The rate-limiting step for CO₂ assimilation at different temperatures is influenced by the leaf nitrogen content in several C₃ crop species

Effects of nitrogen (N) supply on the limiting step of CO₂ assimilation rate (A) at 380μmolmol^-1 CO₂ concentration (A₃₈₀) at several leaf temperatures were studied in several crops, since N nutrition alters N allocation between photosynthetic components. Contents of leaf N, ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) increased with increasing N supply, but the cyt f/Rubisco ratio decreased. Large leaf N content was linked to a high stomatal (g_s) and mesophyll conductance (g_m), but resulted in a lower intercellular (C_i) and chloroplast CO₂ concentration (C_c) because the increase in g_s and g_m was insufficient to compensate for change in A₃₈₀. The A-C_c response was used to estimate the maximum rate of RuBP carboxylation (V_cmax) and chloroplast electron transport (J_max). The J_max/V_cmax ratio decreased with reductions in leaf N content, which was consistent with the results of the cyt f/Rubisco ratio. Analysis using the C₃ photosynthesis model indicated that A₃₈₀ tended to be limited by RuBP carboxylation in plants grown at low N concentration, whereas it was limited by RuBP regeneration in plants grown at high N concentration. We conclude that the limiting step of A₃₈₀ depends on leaf N content and is mainly determined by N partitioning between Rubisco and electron transport components.