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

Bing Cao (1,2), Qing-Lai Dang (1,3) and Shouren Zhang (1,4)

1. Faculty of Forestry and the Forest Environment, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada / 2. Department of Forestry and Horticulture, School of Agriculture, Ningxia University, Xixia District, Yinchuan, Ningxia 750021, China / 3. Corresponding author (qinglaidang@hotmail.com) / 4. Laboratory of Quantitative Vegetation Ecology, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China / Received May 10, 2006; accepted August 22, 2006; published online March 1, 2007

Summary

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 N l^–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).

Keywords: boreal trees, climate change, CO₂–nitrogen interaction.

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