The impact of drought and heat stress at flowering on maize kernel filling: Insights from the field and laboratory

Drought and exposure to high temperature at flowering strongly affect maize production. However, previous studies mostly focused on the effects of the stressors on kernel number. The impacts of drought and heat in isolation and combination at flowering on maize kernel filling and final kernel weight have received limited attention. Here we investigated the effects of three stressors (drought, heat, and drought plus heat) at flowering phase (from tasseling to seed setting completed) on maize kernel filling in field site plots covered with a rainout shelter and under laboratory conditions with kernel in vitro culture. In two experiments, the kernel weight of plants exposed to flowering stressors was significantly decreased relative to controls. In the field experiment, exposure to stressors decreased kernel weight by 5.0–8.3% for the basal kernel and 5.6–11.0% for the apical kernel. Accelerated leaf senescence and decreased rate of photosynthesis were observed in the field. Kernel weight was also reduced by 18.0–37.6% in the laboratory with kernel in vitro culture, suggesting that early stressors at flowering led to a sink (kernel) limit. Carbohydrate metabolism and starch biosynthesis were impaired by drought and heat individually and their combination. During the early kernel filling stage, the activities of sucrose invertase, sucrose synthase, ADP-Glc pyrophosphorylase, and starch synthase were decreased at 10 and 20 days after pollination (DAP) in the stress treatment. For both basal and apical kernels, the correlation analyses indicated the mean kernel filling rate correlated positively with zeatin riboside content at DAP 10 and indole-3-acetic acid content at DAP 20. This study provides useful information to support the development of drought- and heat-tolerant hybrids and effective management strategies to improve crop yields.