Differential sensitivities of photosynthetic processes and carbon loss mechanisms govern N-induced variation in net carbon assimilation rate for field grown cotton (Gossypium hirsutum L.).

Nitrogen (N) deficiency limits net carbon assimilation rates (AN), 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 AN 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. RuBP regeneration and carboxylation, AN, 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 AN 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.