LEAF NITROGEN-CONTENT AS A PREDICTOR OF PHOTOSYNTHETIC CAPACITY IN AMBIENT AND GLOBAL CHANGE CONDITIONS

Leaf assimilation capacity in Lolium perenne, grown in elevated CO2 le vel (700 mu mol mol(-1)) and/or increased air temperature (ambient + 4 degrees C) could be predicted from leaf N content expressed on an are a basis, although the linear relationships between maximum carboxylati on rate (Vc(max)) Or maximum electron transport rate (J(max)) and leaf N depended on treatment. The model, based on Farquhar, Von Caemmerer & Berry (1980) showed negative long-term effects of increased air temp erature on Vc(max) and J(max) while longterm exposure to increased CO2 level affected only Vc(max). Acclimation responses to these global ch anges 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 affected by re duction of leaf N in leaves developed in ambient air temperature, whil e part of the CO2 benefit was lost in leaves developed in increased ai r temperature. This suggests that N-deficient ecosystems maintain the potential to respond to elevated CO2 concentration, unless other proce sses than the primary carbon metabolism become limiting at low N suppl y. 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 vi ce versa). Hypotheses on interaction between CO2 level, nitrogen statu s of the leaf and light intensity are formulated to support these find ings.