The continuous damage to nitrogen metabolism caused by heat stress limited the young ear growth and yield of maize (Zea mays L.)

Extreme high-temperature events triggered by global warming have posed a significant threat to maize production. Plant nutrients such as nitrogen (N) offer an opportunity in increasing heat stress tolerance and sustaining maize productivity. However, it remains unclear how maize growth responds to N and carbon (C) metabolism under heat stress. In this study, the heat-tolerant hybrid Zhengdan958 (ZD) and the heat-sensitive hybrid Xianyu335 (XY) were subjected to heat stress (day/night: 38 °C/30 °C) for seven consecutive days at the twelfth-leaf stage (V12) and the tassel stage (VT) respectively. The maize grown under natural temperature conditions serving as the control. Heat stress strongly inhibited maize organ growth compared with the control. Especially, the dry weight of young ears suffered more severe damage than that of other organs. During the heating process, the ¹⁵N uptake, ¹³C assimilation, and their redistribution to young ear was significantly reduced. After heat stress relief, plant growth rate was not recovered due to continuous decline in C assimilation capacity of leaf N (C/N_leaf), N utilization efficiency (NUtE) and C accumulation. The activities of enzymes related to N and C metabolism, such as sucrose synthase, nitrate reductase, glutamate dehydrogenase, glutamine oxoglutarate aminotransferase reduced averagely by 24.5 %, 23.2 %, 28.9 %, and 27.8 % under heating process, respectively, and failed to be effectively restored in a long time after heat stress relief. In addition, the total accumulation of non-structural carbohydrates (NSC) in Zhendan958 under heat stress at the V12 and VT decreased by 26.5 % and 33.0 % compared with control plants, while that of Xianyu335 decreased by 34.4 % and 42.5 % respectively. Meanwhile, heat stress strongly inhibited the distribution and remobilization of NSC to kernels. The negative impacts induced by heat stress ultimately led to a decrease in maize yield. Compared with the control, the kernel yield of Zhendan958 under heat stress at the V12 and VT stages reduced by 54.6 % and 71.9 %, while those of Xianyu335 reduced by 74.4 % and 83.44 % respectively. The results indicated that the reduction of N distribution and the continuous damage to N metabolism enzyme activities under heat stress were the main pathways inhibiting the growth of young ears and reducing kernel yield, respectively. In contrast, heat-tolerant hybrid promote the growth of young ears under heat stress and reduce yield losses by improving N metabolism.