Effects of Nitrogen on Nitrogen Accumulation and Distribution, Nitrogen Metabolizing Enzymes, Protein Content, and Water and Nitrogen Use Efficiency in Winter Wheat Under Heat and Drought Stress After Anthesis

The diurnal variation of temperature was simulated based on the growth chamber, this study aimed to investigate the effects of nitrogen (N) on dry matter accumulation, N accumulation and distribution, activities of N metabolism-related enzymes, protein content, yield, and water and N use efficiency of winter wheat plants under heat, drought and combined stress.

The experiments were carried out based on growth chambers with Xiaoyan 22 as test material. The experiment consisted of three blocks, in which two temperature treatments (high temperature: H; suitable temperature: S) were assigned as the main plot, two watering treatments (drought: D; sufficient water supply: F) were arranged split-plot, and three N supply levels (low N: N₁; medium N: N₂; high N: N₃) were arranged split-split plot to form a completely randomized block design to investigate the response of the growth and physiological characteristics, yield, and water and N use efficiency in wheat plants to heat, drought stress and different N applications.

Heat, drought and combined stress resulted in the decrease in ADW (aboveground dry weight) and ANA (aboveground nitrogen accumulation). At maturity, the ANA of N₃ supply under HD and SD was higher 7.26% and 6.82% than that under N₁ supply, respectively. Heat, drought and combined stress resulted in the increase in NRR, and the average NRR of three N supplies under HD increased by 38.21% compared with the control, while increasing N supply further expanded this increasing effect. Heat, drought and combined stress led to decrease in N distribution rate of panicle at maturity, especially combined stress. PY decreased significantly when exposed to heat, drought and combined stress. Compared with the control, the decrease of PY under drought stress conditions (7.37%) was more obvious than that under heat stress conditions (3.94%). Under individual and combined stress treatments, PY was significantly increased under N₂ supply. Furthermore, GS and NR activities decreased under individual heat and drought stress, which were significantly increased under regulation of N₂ supply. The NR and GS activities of N₁ supply under HD were 23.81% and 23.07% higher than that of N₃ supply, respectively. Compared with the control, the reduction in grain number per spike, 1000 grain weight and yield under drought stress conditions was greater than that under heat stress. N₂ supply had an obvious positive effect on these parameters of the two stress treatments, and WUE_g and WUE_b were significantly improved under N₂ supply. Adequate water supply under N₂ had 19.09% and 19.44% higher NUE_g than drought and combined stress under N₃, respectively. This indicates adequate water supply under medium N could effectively alleviate the decrease of NUE_g induced by drought and heat stress. The increase of NUE_g and NUE_b might be attributed to increase of GS and NR activities by appropriate N supply. Principal component analysis indicated that TGW and ADW of wheat were more closely related to yield under stresses conditions.

The results showed that combined effect of drought and heat stress was more detrimental than individual stresses. Under individual heat and drought stress, an appropriate N supply could increase the activities of N metabolism enzymes and maintain higher N metabolism capacity, improve GNA and PY, and would be much more beneficial to increasing grain yield, water and N use efficiency in wheat production. However, when wheat was subjected to the combined stress after anthesis, compared with low N supply, increasing N supply had a restrictive effect on wheat yield formation as well as water and N utilization capacity, while N supply should be appropriately reduced.