Excessive nitrogen application under moderate soil water deficit decreases photosynthesis, respiration, carbon gain and water use efficiency of maize.

The impact of water stress and nitrogen (N) nutrition on leaf respiration (R), carbon balance and water use efficiency (WUE) remains largely elusive. Therefore, the objective of the present study was to investigate the effect of soil water and N stresses on growth, physiological responses, leaf structure, carbon gain and WUE of maize. The plants were subjected to different soil water and N regimes to maturity. The results showed that the photosynthesis (An) and stomatal conductance (Gs) decreased significantly under the water stressed treatments across the N treatments mainly ascribed to the decreased plant water status. The moderate water stress reduced the photosynthetic capacity and activity and also caused damage to the structure of leaves, resulting in the significant reduction of An, and thus decreased WUEi. The dark respiration (Rd) was significantly decreased due to the damage of mitochondria, however, the Rd/An increased significantly and the carbon gain was seriously compromised, eventually inhibiting biomass growth under the moderately water stressed treatment. Increasing N dose further aggravated the severity of water deficit, decreased An, Gs and WUEi, damaged the structure and reduced the number of mitochondria of leaves, while increased Rd/An considerably under moderate water stress. Consequently, the biomass accumulation, carbon gain and plant level WUEp in the moderately water stressed treatment decreased markedly under the high N supply. Therefore, excessive N application should be avoided when plants suffer soil water stress in maize production.