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To study the effects of elevated CO 2  concentration ([CO 2 ]) 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 N l −1 ) in either ambient [CO 2 ] (360 μmol mol −1 ) or elevated [CO 2 ] (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 [CO 2 ] and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration. When measured at the growth [CO 2 ], elevated [CO 2 ] 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 [CO 2 ] decreased as photosynthetic down-regulation increased from 60 days to 80 days of treatment. Elevated [CO 2 ] 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 [CO 2 ] 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 CO 2  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 [CO 2 ] than in ambient [CO 2 ] (31.88 versus 37.00 mg g −1 ). Elevated [CO 2 ] 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 [CO 2 ] than in ambient [CO 2 ] (1.913 versus 1.547 g N m −2 ).

Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO 2 ] in white birch seedlings