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.
Foehn-induced dry wind during grain filling increased ring-shaped chalky kernels in rice {Oryza sativa L.) plants. The objective of this study was to determine physiological mechanisms of the occurrence of ring-shaped chalky kernels. Rice plants were subjected to water deficit in a paddy field after shade by applying dry high-speed wind. Additionally, a growth chamber experiment was conducted with plants in pots to measure the water status under the dry wind condition for 24 h by combining in situ turgor {^'^) assay in developing endosperms with the water potential measurements. The dry (high vapor pressure deficit [VPD]) wind treatment produced the largest number of ring-shaped chalky kernels due to poor starch accumulation, compared with shade or low-VPD wind treatment. The inner endosperm cells, where a high frequency of chalkiness was observed, spatially maintained y by osmotic adjustment before the chalky formation with no decline of grain weight. Dry wind reduced photosynthesis due to a partial stomatal closure after water deficit developed. However, these responses, including those related to the plant water status, returned to a level similar to those of the control plants in a day after the dry wind was stopped. We conclude that (i) H'^ maintenance by osmotic adjustment contributes to grain development under water deficit under foehn conditions and (ii) osmotic adjustment has a role in temporally inhibiting starch accumulation in endosperms, resulting in ring-shaped chalky kernels under foehn-induced water deficit conditions.
We designed an inter-latitudinal temperature gradient chamber (TGC) experiment to obtain data for analyzing the effects of global warming on paddy rice production in Japan. TGC experiments were carried out at three sites of different latitudes: Sapporo, Morioka and Kumamoto. The TGCs at each site were controlled to maintain an air temperature gradient of 2 with water temperature treatment (heated 2 and unheated, except for Kumamoto) to grow eight rice varieties: Nikomaru, Reiho, Nipponbare, Koshihikari, Hitomebore, Sasanishiki, Akitakomachi and Kirara397. The higher temperature accelerated the heading date of all varieties in Sapporo and Morioka, but not Kumamoto. The relationship between the cumulative air temperature from the 5th leaf stage to the heading stage (CAT5H) and the temperature gradient in the TGC at each site suggested that the difference in CAT5H among the regions was attributable to day length, and the fact that Kirara397, Akitakomachi, Sasanishiki and Hitomebore have little sensitivity to the photoperiod, whereas Reiho, Nikomaru and Koshihikari exhibit a greater sensitivity to the photoperiod, although the critical day length of Koshihikari was longer than that of the other two varieties. The development of the rice plant depends on both air and water temperatures and the degree of dependency is based on the type of maturity. These results confirm that the effect of global warming on the development of the rice plant and paddy rice production will differ by region (latitude) and variety. Thus, inter-latitudinal TGC experiments are useful for gathering data to estimate the effects of global warming on paddy rice production.
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