KEY WORDS: CO2 and plants, CO2 and photosynthesis, CO2 and stomata, CO2 and respiration, plants and climate change ABSTRACTThe primary effect of the response of plants to rising atmospheric CO 2 (C a ) is to increase resource use efficiency. Elevated C a reduces stomatal conductance and transpiration and improves water use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency. Acclimation of photosynthesis during long-term exposure to elevated C a reduces key enzymes of the photosynthetic carbon reduction cycle, and this increases nutrient use efficiency. Improved soil-water balance, increased carbon uptake in the shade, greater carbon to nitrogen ratio, and reduced nutrient quality for insect and animal grazers are all possibilities that have been observed in field studies of the effects of elevated C a . These effects have major consequences for agriculture and native ecosystems in a world of rising atmospheric C a and climate change. CONTENTS

Abbreviations: t, Rubisco specificity factor (dimensionless); G*, CO 2 compensation point in the absence of dark respiration (mmol mol -2 ); DH a , energy of activation (kJ mol -1 ); A, net rate of CO 2 uptake per unit leaf area (mmol m -2 s -1 ); c, scaling constant (dimensionless); C i , intercellular CO 2 concentration (mmol mol -1 ); K c , Michaelis constant for CO 2 (mmol mol -1 ); K o , Michaelis constant for O 2 (mmol mol -1 ); R, molar gas constant (kJ K -1 mol -1 ); R d , Mitochondrial respiration rate in the light (mmol m -2 s -1 ); T k , Leaf absolute temperature (K); v c , carboxylation velocity (mmol m -2 s -1 ), V c,max , maximum RuBP saturated rate of carboxylation (mmol m -2 s -1 ); v o , oxygenation velocity (mmol m -2 s -1 ); V o,max , maximum RuBP saturated rate of oxygenation (mmol m -2 s -1 ); W c , RuBP saturated rate of carboxylation (mmol m -2 s -1 ); PPFD, photosynthetically active photon flux density (mmol m -2 s -1 ).

Abbreviations: t, Rubisco specificity factor (dimensionless); G*, CO 2 compensation point in the absence of dark respiration (mmol mol ).

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); PPFD ¢ , daily integral of PPFD; Rubisco, ribulose-1,5-bisphosphate carboxylase oxygenase; T leaf , leaf temperature ( ∞ C); TNC, total non-structural carbohydrate (mmol m -2 ); VPD, vapour pressure deficit (kPa).

Although C4 plants are considered to have higher conversion efficiency and productivity than C3, plants, this advantage may not be realized under sub‐optimal conditions. Two perennial C4 rhizomatous grasses of cool temperate origin, Miscanthus × giganteus and Spartina cyno‐suroides, have been suggested as potential fuel crops for north‐western Europe. The conversion efficiencies of these species were examined for 2 years in fertilized, irrigated, replicated plots in south‐eastern England. In the second year, the energy conversion efficiencies for shoot and total biomass production were 0.040 and 0.051, respectively, for S. cynosuroides, with significantly higher values of 0.060 and 0.078, respectively, for M. × giganteus. The M. × giganteus crop attained shoot productivity of 2.87 kg m−2 between April and September, exceeding the highest values typically obtained with intensively managed C3, crops. Canopy development was early, and high interception and conversion efficiencies were maintained over most of the growing season. This study provides the evidence that the superior potential light conversion efficiencies associated with C4 photosynthesis can be realized under cool temperate conditions and that such climatic conditions do not Inherently impair the C4 process.

Repression of photosynthetic genes by increased soluble carbohydrate concentrations may explain acclimation of photosynthesis to elevated CO, concentration. This hypothesis was examined in a field crop of spring wheat (Triticum aestivum L.) grown at both ambient (approximately 360 pmol mol-') and elevated (550 pmol mol-') atmospheric CO, concentrations using free-air CO, enrichment at Maricopa, Arizona. The correspondence of steady-state levels of mRNA transcripts (coding for the 83-kD photosystem I apoprotein, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, phosphoglycerokinase, and the large and small subunits of ribulose-l,5-bisphosphate carboxylase/oxygenase) with leaf carbohydrate concentrations (glucose-6-phosphate, glucose, fructose, sucrose, fructans, and starch) was examined at different stages of crop and leaf development and through the diurna1 cycle. Overall only a weak correspondence between increased soluble carbohydrate concentrations and decreased levels for nuclear gene transcripts was found. The difference in soluble carbohydrate concentration between leaves grown at elevated and current ambient CO, concentrations diminished with crop development, whereas the difference i n transcript levels increased. In the flag leaf, soluble carbohydrate concentrations declined markedly with the onset of grain filling; yet transcript levels also declined. The results suggest that, whereas the hypothesis may hold well i n model laboratory systems, many other factors modified its significance in this field wheat crop.Short-term exposure of terrestrial plants to [CO,] above the current ambient concentration of about 360 pmol mol-' causes an immediate increase in photosynthesis and primary production in C, species (Kramer, 1981;Cure and Acock, 1986). However, growth for prolonged periods at elevated [CO,] may result in physiological acclimation involving a decrease in their capacity for CO, assimilation (Peet et al., 1986;Sage et al., 1989;Yelle et al., 1989aYelle et al., , 1989b that is frequently associated with increased starch and soluble carbohydrate concentration in the source leaves Sasek et al., 1985;Stitt, 1991;Long and Drake, 1992). Severa1 lines of indirect evidence suggest that regulation of the expression of photosynthetic genes, via increased soluble carbohydrate concentration, may underlie acclimation to growth in elevated [CO,] (Sheen, 1990, 1994Stitt, 1991;Krapp et al., 1993;Webber et al., 1994). Decreased expression of severa1 photosynthetic genes has occurred when sugar concentrations have been increased manipulatively by directly feeding mature spinach leaves through the transpiration stream (Krapp et al., 1991), by expression of yeast-derived invertase in transgenic tobacco plants, which directs the gene product to the cell wall to interrupt export from source leaves (Stitt, 1991), and by cooling the petiole to decrease the rate of phloem transport in intact tobacco plants ( Krapp et al., 1993). Sheen (1990) demonstrated a possible mechanism for this by showing that addition of solub...