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 (AN). A number of studies have investigated the underlying physiological limitations to AN 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 AN in field-grown cotton. Water-deficit stress caused substantial reductions in AN, but reductions in AN 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 AN due to water-deficit stress. Reductions in Rubisco carboxylation and RuBP regeneration were the greatest drivers of N deficiency-induced decline in AN. Lower CO2 diffusion and Rubisco carboxylation were main constraints to AN 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 AN. 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 AN were dependent on the type of prevailing stress. Response of photosynthesis to water-deficit stress was dependent on nitrogen rate. CO2 diffusion, RuBP regeneration, and Rubisco carboxylation were major limitations to photosynthesis under water deficit. Rubisco carboxylation and RuBP regeneration co-limited photosynthesis under nitrogen deficiency conditions. CO2 diffusion and Rubisco carboxylation limited photosynthesis under combined water deficit and nitrogen deficiency. Non-photochemical quenching and/or photorespiration prevented photodamage to photosystem II under stress conditions.