Responses of maize with different growth periods to heat stress around flowering and early grain filling

High temperature around flowering and early grain filling greatly reduced maize yield. Hybrids responded differently to high temperature stress, but the underlying mechanisms are unclear, particularly from the perspective of growth season that is widely used to adapt different environments. For this, 162 maize inbred lines with different growing degree days (GDD) were planted at six different environments by adjusting sowing date over four years of 2015 – 2018, including two control treatments and four high temperature treatments during flowering and early grain filling. Yield was significantly positively correlated with GDD but negatively correlated with killing degree days (KDD) across six environments. Daily maximum temperature during flowering and early grain filling was a more important determinant of yield (R2 = 0.689) than solar radiation (R2 = 0.265) and precipitation (R2 = 0.288). Grain yield significantly reduced at high temperature mainly as a result of reduced kernel number per ear (KN/ear). KN/ear was reduced by 11 – 100% among inbred lines at heat stress, and the reductions followed the similar pattern for inbred lines that had different GDDs. Based on GDD, 162 inbred lines were divided into short – (SS) and long – season (LS) groups. Kernel number reductions (% / °C) in SS and LS groups were 14.4 and 12.0, respectively, with significant difference between groups. This revealed that long – season maize lines showed a higher heat tolerance than short – season lines in kernel number formation by maintaining a relatively stable anthesis – silking interval. SS maize was more tolerant to heat stress during early grain filling than LS maize in kernel weight development due to a shorter exposure to heat stress. It is feasible to select of or breed for heat tolerant maize in different regions where maize has different growth seasons. A proper sowing date, along with heat tolerant hybrids should be considered to offset the adverse effects of warming climate.