Does down-regulation of photosynthetic capacity by elevated CO2 depend on N supply in Dactylis glomerata?

Dactytlis glomerata was grown hydroponically in a controlled environment at ambient (360 mu l I (- 1)) or elevated (680 mu l I (- 1)) CO, and four con centrations of nitrogen (0.15, 0.6, 1.5 and 6.0 mM NO3-), to test the hypot hesis that reduction of photosynthetic capacity at elevated [CO2] is depend ent on N availability and mediated by a build-up of non-structural carbohyd rates. Photosynthetic capacity of the youngest fully expanded leaf (leaf 5, 2 days after full expansion) was reduced in CO2-enriched plants at low, bu t not high N supply and so the stimulation of net photosynthesis by CO2 enh ancement was less at low than at high N supply, CO2 enrichment resulted in a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) con tent on a leaf area basis at 0.6 and 1.5 mM NO3- but not at 0.15 and 6.0 mM NO3- and had no effect on the total N content of the leaf on an area basis , However, decreases in Rubisco content could be primarily accounted for by a decrease in total N content of leaves, independent of [CO2]. A doubling of the Rubisco content by increasing the N supply beyond 0.6 mM had only a marginal effect on the maximum carboxylation velocity in vivo, suggesting t hat the fraction of inactive Rubisco increased with increasing N supply. Al though CO2-enriched plants accumulated more non-structural carbohydrates in the leaf, the reduction of photosynthetic capacity at low N supply was not mediated simply by a build-up of carbohydrates. In D. glomerata, the photo synthetic capacity was mainly determined by the total N content of the leaf .