High temperature‐induced grain sterility in rice is becoming a serious problem in tropical rice‐growing ecosystems. We studied the mechanism of high temperature‐induced grain sterility of different rice (Oryza sativa L) cultivars at two relative humidity (RH) levels. Four varieties of Indica and Japonica rice were exposed to over 85 % RH and 60 % RH at 36/30 °C, 34/30 °C, 32/24 °C and 30/24 °C day/night air temperatures from late booting to maturity inside sunlit phytotrons. Increasing both air temperature and RH significantly increased spikelet sterility while high temperature‐induced sterility decreased significantly with decreasing RH. Neither Indica nor Japonica rice types were superior to the other in the response of their spikelets to increased air temperature and RH. Increased spikelet sterility was due to increased pollen grain sterility which reduced deposition of viable pollen grains on stigma. Reduction in sterility with decreased RH was more due to decreased spikelet temperature than to air temperature. Thus the impact of RH should be considered when interpreting the effect of high temperature on grain sterility. Spikelet fertility was curvilinearly related to spikelet temperature. Grain sterility increased when spikelet temperature increased over 30 °C while it became completely sterile at 36 °C. The ability of a variety to decrease its spikelet temperature with decreasing RH could be considered as avoidance while the variability in spikelet sterility among varieties at a given spikelet temperature could be considered as true tolerance.

Spikelet sterility in rice (Oryza sativa L.) induced by high temperatures is a major concern given global warming predictions. We studied differences among eight rice cultivars in spikelet fertility at five different temperature levels in temperature gradient chamber (TGC) experiments. Six japonica and two indica cultivars were exposed to high-temperature gradients in TGCs during the 2005 flowering season. Spikelet sterility increased with temperature in TGCs and differed among cultivars because of both variations in temperature tolerance and timing of heading. The correlation between spikelet fertility of individual panicles and both air temperature and panicle temperature during flowering was analyzed to compare tolerances among cultivars. The temperature (T 75 ) at which spikelet fertility was 75 % of maximum ranged from 34 to 39°C air temperature and differed significantly among cultivars. Indica varieties had higher T 75 values than japonica varieties. The T 75 values based on panicle temperature also differed among cultivars, but the difference between indica and japonica varieties were less significant. We concluded that the higher temperature tolerances of indica cultivars in our experiments could be attributed to lower spikelet temperatures, and cultivars with similar spikelet temperatures still had different heat tolerances due to differences in pollination ability.

Almost complete and uniform grain sterility in rice over large areas has become a serious problem in the warm and humid lowland region in Sri Lanka. In view of this, the influence of high temperature a t low and very high relative humidity (RH) levels and normal temperature at very low and normal RH levels on the spikelet surface temperature and grain sterility in rice at heading were studied under controlled environments. Almost complete grain sterility in rice was induced by high temperature (35OC day/30°C night) when coupled with high RH (85-90%) at heading. Reduction in RH by 30% at high temperature resulted in decrease in grain sterility but made no significant increase in the completely filled grain percentage which was negligibly low at both very high and low RH levels. Under the normal temperature of 30°C da~125~C night, about twofold increase in grain sterility at very low RH (35-40%) over the lowest recorded 13.8% grain sterility at normal RH (65-70%) was observed. Percent partially filled grains were not found to be influenced by both temperature and RH of the surrounding atmosphere. The spikelet surface temperature in rice was always several degrees less than the atmospheric temperature and increase in atmospheric temperature a t heading resulted in increase in spikelet surface temperature. At a given atmospheric temperature, higher the RH, higher the spikelet surface temperature at heading. Spikelet surface optimum and critical high temperatures for grain sterility appeared to be around 26OC and between 26OC and 31°C but close to 31°C, respectively.

Prediction of phenological development of wheat (Triticum aestivum L.) is important for proper agronomic management and schedule planning. A model to calculate developmental rate (DVR) for short-day plant was applied for long-day plant to estimate the timing of heading and maturity of winter wheat using the concept of DVR. The DVR from emergence to heading was expressed using a function of daily mean air temperature and photoperiod. The DVR from heading to maturity was expressed using a function of daily mean air temperature only. The model was examined based on a field experiment in Kumamoto, Japan, in the 2003 and 2004 growing seasons. Three cultivars of winter wheat (Chikugoizumi, Saikai-185 and Iwainodaichi) were used to examine how the parameter values differ with cultivar. Calculated values for duration from emergence to heading and from heading to maturity showed good agreement with observed data over a wide range from 35-170 days and 30-55 days, respectively. Root mean square errors of the model estimation were 4.2-4.9 days for heading date and 1.7-2.8 days for maturity date, respectively. Phenological differences among the cultivars were mainly reflected in parameters expressing the DVR response to temperature. From emergence to heading, the temperature when the DVR is half of the maximum rate (Thv) differed with cultivar from 3 to 9 . The Thv value was higher for a cultivar with a low degree of winter habit (Chikugoizumi), and lower for cultivars with a high degree of winter habit . From heading to maturity, a parameter (Am) that expressed the DVR slope with change in temperature showed a higher value for Chikugoizumi, and lower values for Saikai-185 and Iwainodaichi. The model will be useful for predicting the phenological development of winter wheat and understanding the phenological response to climate change.

No abstract