Leaf Nitrogen Content as a Predictor of Photosynthetic Capacity in Ambient and Global Change Conditions

Abstract

Leaf assimilation capacity in Lolium perenne, grown in elevated CO2 level (700 ,umol mol-1) and/or in- creased air temperature (ambient + 4?C) could be predicted from leaf N content expressed on an area basis, although the linear relationships between maximum carboxylation rate (Vcmax) or maximum electron transport rate (Jmax) and leaf N depended on treatment. The model, based on Farquhar, Von Caemmerer & Berry (1980) showed negative long-term effects of increased air temperature on Vcmax and Jmax, while long- term exposure to increased CO2 level affected only Vcmax. Acclimation responses to these global changes therefore could not be explained by changes in N-content alone, but also in terms of changes in photosynthetic nitrogen use efficiency. Stimulation of photosynthesis by elevated CO2 was not affec- ted by reduction of leaf N in leaves developed in ambient air temperature, while part of the CO2 benefit was lost in leaves developed in increased air temperature. This suggests that N-deficient ecosystems maintain the potential to respond to elevated CO2 concentration, unless other processes than the primary carbon metabolism become limiting at low N supply. Similar to nitrogen content, changes in photon flux density did not change the CO2 benefit either, unless a transition occurred from one limiting process to another (electron transport to carboxylation or vice versa). Hypotheses on interaction be- tween CO2 level, nitrogen status of the leaf and light intensity are formulated to support these findings.