factors are limiting photosynthesis or not. For example, an increase occurs regardless of whether light is limiting or saturating (Long and Drake 1991). This results because an increase in c_a diverts ATP and NADPH generated on the photosynthetic membrane to the anabolic reactions of the Calvin cycle and away from the catabolic reactions of the photosynthetic C₂ pathway leading to photorespiration (Long and Drake 1992).

Photosynthesis is not only the major physiological process by which plants sense change in c_a (Mott 1990), but is also the process by which plant production is most likely to be affected by change in c_a. Other physiological processes influencing gaseous fluxes are also affected independently by change in c_a. In both C₃ and C₄ plants, stomatal aperture and hence conductance (g_s) and transpiration per unit leaf area decrease with increasing c_a (Mott 1990). This effect of rising c_a on stomata is independent of the effect on photo respiration. Increase in c_a has also been shown to depress night-time respiration (R_d) (Bunce 1990). Many other effects have been attributed to increased CO₂ (Bazzaz 1990). Relative stimulation of CO₂ uptake and dry matter production have frequently been shown to occur under conditions of environmental stress, suggesting that elevated CO₂ may ameliorate stress (reviewed: Long and Drake 1992). Amelioration of stress effects may be attributed to an increased capacity for photosynthesis, an increased supply of photosynthate and/or an increased water and nitrogen use efficiency (Idso 1989). There are also examples where elevated c_a has increased the negative effects of nitrogen and phosphorus deficiency on growth or crop yield (Conroy et al. 1988; Mitchell et al. 1993).

6.1.2 Effects of rising CO₂ on photosynthesis and predictive models for natural vegetation

Prediction of crop and natural ecosystem production in a changed climate has often reflected the view that there would be little stimulation of growth with an increase in c_a. The models of Parry and Carter (1988, 1990), used for predicting regional patterns of change in crop production, make no allowance for any direct effect of rising c_a. The model of Esser (1987) underlies the 'Osnabruck Biosphere Model', which has been used to assess future changes in global primary productivity, yet it ignores the direct effects of rising c_a on photosynthetic carbon uptake. The '1990-Greenpeace Assessment' (Schimel 1990) used the CENTURY model to predict the likelihood of ecosystem feedbacks on rising atmospheric CO₂, and considered only the indirect effect of rising c_a on the water use efficiency of vegetation, not its direct effects on photosynthesis (Ågren et al. 1991). Failure to include a direct stimulation of carbon gain by rising c_a may have various roots:

1. observations which show photosynthesis to decline following transfer to an elevated c_a (Bazzaz 1990). Downward adjustments of photosynthetic enhancement during growth in an elevated c_a are usually referred to as 'photosynthetic acclimation' (Gunderson and Wullschleger 1994);

2. the view that production in natural ecosystems is limited by factors other than c_a (Melillo et al. 1990; Schimel 1990);

3. the argument that decrease in photorespiratory losses resulting from rising c_a may be canceled by an increase in photo respiration resulting from the projected concurrent rise in mean temperature (Eamus 1991).

The empirical evidence that crops grown in elevated c_a show increased dry matter production is overwhelming, however. Kimball (1983) cited over 450 studies in which crops were grown at an elevated c_a. He concluded that average crop yield is increased by 34% and transpiration is decreased by 34%. Of course it may be argued that increased yield is the indirect result of decreased transpiration, rather than increased photosynthesis. Other predictions have considered that the effects of elevated c_a are adequately accounted for by multiplication with a constant or ß-factor (reviewed: Hendrey, 1992). Multiplication with a single constant ignores the interactions of c_a with other key variables. Yet these interactions can dramatically modify or even reverse the response to increase in c_a -most are well understood at a mechanistic level (Long 1991).

Here it is demonstrated that the acclimation of photosynthesis to elevated c_a is often associated with artificial environments and less common in field experiments. Where acclimation does occur, it seems to be an adaptive response which benefits the plant, and the positive effect of CO₂ on photosynthesis tends to be sustained in the long term. It is further demonstrated that failure to take account of the interactions of c_a with other environmental parameters can lead to serious errors in the prediction of increase in photosynthesis with rising c_a.

6.2 A MECHANISTIC MODEL OF LEAF AND CANOPY PHOTOSYNTHESIS

In order to reliably predict changes in photosynthesis in response to elevated c_a, a mechanistic approach to modeling the effect of CO₂ on leaf biochemistry is required (Reynolds and Acock 1985). This has been achieved with a mechanistic model of photosynthetic gas exchange, WIMOVAC, which scales from C₃/C4 biochemistry to canopy CO₂-uptake (Humphries and Long 1995). Figure 6.1 illustrates the sub models of WIMOVAC used, their interconnection and the original sources of the equations used. The mechanistic model of leaf photosynthesis developed by Farquhar et al. (1980) and Farquhar and von Caemmerer (1982) has been widely used and validated (Long 1985; Long & Drake 1992; Harley et al. 1992). Equations originally derived by Farquhar et al. (1980) and given in Long and Drake (1992) were modified in WIMOVAC to include a potential phosphate limitation arising from the sequestration of cytosolic phosphate in sugar phosphates and failure of inorganic phosphate supply to keep up with triose phosphate production in the Calvin cycle (Sharkey 1985b). WIMOVAC was used to predict leaf photosynthetic rates of CO₂-uptakeand the significance of changes in temperature and c_a via their effects at the level of carboxylation and oxygenation of RubP. Parameters for the leaf biochemistry module, except where stated otherwise in individual figure legends, are as given by Farquhar et al. (1980) for a leaf temperature of 25°C

Figure 6.1 WIMOVAC mechanistic model for the prediction of leaf/canopy carbon and water vapor exchange from biochemical mechanisms and leaf microclimatic relations. Submodels, their interconnections and primary sources of equations

The biochemical model uses c_i (intercellular CO₂ concentration) rather than c_a as a driving variable. Assimilation of CO₂ and stomatal conductance is assumed to determine c_i. To be useful in predicting leaf response to varying environmental , conditions, the biochemical model of CO₂ assimilation must be integrated with a model of stomatal behavior. A mechanistic understanding of the control processes involved in regulating g_s remains elusive, but a successful and robust phenomenological model of the response of g_s to c_i, A and humidity of the atmosphere is provided by Ball et al. (1987). Harley et al. (1992) adapted this model to use c_a rather than c_a (CO₂ concentration at the leaf surface) and relative humidity of the air outside the boundary layer. This revision is used in WIMOVAC. Because A and g_s are interdependent, the value of c_i is determined in WIMOVAC in an iterative fashion.

In order to examine the effects of variation in temperature, model equations for predicting diurnal change in air temperature were obtained from Spain and Keen (1992). In addition, solubility's for O₂ and CO₂ were recalculated relative to their values at 25°C using polynomial relationships fitted to tabular values of solubility at different temperatures (Linke 1965; Kaye and Laby 1973). Jordan and Ogren (1984) provided data on the response of the kinetic constants and specificity of Rubisco to temperature, which are used here.

Effects of varying leaf nitrogen content on the biochemistry of leaf processes have been introduced into WIMOVAC using data supplied by Field (1983). Field (1983) and Harley et al. (1992) both proposed a linear relationship between leaf nitrogen content and maximum RubP-saturated carboxylation velocity (V_c,max), light-saturated potential rate of electron transport (J_max) and R_d. All leaves, either singularly or as part of a canopy, were assumed here to have a leaf nitrogen concentration of 2 g m^-2 unless otherwise specified.

Although leaf gas exchange provides many insights into plant adaptation to the environment, canopy processes are critical in determining crop and biome- level productivity (Norman 1980). To evaluate the significance of changes inferred at the leaf level to the canopy level, a simplified model of canopy photosynthesis has been used (Long 1991; Long and Drake 1992). Norman (1980) showed that by treating a canopy as two populations of leaves, sunlit and shaded, and by calculating the mean irradiance and in turn mean assimilation rate for each leaf population, the estimated canopy photosynthesis differed little from more complex canopy models. However, this division into sunlit and shaded leaves did provide a substantial improvement in prediction over models which simply assumed an exponential decline in light through homogeneously lit canopy layers. These equations, used in WIMOVAC, are given by Forseth and Norman (1993) and Long and Drake (1992). The parameters used here, unless otherwise stated in individual legends, are as given in Long and Drake (1992).

To facilitate investigation of elevated c_a and concomitant temperature effects on predicted canopy water usage, an expression has been introduced into the canopy sub model of WIMOVAC for the calculation of instantaneous transpiration of water vapor from the canopy, according to Penman (1948) and Monteith and Unsworth (1990). The expression has been combined with a model to describe the transfer of water vapor from the evaporating surface to the bulk air stream in terms of the aerodynamics of the turbulent air above the canopy. This is based upon a boundary layer conductance model proposed by Campbell (1977), and the theory behind this is discussed in some detail by Thornley and Johnson (1990). Transpiration rates for both the sunlit and shaded leaves within the canopy are considered according to the appropriate light and temperature micro climate conditions within the canopy. A derivation of the Penman (1948) equation in which transpiration is eliminated, is used to predict the difference between canopy leaf temperature and the ambient air temperature outside the canopy. Parameters for both the transpiration and leaf temperature modules were as given by Campbell (1977). However, the expression relating apparent sink momentum to canopy height, given by Campbell (1977), was corrected here and has the form, d = 0.77h where d is the apparent sink momentum and h is the canopy height in meters. Leaf transpiration and leaf temperature are not independent quantities, and so an iterative procedure is used here to establish their respective equilibrium values in WIMOVAC.

6.3 ELEVATED c_a AND PHOTOSYNTHETIC ACCLIMATION

6.3.1 Instantaneous responses of photosynthesis to CO₂

Instantaneous responses of photosynthesis to an elevated c_a are obtained when plants grown under the current ambient c_a are transferred to an elevated c_a (Figure 6.2). The simulation shows that an increase in c_a would increase A at all light levels from below the light compensation point for photosynthesis (LCP) to the maximum photon fluxes experienced in the natural environment.

6.3.1.1 Light-limited photosynthesis

Rubisco catalyses both the carboxylation and oxygenation of RubP; CO₂ is a competitive inhibitor of oxygenation and O₂ a competitive inhibitor of carboxylation. Uptake of CO₂ via carboxylation of RubP leads to carbohydrate synthesis through the Calvin cycle. Uptake of O₂ via oxygenation of RubP leads to carbohydrate anabolism via the photosynthetic carbon oxidation cycle, with the resultant release of CO₂ in photorespiration. Rising c_a will favor photosynthetic carbon accumulation at the expense of photorespiration through competitive inhibition of RubP-oxygenation. Maximum quantum yield (f) is the initial and maximum slope of the hyperbolic response of A to absorbed photosynthetically active photon flux (labs). Light limitation restricts the supply of intermediates required in RubP-regeneration, and the rate of RubP supply limits CO₂-uptake as a consequence.f increases after an instantaneous rise in c_a since suppression of photorespiration allows the use of a greater proportion of the available RubP in carboxylation (Figure 6.2).

Figure 6.2 Predicted rates of leaf CO₂-uptake (A in µmol m^-2 S^-1) with photon flux density µmol m^-2S^-1) for four atmospheric CO₂ concentrations (c_a in µmol mol^-1 of CO₂ in air). Inset highlights early response of CO₂-uptake at low photon flux values

Figure 6.3 The simulated response of light-saturated rates of leaf CO₂ uptake (A_sat in µmol m^-2 S^-1) with intercellular CO₂ concentration (c_i, µmol mol^-1 of CO₂ in air). The solid line indicates the curve based on leaf biochemistry parameters given in Long and Drake (1992) and is typical of the response observed in many C₃ plants grown at current c_a. Arrows indicate the operating points, i.e, the c_i obtained for a given external c_a. The dotted lines joining c_i and c_a on the three curves indicate the supply function. The broken lines illustrate two potential patterns of acclimation to growth in elevated CO₂. The lower line indicates the result of a 30% decrease in V _c,max and V_o,max, simulating a loss of Rubisco activity, and the upper line a 65% increase in J_max, simulating an increase in the maximum capacity for regeneration of RubP

6.3.1.2 Light-saturated photosynthesis

The response of light-saturated A (A_sat) to c_i reflects biochemical limitations to A_sat, independent of any stomatal effects. The A/c_i response shows two distinct phases predicted by the theory of Farquhar et al. (1980) (Figure 6.3): an initial slope at low c_i determined by Rubisco activity, which will be RubP-saturated, and a plateau or shallower slope at higher c_i where photosynthesis is limited by the rate of regeneration of theCO₂ acceptor,i.e. RubP. Fora wide range of C₃ species it has been found that the c_i which they attain under current ambient c_a is often at the transition between RubP-saturated and RubP-limited photosynthesis, i.e. at the point where the quantity of active Rubisco and the capacity for regeneration of RubP are colimiting (Jarvis 1989) (Figure 6.3). An environmentally induced change in capacity for carboxylation of RubP appears to be commonly matched by a change in the capacity for RubP-regeneration, and vice versa, such that the two processes remain colimiting (Evans and Farquhar 1991).

6.3.2 Theoretical acclimation responses to CO₂

Decline in Rubisco activity represents one of the most consistent features of acclimation of the photosynthetic apparatus to elevated c_a so far identified (Idso 1989; Allen 1990). Why should such a decrease occur?

Growth in low nitrogen results in decreased concentrations of total Rubisco and corresponding decreases in the initial slope of the A/c_i response, sometimes termed the 'carboxylation efficiency'. However, in a range of species it has been found that capacity for regeneration of RubP changes in concert, such that the operating c_i remains at the point of inflection of the A/c_i response (Evans and Farquhar 1991). A similar pattern of acclimation in the A/c_i response is observed when plants are grown in low photon fluxes (Evans and Farquhar 1991; Sage 1994). These studies suggest an optimization of the distribution of resources within the chloroplast so that neither active Rubisco nor the apparatus for regeneration of RubP is in excess. If optimization of the distribution of resources between components of the photosynthetic apparatus is a ubiquitous phenomenon, then adjustment might also be expected in plants grown in elevated c_a. If c_i increases proportionately, then limitation would be shifted away from carboxylation to RubP-regeneration. Given the large investment of energy and nitrogen in Rubisco, up to 25% of leaf nitrogen, strong selective pressures for modulation of Rubisco carboxylase levels are likely.

Figure 6.3 illustrates how this might occur. Without any adjustment of the photosynthetic apparatus the A/c_i curve would be unchanged. Considering a typical A/c_i curve for a leaf developed at current c_a (based on the parameters of Long and Drake 1992), the operating c_i when c_a = 350 µmol mol^-1 will be at the point of inflection of the curve (point 'a' on Figure 6.3), assuming c_i/c_a = 0.7 .If c_a is now doubled, c_i moves to 490 µmol mol^-1, and is now on the upper portion of the A/c_i response (point 'b' on Figure 6.3). Here A_sat would be limited by regeneration of RubP and Rubisco Capacity would be in considerable excess. Acclimation of the A/c_i curve so that the inflection of the curve moves to 490 µmol mol^-1 could be achieved in two ways (Figure 6.3). Either the quantity of active Rubisco (V_c,max) is decreased, so that We is decreased to equal W_j at c_i = 490 µmol mol^-1, or J_max is increased, so that W_j is increased to equal W_c at c_i = 490 µmol mol^-1 (point 'c', Figure 6.3). Using the model and parameters of Long and Drake (1992), this increase in the point of inflection of the A/c_i response is achieved either by simulating a 30% decrease in V_c,max, representing an equivalent decline in active Rubisco, or by increasing J_max by 65%, simulating an equivalent increase in capacity for regeneration of RubP.

Yelle et al. (1989) suggested that the decrease in photosynthetic capacity of plants growing in elevated CO₂ is caused by decrease in Rubisco activity. Figure 6.3 shows that a 30% decrease in Rubisco activity could occur without decreasing A_sat when c_a is elevated to 700 µmol mol^-1. Whilst these plants will show a reduced A_sat when measured at a c_a of 350 µmol mol^-1 in comparison to non-acclimated plants because they are Rubisco-limited at this c_a, they should not show a reduced A_sat at 700 µmol mol^-1. This pattern of acclimation would explain why many studies have found that plants grown in elevated c_a show a decrease in photosynthesis relative to control plants when measured at 350 µmol mol^-1, but not when measured at the elevated growth concentration (500-800 µmol mol^-1 ).

To maintain c_i = 0.7 c_a as indicated in Figure 6.3, a 42% decrease in stomatal conductance would be required, which at a constant leaf-air vapor pressure deficit (VPD) would give a 42% decrease in transpiration. Thus acclimation of this nature would allow a substantial decrease of investment of nitrogen into Rubisco and provide a marked increase in water use efficiency, while supporting a 22% increase in A_sat. This would clearly be advantageous for plants growing with limited resources.

In summary, theoretical consideration of A/c_i responses suggests that acclimation of the response to elevated c_a could represent an optimization of Rubisco activity, capacity for regeneration of RubP and stomatal conductance according to other limitations, in particular nitrogen supply.

6.3.3 Is there empirical evidence for photosynthetic acclimation to an elevated c_a under natural conditions?

Acclimation is widely reported in studies of coniferous trees and grassland species. In a review covering 39 tree species, Gunderson and Wullschleger (1994) compared A_sat for plants grown at current ambient c_a and elevated c_a. Acclimation of, photosynthesis measured at the current ambient c_a was apparent as an average 21% decrease in A_sat for plants grown at an elevated c_a compared with plants grown at the current ambient c_a (Figure 6.4). However, when A_sat was measured at the elevated c_a in which the plants were grown, an average enhancement of 44% was observed relative to plants grown and measured at the current ambient c_a (Figure 6.4). The observation is consistent with the theoretical prediction that relatively large decreases in Rubisco activity can occur without affecting the enhancement effect of an elevated c_a on CO₂-uptake. Indeed this is illustrated by Figure 6.3 which shows that a ~ 30% decrease in A at a c_a of 350 µmol mol^-1 would occur in leaves optimally acclimated to a c_a of 700 µmol mol^-1 without any decrease in A when measured at 700 µmol mol^-1, relative to controls.

Figure 6.4(a) Frequency distribution of the ratios of leaf photosynthetic rate of CO₂ uptake (A in µmol m^-2 s^-1) for trees grown and measured at elevated c_a. relative to measurements on controls grown and measured at current ambient c_a. Each observation represents a single study; (b) frequency distribution of the ratios of A for trees grown in elevated c_a and measured at the current ambient c_a. to controls grown and measured at the current ambient c_a. This figure is redrawn from Gunderson and Wullschleger (1994) who surveyed studies of the effects of growth in elevated c_a on photosynthesis in 39 tree species

Eleven of the tree species covered by the review of Gunderson and Wullschleger (1994) were conifers. Average acclimation and enhancement of their photosynthesis after growth at elevated c_a were calculated as above, from the values of A_sat provided by Gunderson and Wullschleger (1994). Average photosynthetic enhancement at the growth c_a was 47% and average acclimatory reduction in photosynthesis measured at ambient c_a was 19%, i.e. these conifers showed no obvious difference from the combined values for the 39 species of trees. C₃ grasses also show evidence of photosynthetic acclimation, and enhancement of photosynthesis at elevated c_a similarly appears to persist. Photosynthesis was enhanced by 35-46% in Lolium perenne grown at an elevated c_a despite photosynthetic acclimation (Nijs et al. 1989; Ryle et al. 1992). These observations are consistent with the view that decreased photosynthetic capacity at current ambient c_a following growth at an elevated c_a simply reflects a remobilization or deactivation of excess Rubisco. This is also consistent with the view that Rubisco levels may be modulated to optimize nitrogen-use within the plant. Why then has earlier work suggested that acclimation reflects a decrease in the ability of plants to respond to increased c_a in photosynthesis?

6.3.4 Photosynthetic acclimation and pot size

Artificial limitation of sink size has probably confounded the summaries of effects of CO₂ offered in previous reviews where pot size was not considered (e.g. Cure and Acock 1986). Drawing upon the observations of Herold (1986) that pot size limits sink demand, Arp (1991) examined the relationship between rooting volume used in different experiments and the apparent stimulation of photosynthesis with growth in elevated c_a. Stimulation of photosynthesis in the long term was far more pronounced if the potential rooting volume was large for a wide range of plants. Thomas and Strain ( 1991) provided experimental evidence that restriction of root development by pot size could accentuate acclimation of photosynthesis to elevated c_a. Long and Drake (1992) accounted for this interaction with rooting volume in summarizing the effects of CO₂ on photosynthesis across a range of studies. Results were separated between those obtained using pots of size greater than 10 dm³, and those with smaller rooting volumes. The survey of 112 studies suggested that when rooting volume is limited, a downward acclimation of A_sat at a given c_i results, but when there is no restriction of rooting volume, A_sat remains unchanged. CO₂-uptake from leaves which developed at an elevated c_a was compared to that of leaves developed at current c_a for common values of c_i, approximating c_i at current ambient c_a. Plants grown at elevated c_a in small pots showed statistically significant average reductions in photosynthesis of 21.4 and 12.5% when measured at the ambient and elevated c_a, respectively. Leaf photosynthetic rates of plants grown and measured in elevated CO₂ were on average higher than in plants grown and measured at current c_a. The effect was a 23.3% increase in photosynthesis for plants grown in small pots but a 50.5% increase for plants in large pots. These increases were statistically significant.

Sage (1994) conducted a detailed survey of the effects of growth at elevated c_a on the A/c_i response of photosynthesis. Heexamined the response of CO₂-uptake of ambient (A_amb) and elevated grown plants (A_elev) to c_i. To assess acclimation of an assimilation ratio independent of stomatal effects he plotted A_elev/ A_amb against c_i. He found that most plants (11 of 15) grown in relatively small pots (5 liters or less) had an assimilation ratio less than 1 across a range of c_i, indicating acclimation of both RubP-saturated and RubP-Iimited carboxylation. Many species (5 out of 13) grown in large pots (10 liters or more) exhibited A/c_i responses indicative of a shift in allocation from Rubisco toward factors limiting RubP-regeneration rate. However, most A/c_i responses indicated no acclimation, or an acclimation response involving proportionate responses in all components limiting to carboxylation. The latter pattern was observed in one field study, whilst two others showed no acclimation. 

Results from pot studies may be relevant in natural habitats where rooting volume is limited. Such habitats may include shallow soils, such as those supporting chalk grassland, where the bedrock limits rooting depth; podsols, such as those supporting boreal forest, where an iron pan limits rooting depth; and cliffs and mountainous areas, where small volumes of soil are trapped in cracks in rock and on ledges. It may also have relevance to dense root mats where the physical presence of competitors' roots may limit potential rooting volume.

6.3.5 Photosynthetic components subject to acclimation responses

Analysis of A/c_i responses suggests that acclimation of the photosynthetic response to elevated c_a is uncommon in situations where rooting volume and nutrient availability are not limiting. Where acclimation does occur, it could represent an optimization of Rubisco activity and g_s, but less commonly the capacity for regeneration of RubP. The following sections examine the occurrence and significance of changes in these three potential limitations to CO₂-uptake.

6.3.5.1 Rubisco

The literature survey of Long and Drake (1992) showed that on average, elevated c_a produced a decrease of about 15% in both Rubisco content and Rubisco activity. Rubisco content was significantly decreased by 8% for plants grown with large rooting volumes, and there was also a significant reduction in the proportion of the enzyme which was active. This pattern of decrease in Rubisco content and activity has been clearly demonstrated for rice grown in a range of CO₂ concentrations. Here the activity of Rubisco per unit area and the amount of Rubisco protein were found to decline linearly with increase in c_a, such that the quantity of Rubisco in leaves developed at 900 µmol mol^-1 was about one-third of that in leaves grown at about 150 µmol mol^-1 (Rowland-Bamford et al.1991). In contrast, growth of wheat in 550 µmol mol^-1 under free-air CO₂ enrichment had no effect on the amount of Rubisco at any stage of plant development, except for the final stage of grain-filling (Nie et al. 1995).

Acclimation of carboxylation efficiency and hence V_c,max may result from a decrease in the total quantity of Rubisco and/or a decrease in the activation state of the enzyme (Long and Drake 1992). In surveying a wide range of studies, Bowes (1991) noted marked between-species variation, but also evidence for and against acclimation within the same species. Whilst the majority of studies have shown a decline in Rubisco as a proportion of total soluble protein with growth in elevated c_a, others found no decrease, and a few an increase (see Long and Drake 1992; Gunderson and Wullschleger 1994). Leaf age also appears to affect acclimation, with young leaves apparently showing less acclimation in Rubisco levels than older ones (e.g. Yelle et al. 1989; Besford 1993), although in another study the effect was found to be greater in young leaves (Vu et al. 1983). 

Given a common, although not complete, pattern of decrease in Rubisco, both as a proportion of total soluble protein and per unit leaf area in response to elevated c_a, the question arises as to what triggers the decrease. One obvious change indicated by almost all studies is an increase in carbohydrate within the leaf, indicating significant increases in both starch and sucrose, in plants grown in both large and small rooting volumes (Long and Drake 1992). Sheen (1990) and Sheen et al. (1992) have demonstrated that both sucrose and glucose can produce repression of rbcs, the nuclear gene which codes for the small subunit of Rubisco. The possibility that the carbohydrate status of the leaf affects Rubisco levels is attractive since it would explain the different patterns of acclimation observed in plants with different sink size and explain how acclimation of capacity for regeneration of RubP may be coordinated with Rubisco activity (Webber et al. 1994). 

So far acclimation of Rubisco has only been considered in terms of quantity of protein and active enzyme, i.e. an effective decrease in maximum RubP-saturated velocity of carboxylation and oxygenation ( V_c,max and V_o,max). An acclimatory decrease in carboxylation efficiency would also be effected if the specificity of the enzyme for CO₂ relative to O₂ (t) changed, occurring if the Michaelisconstant for CO₂ (Kc) decreased.So far no evidence of decrease in t as a phenotypic response to growth in elevated c_a has been reported, and the similarity of t across contrasting species of C₃ .terrestrial plants leads Bowes (1991) to suggest that there have been strong selective pressures for the maintenance of a hight. However, forms of Rubisco with a lowert are found where the enzyme is confined to environments with a low ratio of concentrations of O₂:CO₂ (Bowes 1993). Whilst t in C₃ plants is about 100 at 25 °C,t for Rubisco from seven C₄ plants ranged from 58 to 83 (Jordan and Ogren 1984). This lower specificity would have no effect on the rate of assimilation given the high c_i in the bundle sheath of C4 plants (Long 1983). Similarly,t for Rubisco from anaerobic photosynthetic bacteria Can be as low as 9, and values for aquatic angiosperms can also be low (Bowes 1993). These results suggest that the selective pressure for maintenance of a hight is removed in environments where potential for photo respiration is decreased or absent, C₄ plants have evolved from C₃ plants in relatively recent times (Long 1983); it may therefore be assumed that forms of Rubisco with lower t have evolved in these plants following the removal of the selective pressure for a high t. Whilst there is no evidence of decreased t as a phenotypic response to elevated CO₂, a decrease in t within populations may result if elevated c_a decreases the selective pressure for maintenance of a high t.

In summary, a decrease in the quantity of active Rubisco is common in plants grown in elevated c_a, even when the supply of nitrogen is high and rooting volumes large. Decrease in Rubisco quantity corresponds to a decrease in the initial slope of the A/c_i response, i.e. carboxylation efficiency, such that capacity for regeneration of RubP and carboxylation are colimiting. The resulting colimitation suggests that the decrease represents an acclimatory redistribution of resources, into other parts of the photosynthetic apparatus or other parts of the plant. Such reallocation has been observed in coniferous trees and grasses (Stulen and Hartog 1993). As noted in sections 6.3.2 and 6.3.3, enhancement of photosynthesis at growth c_a typically occurs despite acclimation in Rubisco at elevated c_a.

6.3.5.2 Stomata 

In contrast to photosynthetic CO₂ assimilation within the mesophyll, the mechanism by which stomata respond to c_a remains unclear. Nevertheless, stomata of most species show predictable closure reactions with increase in c_a, and decrease in A and relative humidity (RH). Ball et al. (1987) provide a phenomenological model of the dependency of stomatal conductance on CO₂ concentration (c_a) and humidity (h.) at the leaf surface, and A. This has proved valid for a wide range of C₃ species. Harley et al. (1992) developed a more practical approximation using the ambient RH and CO₂ concentration (c_a): 

c_a where a, a', b, and b' are constants characteristic of the given plant material.

Stomatal aperture and conductance (g_s) generally decrease with increase in c_a, explaining the reduction in leaf trarispiration commonly observed in plants grown in elevated c_a (Morison 1987). The stomata of conifers tend to respond less to elevated c_a than broad-leaved tree and herbaceous species (Morison 1985; Jarvis 1989). The mechanism by which stomata sense change in c_a is still a matter of debate (Mott 1990). Where stomata represent the major limitation to diffusion in the gas phase, leaf transpiration will decline linearly with decrease in g_s, if VPD remains constant. However, this does not automatically imply a parallel decrease in A. The significance of change in g_s to A is difficult to assess because of the nonlinearity of the A/c_i response. Farquhar and Sharkey (1982) suggested that the limitation (l) imposed on photosynthesis by stomata could be assessed by comparing the rate of CO₂ uptake (A) at a given c_a with that which would be obtained (Ao) if there was no diffusive barrier, i.e. g_s=a and therefore c_i = c_a:

Limitation (1) is calculated by determining A and A₀ from the A/c_i response. The theoretical A/c_i responses of Figure 6.3 suggest that leaves developed and measured in a c_a of 350 µmol mol^-1 would have an 1 of 0.133. Increase in c_a to 700 µmol mol^-1 would, by reference to Figure 6.3, decrease l by 64% and g_s by 42%, assuming no acclimation of the curve or assuming a decrease in carboxylation efficiency of up to 30%. If capacity for regeneration of RubP was increased by 65%, so raising the point of inflection of the curve to the operating point when c_a = 700 µmol mol^-1, l would decrease by 53% and g_s by 8%. Thus, given all scenarios of acclimation expected from theoretical considerations, stomatal limitation would be expected to decline with rising c_a.

Because dA/d c_i decreases with increased c_i it follows that maintenance of a constant c_i/c_a will require a decrease in g_s as c_a rises. Typically, an increase in c_a from 350 to 700 µmol mol^-1 will require about 40% decrease in g_s (Figure 6.3) and this agrees closely with values observed for a range of plants (Eamus and Jarvis 1989; Mott 1990). This is also indicated by decreased transpiration per unit leaf area in plants grown in elevated c_a.

Interpretation of A/c_i responses and predictions of the effects of elevated c_a on CO₂-uptake via models developed from that of Farquhar et al. (1980) have assumed that stomatal apertures are homogeneous within leaves. Water stress and treatment of leaves with abscissic acid (ABA) have been shown to induce stomatal heterogeneity (reviewed: Terashima 1992). In heterobaric leaves, i.e. leaves in which the intercellular air space system is divided into discrete patches by the pattern of leaf venation, differential closure of the stomata associated with these patches would result in heterogeneity of c_i and A. Cheeseman (1991) used a computer simulation to determine the effects of heterogeneous stomatal opening on the A/c_i response. His analysis suggested that variation in stomatal aperture between patches must exceed a coefficient of variation of 100% of the mean before the estimated A/c_i response differs significantly from the true response of the mesophyll. One effect of heterogeneity will be to broaden the point of transition between carboxylation and RubP-regeneration limitation, diminishing the advantage of an acclimatory change to optimize the balance between limitations. Decreased transpiration at elevated c_a is likely to improve plant water status and decrease ABA levels. Since declining water status and increased ABA levels are known to induce stomatal heterogeneity, reversal of these changes by increased c_a may tend to decrease the incidence of patchiness rather than vice versa. Further, the few field studies so far conducted suggest that stomatal heterogeneity may be rare under natural, as opposed to laboratory induced, drought conditions (Terashima 1992; Wise et al. 1992).

The above analyses have assumed that changes in g_s simply reflect changes in the A/c_i response and change in c_a. This ignores the possibility that the direct response of stomatal movement to c_a or the stomatal apparatus may be modified by development at elevated c_a. An acclimatory response of stomatal conductance could involve a change in sensitivity to c_a or a change in stomatal size or number .

ABA increases the sensitivity of the stomata to CO₂ concentration and is known to accumulate in leaves in response to drought. Since transpiration generally falls when c_a is elevated, a lower accumulation of ABA and decreased sensitivity to c_i  might be expected. Plants of Pinus radiata, P. menziesii (Hollinger 1987) and cotton (Harley et al. 1992) grown in elevated c_a showed decreased sensitivity of g_s to CO₂. In these plants, decrease in g_s with increase in c_a is less in the plants grown in elevated c_a. Thus, slightly increased rates of photosynthesis and transpiration than those predicted are found. Evidence is contradictory for the observation that stomata acclimate to elevated c_a under well-watered conditions, independently of acclimation of photosynthesis (Eamus 1991; Long and Drake 1992; Sage 1994). Black spruce seedling grown under elevated c_a had a lower g_s over a wide range of c_i than those grown under ambient c_a (Johnsen 1993).

Acclimation might also occur by change in stomatal numbers, which in the absence of variation in stomatal dimensions, will determine the maximum g_s that a portion of leaf could attain. One expectation might be that at increased c_a fewer stomata are required, since the rate of CO₂ diffusion into the leaf will be a decreasing limitation to photosynthesis as c_a rises. Studies have been conducted investigating responses of stomatal numbers to elevated c_a over experimental periods of weeks to the 1000s of years covered by paleoecological studies. Experimental evidence shows no clear developmental acclimation of stomata. Reported changes in stomatal density with growth at elevated c_a include increases, decreases and no change (Long and Drake 1992; Gunderson and Wullschleger 1994). Long-term studies drawing on herbarium material and paleoecological evidence are more conclusive, showing an inverse relation between variation in c_a and variation in stomatal numbers (Beerling et al. 1992; Beerling and Chaloner 1993a, b; Woodward 1993). Pearson et al. (1995) have shown an asymmetric response of stomatal conductance between the two surfaces of the leaf of Rumex obtusifolius with growth in elevated c_a, with the functional result that a leaf which is amphistomatous at the current c_a would become hypostomatous at elevated c_a. This could effect a substantial increase in instantaneous water use efficiency (w).

In summary, stomata appear to become less limiting to photosynthesis with elevation of c_a, even though g_s is invariably decreased. Acclimation of stomatal aperture to elevated c_a may occur, allowing greater conductance at a particular c_a. The response could be driven by changes in leaf ABA resulting from reduced transpiration rates. Evidence for acclimation of stomatal development to elevated c_a is conflicting, but evidence showing evolutionary adaptation of stomatal numbers is clearer. Over a longer timescale, continued evolutionary adaptation to rising c_a might be expected in the form of decreased stomatal density.

6.3.5.3 Factors affecting RubP-regeneration rate

At the leaf level, optimization suggests that at elevated c_a investment would be shifted away from Rubisco toward increased capacity for regeneration of RubP, such that the two remain colimiting, i.e. there is no over-investment in anyone part of the photosynthetic apparatus. This is illustrated in Figure 6.3. However, when the plant as a whole is considered when nitrogen is limiting, it may be expected that optimization would redistribute nitrogen away from Rubisco to other parts of the plant, e.g. for root growth, rather than into photosynthetic apparatus for regeneration of RubP. Elevated c_a increases photosynthesis even when it is limited by the rate of regeneration of RubP. Thus, not only is less Rubisco required to maintain a given photosynthetic rate when c_a is elevated, but there is a decreased requirement for capacity for regeneration of RubP. It might therefore be predicted that when nitrogen is strongly limiting, optimization of nitrogen distribution would require a decrease in investment in Rubisco and, to a lesser extent, capacity for RubP-regeneration. If, however, resources are not limiting, then an increase in capacity for RubP-regeneration might be expected (upper line of Figure 6.3). The latter pattern has rarely been observed, whilst decreased capacity has (Sage et al. 1989; Sage 1994). Decreased capacity is influenced by the preponderance of studies in which pot sizes may have been limiting, however. Of the three field studies considered by Sage (1994), one showed an increase in capacity for regeneration of RubP, conflicting with the pattern suggested from pot studies.

The model of leaf photosynthesis used in the theoretical predictions of section 6.3.2 is formulated to assume that either Rubisco activity (W_c) or whole chain electron transport ( W_i limit the rate of carboxylation. Although some A/c_i responses (Sage 1994) indicate an increased capacity for regeneration of RubP following acclimation to elevated c_a, there is as yet very little evidence that reduced allocation of nitrogen to Rubisco is matched by any increased allocation to other enzymes of the Calvin cycle or to thylakoid proteins (Mott 1990). Limitation on regeneration of RubP may also be imposed by capacity to utilize the carbohydrate synthesized (Sage 1994), since increase in source strength relative to sink strength is well known to feedback onto photosynthetic rates (Stitt 1991). The sink is conceived as any pool of carbon outside the chloroplast envelope into which the products of photosynthesis are fed. The source strength is the potential of the chloroplasts to provide assimilate for the sinks; following the definition of Patrick (1993). Long and Drake (1992) showed that increased sucrose contents were a consistent feature of plants grown in elevated c_a. Accumulation of sucrose within the leaf has the potential to feed back on the capacity for regeneration of RubP in the short term via cytosolic inorganic phosphate (P_i concentration and enzyme activation, and in the longer term via gene expression. Accumulation of phosphorylated intermediates of sucrose synthesis in the cytosol occurs when chloroplast formation of sugar phosphates exceeds the rate of sucrose synthesis and export. This may starve the chloroplast of the P_i required for the phosphorylation of Calvin cycle intermediates. In an adaptation of the Farquhar et al. (1980) model, Harley and Tenhunen (1991) suggested that P_i-limitation of regeneration of RubP should be considered as a third limitation to CO₂-uptake(Wp)where A will be determined by the minimum of W_j, W_c and W_p. If photosynthesis is limited by return of Pi to the chloroplast, then this will be apparent in C₃ species by a lack of O₂ sensitivity (Sharkey 1985a). The chloroplast envelope triose phosphate translocator exchanges P_i. Decreased return of P_i will inhibit translocation of triose phosphates and promote starch synthesis within the chloroplast. Decreased P_i could also decrease the chloroplast ATP/ADP ratio, which in turn would inhibit Rubisco activase (Bowes 1991), so leading to the decreased Rubisco activities commonly observed in the literature (Long and Drake 1992). P_i-regulation seems to explain many of the changes observed in plants grown in elevated c_a, but it does not provide a universally applicable solution. Empirical evidence suggests that changes in enzymes and metabolites expected with P_i-limitation are not always found when source activity exceeds sink capacity (reviewed: Stitt 1991). An alternative mechanism may be a decrease in activities or quantities of proteins involved in the regeneration of RubP, essentially matching the plant's photosynthetic capacity to its capacity to utilize end-products. Support for this hypothesis comes from the observations that soluble carbohydrates can suppress the expression of a number of photosynthetic genes (reviewed: Webber et al. 1994).

Increase in capacity for regeneration of RubP therefore requires an increase in sink strength relative to the increase in potential source strength within the plant. This is consistent with changes for the range of plants grown in elevated CO₂ and large rooting volumes observed in the literature survey by Long and Drake (1992). Below-ground parts are a major sink for carbon, and increases in the average root: shoot ratio suggested may be indicative of whole-plant physiological responses which lead to an increase in plant sink size. The change in source-sink balance and the potential increase in nitrogen uptake coming from an enlarged root system would both tend to offset acclimation of Rubisco and allow increased RubP-regeneration capacity. Efficient use of nitrogen: resources in the plant may drive acclimatory changes in the photosynthetic apparatus (see section 6.3.2), and changes in plant nitrogen content may also drive changes in root:shoot ratio. Plant tissue nitrogen content tends to be diluted at an elevated c_a because of increases in carbon uptake not matched by nitrogen uptake (Long and Drake 1992). This may also be partly due to a decreased requirement for Rubisco and possibly apparatus for the regeneration of RubP with growth in elevated c_a. Decreases in plant tissue nitrogen content caused by growth in elevated c_a with a limiting nitrogen-supply commonly correspond with an increase in root:shoot ratio (Gutschick 1993). Increases in root:shoot ratio at elevated c_a may thus be a feedback response which attempts to rectify the relative decrease in nitrogen. If so, then total nitrogen in the vegetation, per unit ground area, should rise despite an increase in the C/N ratio of that tissue with growth at elevated c_a. This has been observed with growth of a C₃ tidal marsh community grown over two years at elevated c_a in the field (Curtis et al. 1989). Other morphological changes suggesting an increased sink size in elevated c_a are reported in the literature. These include increased tillering in a tundra sedge (Tissue and Oechel 1987), increased main shoot development in trees (Oberbauer et al. 1985), induction of flowering in trees (Mousseau and Saugier 1992), and increased root nodule activity in legumes (Idso 1989).

In summary, there is limited evidence that acclimation of photosynthesis to elevated c_a can involve an increase in capacity for the regeneration of RubP. Variation in response might be explained by variation in sink strength and the ability of sink strength to increase in elevated c_a. Whilst limitation on RubP- regeneration could theoretically be imposed through P_i-limitation, this is inconsistent with a number of biochemical and physiological observations. Other mechanisms which cause variation in RubP-regeneration should be sought, particularly those involving modulation of levels of proteins involved in the regeneration of RubP.

6.4 INTERACTIONS BETWEEN CO₂ AND OTHER ENVIRONMENTAL PARAMETERS

6.4.1 Temperature

Theoretical considerations lead to the view that simulation of instantaneous CO₂-uptake by elevated c_a will be relatively small at low temperatures, but large at high temperatures (Long 1991). The view is supported both by mechanistic modeling of leaf and canopy photosynthesis, and by empirical evidence.

6.4.1.1 Leaf light-saturated photosynthesis

According to the mechanistic model described in section 6.2, increase in c_a produces a progressive increase in A_sat accentuated at higher leaf temperature (Figure 6.5). On elevation of c_a from 350 to 550 µmol mol^-1, an elevated c_a commonly used in experimental studies, A_sat is predicted to increase by 14,32 and 57% at leaf temperatures of 10, 20 and 30°C, respectively (Figure 6.5). It also follows from this interaction of c_a and temperature (T) that the temperature optimum ( T_opt) of A_sat must increase with c_a. T_opt is predicted to increase by 2, 5 and 6°C with increase in c_a to 450,550 and 650 µmol mol^-1, respectively. The predicted upper temperature at which a positive A_sat may be maintained is similarly increased. The change in these characteristic temperatures underlies the importance of considering rise in c_a not just as a factor which increases photosynthetic rate, but also as one which strongly modifies the response to T. When the interactive effects of increased c_a and temperature are taken into account, a small increase in A_sat is indicated on increasing leaf temperature from 25 to 30°C.  If T was assumed to affect A_sat independently of c_a, i.e. the optimum remains unchanged at elevated c_a' then it would have been predicted that A_sat would fall rather than increase between 25 and 30°C Thus, failure to take account of the interaction would not only fail to predict the magnitude of change in photosynthesis with T but also the direction of change. Increase in T_opt with rising c_a is of particular significance to expected change in climate. The 'IPCC business-as-usual scenario' suggests an increase in mean temperatures of ~3°C, averaged across latitudes, in the second half of the next century (Watson et al. 1990). In temperate climates this would increase the frequency of temperatures which are supraoptimal for A_sat. The results suggest that this inhibitory effect will be partially negated if the temperature increase coincides with an increase in c_a to 500 µmol mol^-1, as projected (Leggett et al. 1992), since this increase in c_a would apparently increase T_opt by 3°C The modeled interactive increase in A_sat and T_opt with elevated c_a and increasing temperature is very similar to that observed with elevation for c_a and temperature for C₃ species (Berry and Bjorkman 1980; Pearcy and Bjorkman 1983; Sage and Sharkey 1987).

Figure 6.5 Predicted light-saturated rates of leaf CO₂-uptake (A_sat  in µmol m^-2 S^-1) with leaf temperature (°C) for four atmospheric CO₂ concentrations (c_a, µmol mol^-1 of CO2  in air)

Figure 6.6 Predicted temperature difference ( ΔT, °C) between leaves at an atmospheric CO₂ concentration (c_a, µmol mol^-l of CO₂ in air) of 350 µmol mol^-1, i.e. current ambient, and leaves in a c_a. of 450, 550 or 650 µmol mol^-1. A total radiant energy flux of 900 W m^-2, an air temperature of 25°C and a relative humidity of 0.7 are assumed. Leaf absorptance or 0.8 and emissivity of 0.9 are assumed. Stomatal conductance and energy balance are calculated by the procedures described in section 6.2

Predicated temporal patterns of temperature increase with global warming suggest that in the boreal forest biome, temperature will be large in autumn and spring (Mitchell et al. 1990), when T is suboptimal A_sat. Thus, in this situation interaction of rising c_a and T will be strongly synergistic. Growth of plants in different temperatures can result in an acclimatory shift in the response to A_sat to T. However, the relative stimulation of A resulting from increase in c_a would remain unchanged (Berry and Bjorkman 1980).

Will an increase in c_a, in the absence of any increase in air temperature, affect leaf temperature? As noted in section 6.1.1 transpiration per unit leaf area is decreased at elevated c_a, and thus latent heat loss will similarly be decreased. From energy budget considerations, increase in c_a from 350 to 550 µmol^-1 with a photon flux of 1500 µmol m^-2 s^-1 would increase the temperature of a 'typical' C₃ leaf by 0.4°C (Figure 6.6). Empirical evidence from three field-rooted species grown in open-top chambers suggests a strong positive correlation between reduction in g_s at elevated c_a, and concomitant leaf temperature rises (Idso et al.1993). Leaf temperature increases have also been measured at elevated c_a the natural micrometeorological conditions provided by free-air CO₂ enrichment (FACE) of cotton crop. Here foliage temperature was on average 0.8°C warmer 550 µmol mol^-1 c_a than at current ambient c_a (kimball et al.1992).

Figure 6.7 Predicted response of the maximum quantum yield of CO₂-uptake (f to leaf temperature (°C ) at four atmospheric CO₂ concentrations (c_a, in µmol mol^-l of CO₂ in air)

Thus, even in the absence of any increase in mean air temperatures, some increase in mean leaf temperature is expected. For leaves which commonly experience suboptimal temperatures, e.g, boreal forest trees and temperate grassland species in the spring, this indirect effect of elevated c_a would produce a further stimulation of photosynthetic rate (Long 1991; Long and Drake 1992; see (Figure 6.5).

6.4.1.2 Light-limited leaf photosynthesis

Experimental observation has established that f in C₃ species decreases with increase in T (Ehleringer and Bjorkman 1977). The biochemical basis of this decrease is that increased temperature decreases the ratio of carboxylation to oxygenation rates (V_c/V_o). With a decrease in V_c/V_o an increasing proportion of the limiting supply of NADPH and ATP produced by electron transport is diverted into photorespiration. Because CO₂ is a competitive inhibitor of the oxygenation reaction, progressive increase in c_a decreases the rate of decline in V_c/V_o and hence f, Figure 6.7 illustrates from the mechanistic model WIMOVAC how various increases in c_a would affect the efficiency of light-limited photosynthesis at a range of temperatures. Long and Drake (1991) showed for plants grown for three years at elevated Ca, that in shoots grown and measured at a c_a of 700 µmol mol^-1 at a leaf temperature of 25°C was 0.080, 25% higher than the value of 0.064 for shoots grown and measured at the current 144 ambient c_a. This increase agrees closely with the predictions modeled from consideration of the underlying biochemistry (Figure 6.7). Since f declines with temperature, increased temperature will in part counteract the individual effect of increased c_a. For example, if the increase in c_a were accompanied by a 4°C increase in leaf temperature, then f would increase to only 0.077 with a doubling of c_a  i.e. a 20% as opposed to a 25% increase. Unlike light-saturated photosynthesis, light-limited photosynthetic rate by definition depends on the quantity of light received and the efficiency with which it is transformed, and becomes independent of both quantities of Rubisco and capacity for regeneration of RubP . The light-limited rate of CO₂-uptake increases with increase in c_a, simply because the efficiency of energy transformation is increased by the inhibition of photorespiration. At a constant c_a quantum efficiency has been shown to be remarkably constant in C₃ terrestrial plants regardless of their taxonomic and ecological origins (Long et al. 1993). This experimental observation reflects the constancy of the photosynthetic mechanism across C₃ species. Thus, the rate of light-limited photosynthesis may be predicted mechanistically for C₃ vegetation without any need for parameterization, in contrast to light-saturated photosynthesis.

If R_d remains constant and f rises with increase in c_a it follows that the LCP must decrease. At all temperatures, LCP is depressed by increase in c_a; again the effect being greatest at high temperatures (e.g. Long and Drake 1992). Such decreases are of considerable significance in warm and shaded environments, where they would significantly alter the number of leaf layers that a canopy could maintain above the LCP for net carbon gain. They would greatly increase the photosynthetic productivity of forest ground-flora and understorey plants existing at photon fluxes which are little above the LCP in the current ambient c_a. Decreases in LCP in accordance with these predictions were observed in Scirpus olneyi growing in an elevated c_a (Long and Drake 1991).

6.4.1.3 Canopy photosynthesis

The mechanistic model WIMOVAC, described in section 6.2, uses predictions of A to calculate net canopy CO₂ accumulated over 24 hours (A_c,tot), where the quantity of an angular distribution of light is continually varying with change in sun angle. For a canopy of leaf area index 5, a substantial portion of the leaf area will be shaded even during the course of a sunny summer day, and thus the canopy response to T will not be dependent solely on the response of light- saturated leaf photosynthesis (A_sat) to temperature. Nevertheless, the pattern of response of A_c,tot, to temperature with increase in c_a is similar to that described for A_sat (Figure 6.8). For a rise in c_a from 350 to 550 µmol mol^-1, A_c,tot is predicted to increase by 15, 30 and 74% at mean daily temperatures of 10, 20 and 30°C, respectively for a summer day at latitude 52° N. The optimum mean daily temperature of A_c,tot, is predicted to rise by 10°C and become more sensitive to temperature. This is a larger increase than predicted at the single leaf level (Figure 6.5) and may be accounted for by the differing temperature relations of light-saturated and light-limited photosynthesis (cf. Figure 6.7), both of which affect the daily integral of canopy CO₂-uptake. However, as suggested at the leaf level, the predictions for canopy photosynthesis suggest that the enhancement of canopy photosynthesis induced by elevated c_a will be minimal in cold climates and maximal in the hottest climates (Figure 6.8). This expectation is consistent with experimental observations. Grulke et al. (1990) found little stimulation of net canopy CO₂-uptake (A_c) in Arctic tussock tundra, whilst model pasture ecosystems in N. Germany and a C₃ wetland vegetation in Maryland showed average increases of 25-45 and 88% respectively, with a doubling of c_a in the field (Overdieck 1990; Drake and Leadley 1991).

Figure 6.8 Predicted rates of canopy CO₂-uptake integrated over a 24 hr period (A_c,tot) with mean daily temperature (°C) and four atmospheric CO₂ concentrations (c_a, in , µmol mol^-1 of CO₂ in air). Figures are for a canopy of leaf area index of 5 and a ratio of horizontally to vertical projected leaves of 1, on Julian day 190, assumingclear sky conditions (atmospheric transmittance = 0.75) at a latitude of 0°A diurnal temperature range of ± 5°C is also assumed

6.4.1.4 Conclusion

Theoretical considerations suggest that the relative stimulation of both light- limited and light-saturated C₃ photosynthesis resulting from elevated c_a will increase with temperature. These are consistent with field observations. The conserved nature of the mechanisms of C₃ photosynthesis and its temperature relations provides a strong basis for the development of canopy and vegetation scale models across biomes. The validity of this approach will depend largely on the significance and understanding of second-order effects determined by nutrient limitation, water availability and acclimation, which may alter light- saturated photosynthesis.

6.4.2 Nutrient limitation

Primary production in natural and crop ecosystems is commonly limited, at least for a part of the growing season, by nutrient supply; most commonly the supply of nitrogen or phosphorus. Elevated c_a. over long periods could lead to secondary feedbacks at the ecosystem level, and impose further nutrient limitation (Bazzaz 1990). Indeed this has led to the suggestion that nutrient limitation would negate any stimulation of carbon gain, in the longer term, with rising c_a. However, two factors may act against this. First, photosynthesis is more resource-use efficient in elevated c_a because photorespiration is inhibited, so increasing the overall efficiency of net photosynthesis. Second, continued anthropogenic release of nitrogen oxides and ammonia to the atmosphere is increasing nitrogen in many ecosystems (Morris 1991). The interaction between nutrient limitation and elevated c_a on photosynthesis could be of great importance when predicting future photosynthetic production. What might be expected if there is a decrease in either the availability of nitrogen or the amount per unit leaf area?

6.4.2.1 Theoretical expectations

Field (1983) and Harley et al. (1992) proposed a linear relationship between leaf nitrogen content and both the maximum activity of Rubisco ( V_c,max) and capacity for regeneration of RubP (J_max). Figure 6.9 couples these relationships with the mechanistic model of photosynthetic response to c_a. It shou]d be noted that this is a worst case scenario, since it ignores the possibility that there might be a decline in V_c,max without a decline in J_max (see section 6.3.2).Figure 6.9 suggests that whilst A_sat declines almost linearly with leaf nitrogen content, the proportionate increase in photosynthesis with increase in c_a is predicted to be similar at all nitrogen contents from 0.5 to 4 g [N] m^-2. However, if growth at elevated c_a leads to decreased leaf nitrogen contents over prolonged periods, then Figure 6.9 predicts that the gain resulting from increase in c_a will be partially offset by the decreased nitrogen content. 

Figure 6.9 The predicted response of CO₂-uptake (A, µmol m^-2 S^-2) to leaf nitrogen content (g N m^-2 ) at four leaf-N concentrations. A leaf temperature of 25°C, relative humidity of 0.7 and photon flux of 1500 µmol m^-2 S^-1 are assumed

Will the gain through the direct effect of increased c_a. on photosynthesis be fully offset by decrease in leaf nitrogen content? This will depend first on the extent of nitrogen decrease and secondly on temperature. Decrease in nitrogen will affect photosynthetic capacity at all temperatures, but the direct stimulation of photosynthesis by increased c_a will be greater at higher temperatures. Figure 6.10 shows that if leaf nitrogen is decreased by 50% with growth at 500 µmol mol^-1 c_a, then net photosynthetic CO₂-uptake would be lower in the elevated c_a than at the current ambient concentration at all temperatures below 30°C. At 5°C there would be a c_a 40% loss of photosynthetic rate. However, the leaf could lose 40% of its nitrogen with growth in elevated CO₂ at temperatures above 14°C, and still show a 50% increase in A at 35°C.

6.4.2.2 Empirical evidence

Recent literature reviews suggest that photosynthetic acclimation to elevated c_a tends to be more pronounced under nutrient limitation (Bazzaz 1990; Gunderson and Wullschleger 1994; Sage 1994), although there are examples of the converse, i.e. acclimation is less pronounced or absent under nutrient limitation (Bunce 1992; Yuncong and Gupta 1993). Nutrient limitation has been suggested as the driving force for acclimation in general, since acclimation tends to increase nitrogen-use efficiency, but experimental evidence supporting the hypothesis is not conclusive (Bunce 1992; Gunderson and Wullschleger 1994). 

Figure 6.10 The upper line illustrates the ratio of CO₂-uptake (A, µmol m^-1 S^-1) at the current ambient CO₂ concentration (c_a = 350 µmol mol^-1) relative to A at an elevated c_a of 550 µmol mol^-1, versus temperature, for leaves with a nitrogen content of 2 g m^-2. Growth in elevated c_a may lead to a dilution of leaf nitrogen. The lower lines compare A at c_a = 350 µmol mol^-1 with A at c_a = 550 µmol mol^-1, but illustrate the effect of a progressive decrease in the nitrogen content of the leaves grown at the elevated c_a

Photosynthetic response was correlated with nitrogen-availability in Pinus taeda seedlings; acclimation being greatest where nitrogen-limitation was the greatest. In the same study, phosphorus-limited plants with an ample nitrogen- supply showed a greater enhancement of photosynthesis under elevated c_a than plants with an ample phosphorus supply (Tissue et al. 1993).

Studies of cotton suggest that the type of acclimation response occurring can depend on nutrient supply (Wong 1979). Plants grown at a c_a of 640 µmol mol^-1 and a strongly limiting supply of nitrate showed a marked decrease in the initial slope of their A/c_i curve without any increase in the c_i-saturated rate, relative to plants grown in a c_a of 330 µmol mol^-1. Plants grown with a good supply of nitrate and in a c_a of 640 µmol mol^-1 showed little decrease in the initial slope of the A/c_i curve relative to those grown in a c_a of 330 µmol mol^-1, but an increase in A_sat at high c_i, indicating an increased capacity for regeneration of RubP.

6.4.2.3 Conclusion

Although it is commonly suggested that nutrient limitation may negate the stimulation of carbon gain by elevated c_a (e.g. Melillo et al. 1990), theoretical considerations suggest that elevated CO₂ will allow C₃ plants to be more efficient in their use of nutrients. In a doubled c_a, an average C₃ leaf could lose 30-40% of its Rubisco activity before the light-saturated rate of photosynthesis is affected. Decreased Rubisco levels in leaves developed in elevated c_a, rather than reflecting a loss of photosynthetic capacity, may represent a redistribution of limiting resources, in particular nitrogen (Webber et al.1994).

6.4.3 Interaction with water

6.4.3.1 Water use efficiency

Figure 6.11 The predicted daily water use efficiency with respect to air temperature for a canopy of leaf area index 5 in a range of CO₂ concentrations (c_a, µmol mol^-1 ). A constant relative humidity of 0.7 is assumed. Other assumptions are as in Figure 6.9

Increased CO₂-uptake, and decreased transpiration due to reduced g_s, would both lead to improvements in water use efficiency with rising c_a. Instantaneous water-use efficiency (w) is defined as the ratio of the rate of CO₂-uptake to rate of water loss. Figure 6.11 uses WIMOVAC to predict the daily w for a canopy at four c_a with air temperatures from 5 to 35°C, assuming a constant RH of 0.7 in the ambient air. Instantaneous water-use efficiency is predicted to rise with increase in c_a at all temperatures, but the relative rise due to elevated c_a is predicted to increase with temperature. The predicted rise in w at a c_a of 550 relative to 350 µmol mol^-1 is 83%, but only 21% at 10°C Figure 6.11 suggests a near linear increase in w with increase in c_a at any given temperature, in agreement with the relationship described by Morison (1993) in a survey of studies covering 20 species.

Although there are no field data on the interactive effects of elevated c_a and temperature, there is much information showing an increase in w. Elevated c_a leads to increased water use efficiency, especially in the short term, and this can result in improved drought-resistance through avoidance of water-stress. Increased CO₂-uptake and decreased transpiration due to reduced g_s at elevated c_a both lead to improvements in w. Averaging over 29 observations of 20 C₃ species, Morison (1993) found that w was approximately doubled in plants grown at elevated c_a. Improvements in w can lead to the improved water status of plants grown at elevated c_a when subjected to water-stress. Black spruce seedlings were more drought-tolerant when grown at elevated c_a, as indicated by a higher xylem pressure potential after a drought episode (Johnsen 1993). Enhancement of net photosynthesis for plants growing in the elevated c_a increased from 33 to 333% in drought-treated plants. An interactive effect also occurred in g_s where a 28% reduction at elevated c_a in well-watered plants compared to no difference between plants grown at elevated and control c_a under drought conditions. Similarly, Nie et al. (1992) examined the interactive effect of mild water-stress and elevated c_a on C₃ grasses in a natural prairie ecosystem. Again, leaf transpiration rate was not affected by elevated c_a in water-stressed plants, but was reduced in well-watered plants because of a reduction in g_s at elevated c_a. Both results might be explained by the initial decrease in transpiration at elevated c_a. This would decrease the rate of soil water depletion so that soil and plant water status will be better in the elevated c_a treatment as the drought develops. These plants might therefore be able to maintain a water status sufficient to attain the maximal stomatal aperture for the given c_a for a longer period into the drought. Indeed, Kimball et al. (1995) have recently shown under free-air CO₂ enrichment on a field scale, that for wheat grown under drought conditions (i.e. 50% of the water required for maximum yield) the enhancement of dry matter production and grain yield by elevated c_a, relative to plants grown at the current ambient c_a, was greater than in plants grown with 100% of the water required for maximum yield. On average, the increase in c_a to 550 µmol^-1 decreased the rate of soil moisture depletion and crop water loss by ca 10%. If however, increased c_a is accompanied by increased air temperature, then this gain may be offset by an increased water vapor pressure deficit (VPD).

The literature survey of Long and Drake (1992) suggested that where rooting volumes are large, acclimatory increases in water use efficiency are smaller than in plants grown with small rooting volumes. These results suggest a balance between increased water use efficiency and photosynthesis, where increase in water use efficiency will be least in elevated c_a when other resources allow the plant to make maximum use of the increased supply of CO₂. This is supported by the observations of Wong (1979) on cotton where stimulation of CO₂-uptake at the lowest nitrogen fertilization rate was least and increase in water-use efficiency greatest.

Decreased transpiration may add an important complication to attempts to identify stomatal acclimation to elevated c_a. If canopy transpiration is decreased, then soil moisture may remain higher, inducing a secondary difference in plant water status, which might be interpreted as stomatal acclimation to elevated c_a.

6.4.3.2 Rising water-air vapor pressure deficit (VPD)

Figure 6.12 Predicted light-saturated rate of leaf CO₂-uptake (A_sat at a photon flux of 1500 µmol m^-2 s^-1, illustrating the effect of an increase in c_a from 350 to 550 µmol mol^-1 with different assumptions about temperature and humidity change. First, no change in temperature or VPD, i.e. the saturation (e_s) and ambient humidity (e_a) difference (VPD = 0.9 kPa). Second, with a 4°C rise in temperature and a concurrent increase in e_a to maintain a constant VPD. Third, with a 4°C increase in temperature with no increase in e_a, which would increase the VPD to 1.8 kPa

Because the saturation humidity of the atmosphere ( e_s) rises almost exponentially with temperature, it is likely that an increase in mean global air temperature will also increase VPD, i.e. the difference between actual and saturation humidity. This will be further increased if leaf-air temperature differences increase. An increase in VPD might also result if increased c_a can inhibit transpiration on a regional scale; on the interior of land masses transpiration is the major source of water vapor in the air. Stomatal conductance often declines with increase in VPD. If photosynthetic capacity remains unchanged, then this increase in VPD would force a decrease in c_i, and hence A. How significant would this decrease be and could it offset the increase expected from the direct effects of elevated c_a on photosynthesis? Figure 6.12 explores this question with WIMOVAC. If an increase of c_a to 550 µmol mol^-1 is concurrent with an increase in mean air temperature of 4°C, then in the worst case scenario there would be no increase in ambient humidity. As a result, VPD would rise. Figure 6.12 shows that if there was no increase in e_a, and assuming an RH of 0.7 at 25 °C, with an increase in temperature of +4 °C, VPD would rise from 0.9 to 1.8 kPa. However, this marked increase in VPD is predicted to remove only 3.5% of the gain in A that would result if c_a and T are simultaneously increased from 350 to 550 µmol mol^-1 and 25 to 29°C, respectively.

If acclimation of stomatal aperture or density to elevated c_a occurs, it could lead to even greater benefits under water stress. Another acclimatory response of stomatal conductance to elevated c_a could involve a change in sensitivity to humidity. Sage (1994) cites evidence from crop plants suggesting that stomata close to a greater degree in response to water stress in plants grown at elevated c_a.

6.5 CONCLUSION

In considering the effects of rising c_a on the carbon cycle in terrestrial biomes, photosynthesis occupies a unique position as the point of energy input into the cycle. Photosynthesis is also unique as the one process in which direct effects at the substrate level can be anticipated. The instantaneous effects of c_a on photosynthesis are well understood and the mechanisms highly conserved across terrestrial vegetation. This has provided a sound platform for the development of mechanistic models that are valid at scales from the cell to the landscape. Exploration of the mechanisms show that it is unreasonable to describe the effects of rising CO₂ on photosynthetic carbon uptake by use of a simple multiplier, such as the p-factor. Interactions with temperature, nitrogen and water can all markedly modify the degree and even direction of change. This is further complicated by the effects of growth at elevated c_a on the photosynthetic apparatus. Although much of the 'down-regulation' of photosynthesis may be explained as an artifact of the pot size used or the measurements made, there are species differences and clear interactions with nutrient supply. Understanding these species differences and nutrient effects will be critical to the development of sound models for the prediction of the long-term changes of photosynthetic carbon uptake by coniferous forests and grasslands.

6.6 REFERENCES

Agren, G. I., McMurtrie, R. E., Parton, W. J., Pastor, J. and Shugart, H. H. (1991) State-of the-art models of production-decomposition linkages in conifer and grassland ecosystems. Ecol. Appl. 1, 118-138.

Allen, L. H. (1990) Plant responses to rising carbon dioxide and potential interactions with air pollutants. J. Environ. Qual. 19, 15-34.

Arp, W. J. (1991) Effects of source-sink relations on photosynthetic acclimation to elevated CO₂. Plant Cell Env. 14, 869-876.

Ball, J. T., Woodrow, I. E. and Berry, J. A. (1987)Amodel predicting stomatal conductance and its contribution to the control of photosynthesis under different environmental conditions. In: Biggins, J. (Ed.) Progress in Photosynthesis Research, pp. 221-224. Nijhoff, Dordrecht.

Bazzaz, F. A. (1990) The response of natural ecosystems to the rising global CO₂ levels. Ann. Rev. Ecol. Syst. 21, 167-196. 

Beerling, D. J. and Chaloner, W. G. (1993a) The impact of atmospheric CO₂ and temperature change on stomatal density: observations from Quercus robur lammas leaves. Ann. Bot. 71,231-235.

Beerling, D. J. and Chaloner, W. G. (1993b) Stomatal density responses of Egyptian Olea europaea L. leaves to CO₂ change since 1327 BC;. Ann. Bot. 71,431-435.

Beerling, D. J., Chaloner, W. G., Huntley, B., Pearson, J. A., Tooley, M. J. and Woodward, F. I. (1992) Variations in the stomatal density of Salix herbacea L. under the changing atmospheric CO₂ concentrations of late. and post-glacial time. Phil. Trans. R. Soc. Land. B 336,215-224.

Berry, J. and Bjorkman, O. ( 1980) Photosynthetic response and adaptation to temperature in higher plants. Ann. Res. Plant Physiol. 31, 491-543.

Besford, T. (1993) Photosynthetic acclimation in tomato plants grown in high CO₂. Vegetatio 104/105, 441-448.

Bowes, G. (1991) Growth at elevated CO₂: photosynthetic responses mediated through Rubisco. Plant Cell Env. 14,195-806.

Bowes, G. (1993) Facing the inevitable: plants and increasing atmospheric CO₂. Ann. Rev. Plant Phys. Mol. Bioi. 44, 309-332.

Bunce, J. A. (1990) Short and long term inhibition of respiratory carbon dioxide efflux by elevated carbon dioxide. Ann. Bot. 65, 637-642.

Bunce, J. A. (1992) Light, temperature and nutrients as factors in photosynthetic adjustment to an elevated concentration of carbon dioxide. Phys. Plant. 86, 173-179.

Campbell, G. S. (1977) An Introduction to Environmental Biophysics. Springer, New York.

Cheeseman, J. (1991) PATCHY: simulating and visualizing the effects of stomatal patchiness on photosynthetic CO₂ exchange studies. Plant Cell Env. 14, 593-600.

Conroy, J. P., Kuppers, M., Kuppers, B., Virgona, J. and Harlow, W. R. (1988) The influence of CO₂ enrichment, phosphorus deficiency and water stress on growth, conductance and water use of Pinus radiata D. Don. Plant Phys. 11,91-98.

Cure, J. D. and Acock, B. (1986) Crop responses to carbon dioxide doubling: a literature survey. Agric. For. Met. 38, 127-145.

Curtis,P. S., Drake, B. G.and Whigham,D. F.(1989) Nitrogen and carbon dynamics in C₃ and C4 estuarine marsh plants grown under elevated CO₂ in situ. Oecologia 78, 297-301.

Drake,B. G. and Leadley, P. W. (1991) Canopy photosynthesis of a crops and native plant communities exposed to long-term elevated CO₂. Plant Cell Env. 14, 853-860.

Eamus, D. (1991)The interaction of rising CO₂ and temperatures with water use efficiency: commissioned review. Plant Cell Env. 14, 843-852.

Eamus, D. and Jarvis, P. G. (1989) The direct effects of increase in the global atmospheric CO₂ concentration on natural and commercial temperate trees and forests. Adv. Ecol. Res. 19, 1-55.

Edwards, G. E. and Walker, D. A. (1983) C₃, C₄ Mechanisms, Cellular and Environmental Regulation of Photosynthesis. Blackwell, Oxford.

Ehleringer, J. and Bjorkman, O. (1977) Quantum yield for CO₂ uptake in C₃ and C₄ plants. Plant Physiol. 59, 86-90.

Esser, G. (1987) Sensitivity of global carbon pools and fluxes to human and potential climatic impacts. Tellus 39B, 245-260.

Evans, J. R. and Farquhar, G. D. (1991) Modeling canopy photosynthesis from the biochemistry of the C₃ chloroplast. In: Boote, K. J. and Loomis, R. S. (Eds) Modeling Crop Photosynthesis- from Biochemistry to Canopy, pp. 1-16. Crop Science Society of America, Inc., Madison, Wis.

Farquhar, G, D. and Sharkey, T. D. (1982) Stomatal conductance and photosynthesis. Ann. Rev. Plant Physiol. 33, 317-345. 

Farquhar, G. D. and von Caemmerer, S. (1982) Modeling of photosynthetic responses to environmental conditions. In: Lange, O L., Nobel, P. S., Osmond, C. B. and Ziegler, H. (Eds) Physiological Plant Ecology. 11. Encyclopaedia of Plant Physiology, New Series, pp. 548-577. Springer, Berlin.

Farquhar, G. D., von Caemmerer, S. and Berry, J. A. (1980) A biochemical model of photosynthetic CO₂ assimilation in leaves of C₃ species. Planta 149, 78-90.

Field, C. (1983) Allocating leaf nitrogen for the maximization of carbon gain: leaf age as a control on the allocation program. Oecologia 56, 341-347.

Forseth, I.N. and Norman, J. M. (1993) Modeling of solar irradiance, leaf energy budget and canopy photosynthesis. In: Hall, D.O., Scurlock, J. M. O., Bolhilr-Nordenkampf, H. R., Leegood, R. C. and Long, S.P. (Eds) Photosynthesis and Productivity in a Changing Environment. pp. 207-219. Chapman & Hall, London.

Grulke, N. E., Rieckhers, G. H., Oechel, W. C., Hjelm, U. and Jaeger, C. (1990) Carbon balance in tussock tundra under ambient and elevated atmospheric CO₂. Oecologia 83, 485-494.

Gunderson, C. A. and Wullschleger, S. D. (1994) Photosynthetic acclimation in trees to rising atmospheric CO₂: a broader perspective. Photosynth. Res. 39, 369-388.

Gutschick, V. P. (1993) Nutrient-limited growth rates: roles of nutrient-use efficiency and of adaptations to increase uptake rate. J. Exp. Bot. 44,41-51.

Harley, P. C. and Tenhunen, J. D. (1991) Modeling the photosynthetic response of C₃ leaves to environmental factors. In: Boote, K. J. and Loomis, R. S. (Eds) Modeling Crop Photosynthesis-from Biochemistry to Canopy, pp. 17-39. Crop Science Society of America, Inc., Madison, Wis.

Harley, P. C., Thomas, R. B., Reynolds, J. F. and Strain, B. R. (1992) Modeling photosynthesis of cotton grown in elevated CO₂. Plant Cell Env. 15, 271-282.

Hendrey, G. R. (1992) Global greenhouse studies: need for a new approach to ecosystem manipulation. Critical Rev. Plant Sci. 11,61-74.

Herold, A. (1986) Regulation of photosynthesis by sink activity-the missing link. New Phytol.86, 131-144.

Hollinger, D. Y. (1987) Gas exchange and dry matter allocation responses to elevation of atmospheric CO₂ concentration in seedlings of three species. Tree Physiol. 3, 193-202.

Humphries, S. W. and Long, S.P. (1995) WIMOVAC: a software package for modelling the dynamics of plant leaf and canopy photosynthesis. CABIOS 11, 361-371.

Idso, S. B. (1989) Carbon Dioxide and Global Change: Earth in Transition. IBR Press, Tempe.

Idso, S. B., Kimball, B. A., Akin, D. E. and Kridler, J. (1993) A general relationship between CO₂-induced reductions in stomatal conductance and concomitant increases in foliage temperature. Env. Exp. Bot. 33, 443-446.

Jarvis, P. G. (1989) Atmospheric carbon dioxide and forests. Phil. Trans. R. Soc. Lond. B 324, 369-392.

Johnsen, K. H. (1993) Growth and ecophysiological responses of black spruce seedlings to elevated CO₂ under varied water and nutrient additions. Can. J. For. Res. 23, 1033-1042.

Jordan, D. and Ogren, W. L. (1984) The CO₂/O₂ specificity of ribulose 1,5 bisphosphate carboxylase/oxygenase dependence on ribulose bisphosphate concentration, pH and temperature. Planta 161, 308-313.

Kaye, G. W. C. and Laby, T. H. (1973) Tables of Physical and Chemical Constants. 4th Edn. Longman, London. 

Kimball, B. A. (1983) Carbon dioxide and agricultural yield: an assemblage and analysis of 430 prior observations. Agron. J. 75, 779-788.

Kimball, B.A., Pinter, P.J., Garcia, R.L., LaMorte, R.L., Wall, G.W., Hunsaker, D.J., Wechsung, G., Wechsung, F. and Kartschall, T. (1995) Productivity and water use of wheat under free-air CO₂ enrichment. Global Change Biol. 1, 425-445. 

Kimball, B. A., Pinter, P.J. and Mauney, I. R. (1992) Cotton leaf and boll temperatures in the 1989 FACE experiment. Critical Rev. Plant Sci. 11, 233-240.

Leggett,J., Pepper, W.J. and Swart, R.I. (1992) Emission scenarios for the IPCC: an update. In: Houghton, J.T., Callander, B.A. and Yarney, S.K. (Eds) Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment, pp. 17-39. Cambridge University Press, Cambridge, UK.

Linke, W. F. (1965) Solubilities: Inorganic and Metal-organic Compounds. 14th Edn. American Chemical Society, Washington DC.

Long, S. P. (1983) C4 photosynthesis at low temperature. Plant Cell Env. 6, 345-363. 

Long, S. P. (1985) Leaf gas exchange. In: Barber, I. and Baker, N.R. (Eds) Photosynthetic Mechanisms and the Environment, pp. 453-499. Elsevier, Amsterdam. 

Long, S. P. (1991) Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO₂ concentrations: has its importance been under- estimated? Plant Cell Env. 14, 729-740.

Long,S. P. and Drake, B. G. (1991) Effects of the long-term elevation of CO₂ concentration in the field on the quantum yield of photosynthesis of the C₃ sedge, Scirpusolneyi. Plant Physiol. 96, 221-226.

Long, S. P. and Drake, B. G. (1992) Photosynthetic CO₂ assimilation and rising atmospheric CO₂ concentrations. In: Baker, N.R. and Thomas, H. (Eds) Crop Photosynthesis: Spatial and Temporal Determinants, pp. 69-103. Elsevier Science Publishers BY, Amsterdam.

Long, S. P., Post], W. F. and Bolhar-Nordenkampf, H. R. (1993) Quantum yields for uptake of carbon dioxide in C₃ vascular plants of contrasting habitats and taxonomic groupings. Planta 189, 226-234.

Long, S. P., and Woodward, F. I. (Eds) (1988) Plants and Temperature. Company of Biologists. Cambridge.

Melillo, J., Callaghan, T. V., Woodward, F. I., Salati, E. and Sinha, S. K. (1990) Effects on ecosystems. In: Houghton, J. T., Jenkins, G. J. and Ephraums J. J. (Eds) Climate Change: The IPCC Scientific Assessment, pp. 283-310. Cambridge University Press, Cambridge.

Mitchell, J. F. B., Manabe, S., Meleshko, V. and Tokioka, T. (1990) Equilibrium climate change- and its implications for the future. In: Hought,on, J. T ., Jenkins, G.J. and Ephraums. J. J. (Eds) Climate Change: The IPCC Scientific Assessment, pp. 131-175. Cambridge University Press, Cambridge.

Mitchell, R. A. C., Mitchell, V. J., Driscoll, S. P., Franklin. J. and Lawlor, D.W. (1993) Effects of increasing CO₂ concentration and temperature on growth and yield of winter wheat at two levels of nitrogen application. Plant Cell Env. 16, 521-529.

Monteith, J. L. and Unsworth, M. J. (1990) Principles of Environmental Physics. 2nd Edn. Edward Arnold, London.

Morison, J. I. L. (1985) Sensitivity of stomata and water use efficiency to high CO₂. Plant Cell Env. 8, 467-474.

Morison, J. I. L. (1987) Intercellular CO₂ concentration and stomatal response to CO₂. In: Zeigler, E., Farquhar, G. D. and Cowan. I. R. (Eds) Stomatal Function, pp. 229-251. Stanford University Press, Stanford, Calif.

Morison, J. I. L. (1993) Responses of plants to CO₂ under water limited conditions. Vegetatio 104/105, 193-209.

Morris, J. T. (1991) Effects of nitrogen loading on wetland ecosystems with particular reference to atmospheric deposition. Ann. Rev. Ecol. Syst. 22, 257-279.

Mott, K. A. (1990) Sensing of atmospheric CO₂ by plants. Plant Cell Env. 13,731-737. 

Mousseau, M. and Saugier, B. (1992) The direct effect of increased CO₂ on gas exchange and growth of forest tree species. J. Exp. Bot. 43, 1121-1130.

Nie, D., He. H., Kirkham, M. B. and Kanemasu, E. T. (1992) Photosynthesis of a C₃ grass and a C4 grass under elevated CO₂. Photosynthetica 26, 189-198.

Nie, G.- Y., Long, S.P., Garcia, R.L., Kimball, B.A., LaMorte, R.L., Pinter, P.J., Wall, G. W. and Webber, A.N. (1995) Effects of free-air CO₂ enrichment on the development of the photosynthetic apparatus in wheat, as indicated by leaf proteins. Plant Cell Env. 18, 855-864.

Nijs, I., Impens, I. and Behaeghe, T. (1989) Leaf and canopy responses of Lolium perenne to long-term elevated atmospheric carbon-dioxide concentration. Planta 177, 312-320.

Norman, J. M. (1980) Interfacing leaf and canopy light interception models. In: Hesketh, J. D. and Jones, J. W; (Eds) Predicting Photosynthesis for Ecosystem Models, pp. 49-67. CRC Press, Boca Raton.

Oberbauer, S. F., Strain, R. and Fetcher, N. (1985) Effects of CO₂-enrichment on seedling physiology and growth of two tropical tree species. Physiol. Plant. 65, 352-356.

Overdieck, D. (1990) Direct effects of elevated CO₂ concentration levels on grass and clover in 'model ecosystems'. In: Beukema, J.J., Wolff, W.J. and Brouns, J. J. W. M. (Eds) Expected Effects of Climatic Change on Marine Coastal Ecosystems, pp. 41-47. Kluwer, Dordrecht.

Parry, M. L. and Carter, T. R. (1988) The assessment of effects of climatic variations on agriculture: aims, methods and summary of results. In: Parry, M. L., Carter, T. R. and Konijn, N. T. (Eds) The Impact of Climatic Variation on Agriculture, pp. 11-95. Kluwer, Dordrecht.

Parry, M. L. and Carter, T. R. (1990) An assessment of the effects of climatic change on agriculture. In: Jackson. M., Ford-Lloyd, B. V. and Parry, M. L. (Eds) Climatic Change and Plant Genetic Resources, pp. 61-84. Belhaven, London.

Patrick, J. W. (1993) Sink strength: whole plant considerations. Plant Cell Env. 16, 1019-1020.

Pearcy, R. W. and Bjorkman, O. (1983) Physiological effects. In: Lemon, E. R. (Ed.) CO₂ and Plants: The Response of Plants to Rising Levels of Atmospheric Carbon Dioxide, pp. 65-105. Westview Press, Boulder, Colo.

Pearson, M., Davies, W .J. and Mansfield, T .A. ( 1995) Asymmetric responses of adaxial and abaxial stomata to elevated CO₂: impacts on the control of gas exchange by leaves. Plant Cell Env. 18, 837-843.

Penman, H. L. (1948) Natural evaporation from open water, bare soil and grass. Proc. R. Soc. Lond. B 193, 120-145. 

Reynolds,J. F. and Acock, B. (1985) Predicting the response of plants to increasing carbon dioxide: a critique of plant growth models. Ecol. Modeling 29, 107-129.

Rowland-Bamford, A. J., Baker, J. T., Allen, H. J. and Bowes, G. (1991) Acclimation of rice to changing atmospheric carbon dioxide concentration. Plant Cell Env. 14, 577-583.

Ryle, G. J. A., Powell, C. E. and Tewson, V. (1992) Effect of elevated CO₂ on the photosynthesis, respiration and growth of perennial ryegrass. J. Exp. Bot. 43,811-818.

Sage, R, F. (1994) Acclimation of photosynthesis to increasing atmospheric CO₂: the gas exchange perspective. Photosynth. Res. 39, 351-368.

Sage, R. F. and Sharkey, T. D. (1987) The effect of temperature on the occurrence of 02 and CO₂ insensitive photosynthesis in field grown plants. Plant Physiol. 84, 658-664.

Sage, R. F., Sharkey, T. D. and Seemann, J. R. (1989) Acclimation of photosynthesis to elevated CO₂ in five C₃ species. Plant Physiol. 89, 590-596.

Schimel, D. (1990) Biogeochemical feedbacks in the Earth system. In: Leggett, J. (Ed.) Global Warming. The Greenpeace Report, pp. 68-82. Oxford University Press, Oxford.

Sharkey, T. D. (1985a) O₂-insensitive photosynthesis in C₃ plants. Plant Physiol. 78, 71-75.

Sharkey, T. D. (1985b) Photosynthesis in intact leaves of C₃ plants: physics, physiology and rate limitations. Bot. Rev. 51, 53-105.

Sheen, J. (1990) Metabolic repression of transcription in higher plants. Plant Cell 2, 1027-1038.

Sheen, J., Huang, H., Schaeffner, A. R., Leon, P. and Jang, J.-C. (1992) Sugars, fatty acids, and photosynthetic gene expression. Photosynth. Res. 34, 107.

Spain, J. D. and Keen, R. E. (1992) Temperature and biological activity. In: Keen, R.E. and Spain, J. D. (Eds) Computer Simulation in Biology, pp. 183-200. Wiley-Liss, New York.

Stitt, M. (1991) Rising CO₂ levels and their potential significance for carbon flow in photosynthetic cells. Plant Cell Env. 14, 741-762.

Stulen, I. and Hartog, J. D. (1993) Root growth and functioning under atmospheric CO₂ enrichment. Vegetatio 104, 99-115. 

Terashima, I. (1992) Anatomy of non-uniform photosynthesis. Photosynth. Res. 31, 195-212.

Thomas, R. B. and Strain, B. R. (1991) Source-sink balance as a factor in the photosynthetic acclimation of cotton to long-term elevated CO₂. Plant Physiol. 96, 627-634.

Thornley, J. H. M. and Johnson, I. R. (1990) Plant and Crop Modeling-a Mathematical Approach to Plant and Crop Physiology. Oxford Univ. Press, Oxford.

Tissue, D. T. and Oechel, W. C. (1987) Response of Eriophorum vaginatum to elevated CO₂ and temperature in the Alaskan tussock tundra. Ecology 68, 401-410.

Tissue, D. T ., Thomas, R. B. and Strain, B. R. ( 1993) Long-term effects of elevated CO₂ and nutrients on photosynthesis and Rubisco in loblolly pine seedlings. Plant Cell Env. 16, 859-865.

Vu, C. V., Allen, L. H., Jr and Bowes, G. (1983) Effects of light and elevated atmospheric CO₂ on the ribulose bisphosphate carboxylase activity and ribulose bisphosphate level of soybean leaves. Plant Physiol. 73, 729-734.

Watson, R. T., Rodhe, H., Oescheger, H. and Siegenthaler, U. (1990) Greenhouse gases and aerosols. In: Houghton, J. T., Jenkins, G. J. and Ephraums, J. J. (Eds) Climate Change: The IPCC Scientific Assessment, pp. 1-40. Cambridge University Press, Cambridge.

Webber,A. N., Nie, G.-Y. and Long, S. P.(1994) Acclimation of photosynthetic proteins to rising atmospheric CO₂. Photosynth. Res. 39, 413-426.

Wise, R. R., Ortiz-Lopez, A. and Ort, D. R. (1992) Spatial distribution of photosynthesis during drought in field-grown and acclimated and non-acclimated growth chamber- grown cotton. Plant Physiol. 100, 26-32.

Wong, S. C. (1979) Elevated atmospheric partial pressure of CO₂ and plant growth. 1. Interactions of nitrogen nutrition and photosynthetic capacity in C₃ and C4 plants. Oecologia 44, 68-74.

Woodward, F. I. (1993) Plant responses to past concentrations of CO₂. Vegetatio 104/105, 145-155.

Yelle,S.,Beeson, R. C.,Jr, Trudel, M. J. and Gosselin,A.(1989) Acclimation of two tomato species to high atmospheric CO₂. I. Sugar and starch concentrations. Plant Physiol. 90, 1465-1472.

Yuncong, L. and Gupta, G. (1993) Photosynthetic changes in soybean with and without nitrogen and increased carbon dioxide. Plant Sci. 89, 1-4.

6.7 ABBREVIATIONS