Differential sensitivities of photosynthetic processes and carbon loss mechanisms govern N-induced variation in net carbon assimilation rate for field-grown cotton

Net carbon assimilation is co-limited by RuBP regeneration and Rubisco carboxylation under N deficiency, and lower sensitivities of dark respiration and photorespiration contributed to nitrogen-induced declines in assimilation in cotton. Nitrogen (N) deficiency limits the net carbon assimilation rate (A(N)), but the relative N sensitivities of photosynthetic component processes and carbon loss mechanisms remain relatively unexplored for field-grown cotton. Therefore, the objective of the current study was to define the relative sensitivity of individual physiological processes driving N deficiency-induced declines in A(N) for field-grown cotton. Among the potential diffusional limitations evaluated, mesophyll conductance was the only parameter substantially reduced by N deficiency, but this did not affect CO2 availability in the chloroplast. A number of metabolic processes were negatively impacted by N deficiency, and these effects were more pronounced at lower leaf positions in the cotton canopy. Ribulose bisphosphate (RuBP) regeneration and carboxylation, A(N), and gross photosynthesis were the most sensitive metabolic processes to N deficiency, whereas photosynthetic electron transport processes, electron flux to photorespiration, and dark respiration exhibited intermediate sensitivity to N deficiency. Among thylakoid-specific processes, the quantum yield of PSI end electron acceptor reduction was the most sensitive process to N deficiency. It was concluded that A(N) is primarily limited by Rubisco carboxylation and RuBP regeneration under N deficiency in field-grown cotton, and the differential N sensitivities of the photosynthetic process and carbon loss mechanisms contributed significantly to photosynthetic declines.