Interactive effects of water deficit and nitrogen deficiency on photosynthesis, its underlying component processes, and carbon loss processes in cotton

Drought stress and nitrogen (N) deficiency are important abiotic stresses that severely limit net photosynthetic rate (A_N). A number of studies have investigated the underlying physiological limitations to A_N in response to water deficit or N deficiency; however, the relative sensitivities of photosynthetic component processes and carbon loss processes to combined drought and N deficiency in field-grown cotton (Gossypium hirsutum L.) have not been explored. Therefore, the objective of the present study was to determine the effects of combined water deficit and nitrogen deficiency on the underlying physiological processes driving A_N in field-grown cotton. Water-deficit stress caused substantial reductions in A_N, but reductions in A_N were greater under optimum N conditions (74%) than under N deficiency (22%). Decreased CO₂ diffusion, RuBP regeneration, and Rubisco carboxylation were major contributors to a decline in A_N due to water-deficit stress. Reductions in Rubisco carboxylation and RuBP regeneration were the greatest drivers of N deficiency-induced decline in A_N. Lower CO₂ diffusion and Rubisco carboxylation were main constraints to A_N due to combined water deficit and N deficiency. Regarding carbon loss processes, both dark respiration and photorespiration under water-deficit stress or N deficiency, and only photorespiration under combined water deficit and N deficiency, contributed to declines in A_N. Increased non-photochemical quenching and/or photorespiration prevented photoinhibition of photosystem II under stress conditions. Overall, response of photosynthesis to water-deficit stress was dependent on N availability, and rate-limiting physiological processes contributing to declines in A_N were dependent on the type of prevailing stress.