The effect of leaf nitrogen and temperature on the CO2 response of photosynthesis in the C3 dicot Chenopodium album L.

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.