Low Grain Yield of Maize ( <i>Zea mays</i> L) due to Stunted Tassel and Limited Sugar Transport Under Drought and Heat During Flowering

ABSTRACT Global seasonal heat and drought have become the main limiting factors affecting maize growth and grain yield. Huanong138 (HN138) and Zhengdan958 (ZD958) were used as the experimental material. The greenhouse experiment was conducted to uncover tassels and ear morphological development and grain fertility rate of maize under three stressors (heat (H), drought (D), and heat + drought (H + D)) at tasselling for 14 days. On average of the two hybrids, grain yield decreased by 17.3%, 31.2% and 45.4% in H, D and H + D, respectively, compared with CK. The grain fertility rate of stress treatments significantly reduced by 11.6%–28.7% under natural pollination (H♀ × H♂, D♀ × D♂), and reduced by 6.7%–24.7% under the condition of positive pollination (CK♂ × H or/and D♀), while reduced by 24.2%–48.9% under the condition of reverse pollination (CK♀ × H or/and D♂) than control, respectively. Moreover, pollen quantity, total spikelet number and density of tassels, silk viability significantly decreased, while anthesis silking interval was prolonged by 0.5–3.4 days in two genotypes under individual and combined stress than control. Individual and combined stress treatments suppressed grain sucrose metabolism via coordinated downregulation of sucrose synthase (SS) by 16.1%–59.2% and sucrose‐phosphate synthase (SPS) activities by 2.6%–44.9% in developing grains, concomitantly triggering sucrose translocation to leaves and stalk, suggesting source‐sink redistribution under stress constraints. Notably, the reduction of grain yield mainly resulted from stunted pollen development and limited sugar transport. These provided a strategy for future selected hybrids with the shorter ASI, higher pollen viability and quantity adapting well to climate change.