Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO2] in white birch seedlings.

To study the effects of elevated CO₂ concentration ([CO₂]) on relationships between nitrogen (N) nutrition and foliar gas exchange parameters, white birch (Betula papyrifera Marsh.) seedlings were exposed to one of five N-supply regimes (10, 80, 150, 220, 290 mg Nl^-1) in either ambient [CO₂] (360 µmol mol^-1) or elevated [CO₂] (720 µmol mol^-1) in environment-controlled greenhouses. Foliar gas exchange and chlorophyll fluorescence were measured after 60 and 80 days of treatment. Photosynthesis showed a substantial down-regulation (up to 57%) in response to elevated [CO₂] and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration. When measured at the growth [CO₂], elevated [CO₂] increased the overall rate of photosynthesis (P_n) and instantaneous water-use efficiency (IWUE) by up to 69 and 236%, respectively, but decreased transpiration (E) and stomatal conductance (g_s) in all N treatments. However, the degree of stimulation of photosynthesis by elevated [CO₂] decreased as photosynthetic down-regulation increased from 60 days to 80 days of treatment. Elevated [CO₂] significantly increased total photosynthetic electron transport in all N treatments at 60 days of treatment, but the effect was insignificant after 80 days of treatment. Both P_n and IWUE generally increased with increasing leaf N concentration except at very high leaf N concentrations, where both P_n and IWUE declined. The relationships of P_n and IWUE with leaf N concentration were modeled with both a linear regression and a second-order polynomial function. Elevated [CO₂] significantly and substantially increased the slope of the linear regression for IWUE, but had no significant effect on the slope for P_n. The optimal leaf N concentration for P_n and IWUE derived from the polynomial function did not differ between the CO₂ treatments when leaf N was expressed on a leaf area basis. However, the mass-based optimal leaf N concentration for P_n was much lower in seedlings in elevated [CO₂] than in ambient [CO₂] (31.88 versus 37.00 mg g^-1). Elevated [CO₂] generally decreased mass-based leaf N concentration but had no significant effect on area-based leaf N concentration; however, maximum N concentration per unit leaf area was greater in elevated [CO₂] than in ambient [CO₂] (1.913 versus 1.547 g N m^-2). [ABSTRACT FROM AUTHOR]