Nitrogen mitigates the negative effects of combined heat and drought stress on winter wheat by improving physiological characteristics

Extreme drought stress is often accompanied by heat stress after anthesis in winter wheat. Whether nitrogen (N) can mitigate the damage caused by combined stress on wheat plants by regulating root physiological characteristics is still unclear. Thus, this study aimed to study the effects of combined heat and drought stress on photosynthesis, leaf water relations, root antioxidant system, osmoregulatory, and yield in wheat to reveal the physiological mechanism of N regulating the adverse impacts of combined stress on wheat. Heat and drought stress markedly reduced photosynthesis, leaf water content, root vitality, and bleeding sap. The combination of heat and drought strengthens these changes. Within a certain stress range, the increase in soluble sugar and proline contents and the activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase under combined stress effectively alleviated the oxidative damage. Compared with those under high N application (N3), wheat plants under low N application (N1) maintained higher yield and yield components under combined stress; the number of grains per spike, 1000-grain weight, and yield increased by 13.65%, 9.07%, and 15.33%, respectively, under N1 compared with those under N3 treatment, which may be attributed to the greater maintenance of photosynthesis, leaf water status, root vitality, and antioxidant and osmoregulation capacities. In summary, reduced N application mitigated the damage caused by combined heat and drought stress in wheat by improving root physiological characteristics and enhanced adaptability to combined stress, which is an appropriate strategy to compensate for yield losses.