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

Dactylis glomerata was grown hydroponically in a controlled environment at ambient (360 μl l−1) or elevated (680 μl l−1) CO2 and four concentrations of nitrogen (0.15, 0.6, 1.5 and 6.0 mM NO3−), to test the hypothesis that reduction of photosynthetic capacity at elevated [CO2] is dependent on N availability and mediated by a build‐up of non‐structural carbohydrates. Photosynthetic capacity of the youngest fully expanded leaf (leaf 5, 2 days after full expansion) was reduced in CO2‐enriched plants at low, but not high N supply and so the stimulation of net photosynthesis by CO2 enhancement was less at low than at high N supply. CO2 enrichment resulted in a decrease in ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) content 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 that the fraction of inactive Rubisco increased with increasing N supply. Although 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 photosynthetic capacity was mainly determined by the total N content of the leaf.