Effect of high temperature at ripening stage on the reserve accumulation in seed in some rice cultivars

The resistance to high-temperature stress and the structural appearance of the imperfect grains caused by a high temperature at the ripening stage were studied using 13 selected cultivars of rice. High temperature treatment (daily maximum temperature range, 32-40degreesC) given from the 4th day after heading caused the decrease in panicle weight in all of the cultivars examined. The number of empty grains in the upper and lower parts of a panicle was increased by the high temperature in 10 cultivars. Cultivars KRN, Citanduy, Belle patna and BPB were tolerant to the high-temperature treatment at the ripening stage, and cvs. Koshihikari, Sanlicun, Tainting 67, Yamada-nishiki and Lady Wright were sensitive. Light microscopic observation showed that, the whole endosperm was covered with a nucellar epidermis (NE) under both high and natural temperature (26-31degreesC) conditions at the first week after heading (WAH). Under high-temperature conditions the NE degenerated earlier than under natural temperature conditions. Scanning electron microscopic observation showed that, the endosperm cells of the seeds with a specific gravity (s.g.) of higher than 1.06 had large amyloplasts filled with starch granules. However, the endosperm cells of seeds with a s.g. of 1.00-1.06 had many small amyloplasts containing small single starch granules and had numerous spaces among the amyloplasts. In the endosperm cells at the dorsal side of imperfect grain, layered structures showing progressive decomposition of starch granules were observed.