Modelling the Effects of Post-Anthesis Heat Stress on Seed-Setting Rate in Rice.

Seed-setting rate of rice significantly decreases under high temperature during the reproductive stage largely due to heat-induced spikelet sterility. The effects depend greatly on the timing and severity of the heat stress, but the variation of stage sensitivity in response to heat stress is not yet well simulated by most of the existing crop models. We therefore attempted to determine the quantitative relationships between the seed-setting rates and degree of heat exposure of the crops, taking account of the changes in sensitivity to the heat with crop growth stages. Thus four-year phytotron experiments were conducted, where we exposed three japonica cultivars to heat at different temperature levels at 0, 6 or 12 days after anthesis and the duration varied from 2 to 6 days. Heat stresses were quantified by heating degree-day (HDD), defined as the temperature sum above a critical temperature. Growth stage of each spikelet varied even in the same panicle, because not all flowers opened at the same time. The variation in timing was apparent among upper, middle and lower parts of the panicles: 1 and 3-day delay of anthesis stage for the spikelets on the mid and lower parts of the panicles respectively compared to the upper panicles. Seed-setting rates were well expressed as a logistic function of HDD, but the parameters of logistic curves were different depending on the timing of the treatments and positions of the panicles. To estimate the seed-setting rates in response to heat stress of different severity or duration at any time during the post-anthesis periods, a simulation model that reflects the changes in sensitivity of seed setting to HDD was constructed, calibrated, and validated. The root mean square error and mean bias error of the model validation showed the satisfactory predictability of seed-setting under post-anthesis heat stresses.