Acclimation of photosynthesis to elevated CO 2 has previously been shown to be more pronounced when N supply is poor. Is this a direct effect of N or an indirect effect of N by limiting the development of sinks for photoassimilate? This question was tested by growing a perennial ryegrass (Lolium perenne) in the field under elevated (60 Pa) and current (36 Pa) partial pressures of CO 2 (pCO2) at low and high levels of N fertilization. Cutting of this herbage crop at 4-to 8-week intervals removed about 80% of the canopy, therefore decreasing the ratio of photosynthetic area to sinks for photoassimilate. Leaf photosynthesis, in vivo carboxylation capacity, carbohydrate, N, ribulose-1,5-bisphosphate carboxylase/ oxygenase, sedoheptulose-1,7-bisphosphatase, and chloroplastic fructose-1,6-bisphosphatase levels were determined for mature lamina during two consecutive summers. Just before the cut, when the canopy was relatively large, growth at elevated pCO 2 and low N resulted in significant decreases in carboxylation capacity and the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase protein. In high N there were no significant decreases in carboxylation capacity or proteins, but chloroplastic fructose-1,6-bisphosphatase protein levels increased significantly. Elevated pCO 2 resulted in a marked and significant increase in leaf carbohydrate content at low N, but had no effect at high N. This acclimation at low N was absent after the harvest, when the canopy size was small. These results suggest that acclimation under low N is caused by limitation of sink development rather than being a direct effect of N supply on photosynthesis.Acclimation of photosynthesis to growth in elevated pCO 2 has frequently been shown to be more marked under suboptimal N supply (Drake et al., 1997). Growth in low N limits the development of the shoot and root, and therefore the capacity for utilization of the additional photoassimilate formed under elevated pCO 2 . Low N may therefore exacerbate the accumulation of carbohydrate observed under elevated pCO 2 (Webber et al., 1994; Drake et al., 1997). Alternatively, nitrate accumulation within the plant can alter gene expression (Paul and Driscoll, 1997; Scheible et al., 1997), and could lead to different patterns of acclimation to elevated pCO 2 depending on the N supply. Wheat grown under limiting N supply showed a greater loss of Rubisco in response to elevated pCO 2 than plants grown with free access to N (Rogers et al., 1996). This appeared to result from an accumulation of soluble carbohydrates in leaves, resulting in sugar repression of the expression of the genes encoding the LSU and the small subunit of Rubisco (rbcL and rbcS, respectively) (Stitt, 1991; Sheen, 1994; Krapp and Stitt, 1995; Koch, 1996).Most studies of acclimation to elevated pCO 2 under different levels of N nutrition have been conducted in containers in the laboratory. However, Arp (1991) demonstrated that such restriction of rooting volume might accentuate acclimation to elevated pCO 2 . In addition to the p...

Artificial chalk grassland swards were exposed to either ambient air or air enriched to 600 µmol mol -1 CO 2 , using free-air CO 2 enrichment technology, and subjected to an 8 week simulated grazing regime. After 14 months of treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco) activity (V c,max ) and electron transport mediated ribulose-1,5-bisphosphate (RuBP) regeneration capacity (J max ), estimated from leaf gas exchange, were significantly lower in fully expanded leaves of Anthyllis vulneraria L. (a legume) and Sanguisorba minor Scop. grown in elevated CO 2 . After a change in source:sink balance brought about by defoliation, photosynthetic capacity was fully restored in A. vulneraria and S. minor, but acclimation continued in the grass Bromopsis erecta (Hudson) Fourr. Changes in net photosynthesis (P n ) with growth at elevated CO 2 ranged from a 1·6% reduction in precut leaves of A. vulneraria to a 47·1% stimulation in postcut leaves of S. minor. Stomatal acclimation was observed in leaves of A. vulneraria (reduced stomatal density) and B. erecta (reduced stomatal conductance). The results are discussed in terms of whole-plant resource-use optimization and chalk grassland community competitive interactions at elevated CO 2 .

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