Dissecting the critical stage in the response of maize kernel set to individual and combined drought and heat stress around flowering

Increases in frequency and magnitude of drought and heat events are the prime abiotic constraints, which cause considerable adverse effects on maize kernel set and grain yield. However, there is little information about a comprehensive and systematical understanding to identify the critical stage in the response of kernel numbers per spike (KNS) to individual and combined drought and heat stress around flowering. Here we evaluated three successive phases (florets development, florets pollination and seed setting) of kernel set under drought, heat and combined drought and heat stress (DS, HS and DHS) with two maize hybrids using the field site ponds covered with a rain shelter in 2018 and 2019. As an average in two years, KNS was reduced by 18.9 % in DS treatment, 10.8 % in HS treatment and 37.2 % in DHS treatment compared with the control treatment. The most considerable adverse impacts during the three phases were all observed in DHS treatment, followed by DS and HS treatments. Among the three phases, the impact of stress on seed setting was more substantial than that in the other two phases for both hybrids. Kernel abortion was the main reason for the decrease in KNS due to the stress, which resulted from the prevention of sugar-starch conversion. The starch content in the kernel was reduced by 42 % in DS treatment, 29 % in HS treatment and 58 % in DHS treatment on the 8th day after silking. This finding would supply a reference for breeders to evolve strategies in developing stress-resilient and high-yielding hybrids under climate change.