The Effect of Leaf Nitrogen and Temperature on the CO₂ Response of Photosynthesis in the C₃ Dicot MChenopodium album L

Abstract

The CO₂ response of photosynthesis was studied in the C₃ annual, Chenopodium album L. Both the initial slope of the photosynthetic CO₂ response and the CO₂ saturated rate of photosynthesis were linearly dependent on organic leaf nitrogen content. As leaf nitrogen increased or leaf temperature declined, the CO₂ saturation point of photosynthesis declined. Increasing leaf temperature from 15 to 34°C stimulated the CO₂-saturated rate of photosynthesis but had little effect on the initial slope of the photosynthetic CO₂ response. According to the photosynthesis model of Sharkey (1985 Bot. Rev. 51: 53-105), these results indicate that as leaf nitrogen increased, the capacity for RuP₂ carboxylase and RuP₂ regeneration increased to a greater extent than the capacity of starch and sucrose synthesis to regenerate orthophosphate. As a result, in high nitrogen leaves, photosynthesis appeared to be limited by the capacity to regenerate phosphate at lower CO₂ partial pressures than in low nitrogen leaves. In high nitrogen leaves, increasing temperature appeared to enhance the phosphate regeneration capacity to a greater extent than the capacity of RuP₂ carboxylase. Consequently, while under cool conditions (<20°C), CO₂ assimilation in normal atmospheric air appeared to be limited by the phosphate regeneration capacity, under warm conditions (34°C), RuP₂ carboxylase capacity appeared to limit CO₂ assimilation.