MODELLING THE EFFECT OF ELEVATED AIR TEMPERATURE ON SPIKELET STERILITY OF RICE

Abstract

Heat stress damage to the rice production is expected to increase under the projected global warming climate in Korea. The objectives of this study were to elucidate the effect of elevated air temperature on rice yield and its components experimentally and construct a simulation model to predict the spikelet sterility due to heat stress during the reproductive stage of rice. To assess the impact of the elevated air temperature on the growth and yield of rice, a rice cultivar Hwaseongbyeo was grown under two nitrogen levels in 2008 and two rice varieties, Hwaseongbyeo and Dasanbyeo were grown in 2009 in four plastic houses that were controlled to the temperature regimes of ambient, ambient+1.5oC, ambient+3oC, and ambient+5oC throughout the rice growing season. Heading dates in the elevated temperature treatments were three to five days earlier than those in the ambient temperature treatment. Rice growth was affected by temperature treatments differentially according to the tested cultivars but not according to the nitrogen levels. Hwaseongbyeo (japonica) showed significant reduction of shoot dry weight under ambient+5.0oC treatment compared to the other treatments, while Dasanbyeo (tongil-type) showed significant increase of shoot and root dry weight under the elevated temperature treatments. The number of panicles per pot, and the number of spikelet per panicle and per pot were not significantly different among temperature treatments in both cultivars tested. However, grain yield was significantly lower when the air temperature was raised to the level of 3.0oC and 5.0oC above the ambient air temperature. This lower grain yield in the elevated temperature treatment of ambient+3oC was attributed mainly to the decrease of grain weight that resulted from shortened grain filling period. Moreover, the considerable yield reduction in the treatment of ambient+5oC was caused not only by the lower grain weight but also by the marked the marked decrease in ripened grain ratio due to the high temperature during reproductive stage of rice. In conclusion, the ongoing global warming is expected to decrease the grain yield not only by decreasing the grain filling period in the near future but also decreasing the ripened grain ratio that results from spikelet sterility and early abortion of rice kernel development under the long-term projected climate of Korea. As evidenced in the prior experiment, spikelet sterility due to heat stress during reproductive stage would be a prominent factor to reduce rice yield in the future climate. A simulation model was constructed, calibrated, and validated for assessing the impact of global climate change on spikelet sterility of rice. Based on the literature review on high temperature-induced sterility of rice, a simulation model to predict spikelet sterility was constructed by assuming that the spikelet sterility increases with logistic response to the heating degree hour above 31oC on the day of meiosis of a spikelet (assumed as 10 days before flowering of spikelet), one hour exposure of flowering spikelet to high temperature above a certain degree is enough to induce spikelet sterility, and the probability to induce sterility increase with logistic response to the temperature rise. The model was comprised of equations to estimate the probability distributions of heading date of panicles in the field, flowering date of spikelets on a panicle, and flowering time of spikelets during the daytime, and the sterility response functions to the temperature on the day of meiosis and at flowering time of a spikelet. To collect the data for calibrating and validating the model, rice plant of a japonica Hwaseongbyeo and a tongil-type Dasanbyeo were grown under ambient temperature and then transferred at 20 days before heading and at initial heading to the plastic houses controlling the temperature to ambient, ambient+1.5oC, ambient+3.0oC, ambient+5.0oC, and ambient+7.0oC for the temperature exposures in 2009. Rice plants exposed to temperature treatments were sampled at 20 days after initial heading for evaluating spikelet sterility. Heading date and sterility were recorded for each panicle. Eight panicles (two panicles from 4 pots) for each temperature treatment were selected and the number of anthesized spikelet was recorded every day from the initial heading to the end of flowering to know the flowering date distribution of spikelets on a panicle. Also, the number of opened spikelets was counted every 30 minutes during the daytime for two days for the four fully-headed panicles from each temperature treatments. The peak heading occurred on four to six days after initial heading and the heading date distribution of panicles was well fitted to Poissons equation. The flowering peak of spikelets on a panicle occurred at around 5 days after heading regardless of temperature and cultivar and the flowering distribution was well fitted to the normal distribution function with standard deviation of 1.41 and 1.73 day in Hwaseongbyeo and Dasanbyeo, respectively. The peak anthesis in a day occurred around 11oclock regardless of temperature treatments, being a little earlier in Hwaseongbyeo than in Dasanbyeo. The flower opening time distribution followed the normal distribution with standard deviation of 0.99 and 0.97 day for Hwaseongbyeo and Dasanbyeo, respectively. The fertility responses to high temperature on the day of spikelet meiosis and at the time of spikelet anthesis were well fitted to logistic functions of heating degree hour above 31oC on 10 days before spikelet anthesis and air temperature at the time of spikelet flowering, respectively. The model integrating the above equations predicted the high temperature sterility with reasonable accuracy for the independent experiment. The developed model for predicting spikelet sterility would have accuracy enough to be incorporated into crop growth simulation model for assessing the impact of climate change on rice growth and yield.