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 380 mu mol mol-1 CO2 concentration (A(380)) at several leaf temperatures were studied in several crops, since N nutrition alters N allocation between photosynthetic components. Contents of leaf N, ribulose 1 center dot 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 A(380). 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 C-3 photosynthesis model indicated that A(380) 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 A(380) depends on leaf N content and is mainly determined by N partitioning between Rubisco and electron transport components.