High temperature sensitivity of kernel formation in different short periods around silking in maize

Anthesis stage has been proved a typically sensitive period to heat stress in maize (Zea mays L.), but effects of short–term heat stress around anthesis (tasseling–silking period) are not known. A temperature–controlled experiment was conducted using one heat–sensitive maize hybrid and two temperature levels, 40/30 °C (heat stress) and 32/22 °C (control) in eight different short–term episodes around silking (silking represented “0”) which were from 15 d before silking (BS) to 15 d after silking (AS) with 5–d (15–10, 10–5, 5–0 d BS and 0–5, 5–10, 10–15 d AS) and 15–d (15–0 d BS and 0–15 d AS) intervals. The yield losses under 15–d heat stress (average of –25.9 %) around silking were higher than 5–d (average of –16.1 %), and the post–silking time (average of –22.1 %) was more sensitive than pre–silking (average of –15.1 %). Wherein, the first 5 d post–silking short–term heat stress resulted in the largest yield loss (–28.1 %) as a function of kernel number reduction among the treatments with 5–d period heat stress, and yield reduced by 30.4 % in 15 d post–silking heat stress. Kernel number was more closely correlated with pollen shedding number (r = 0.58) than with pollen viability (r = 0.18) in heat stress. The decreased pollen shedding number was a result of suppressed anther dehiscence rather than pollen grain production. The damaged pollen morphology during development reduced pollen viability. Maize leaves maintained a stable photosynthesis at heat stress could be related to the increased stomatal conductance and transpiration, showing a higher heat stress tolerance. Thus, pollen shedding, pollination, pollen tube growth, and early kernel formation in the period shortly after silking are more sensitive to heat stress in maize. A proper crop management (i.e., slightly adjusting sowing date) that can avoid overlapping 5 d post–silking growth with heat stress is recommended. When selecting of or breeding for heat tolerant maize hybrids, the short growth phase from silking to kernel formation should be closely concerned.