Characteristics of Opaque and Translucent Parts of High Temperature Stressed Grains of Rice

Abstract

Global warming is one of the most serious environmental problems we are having today. The average temperature of Japan actually increased about 1 C during the last 100 years. High temperature stress during rice ripening periods causes a decrease in not only grain yield but also grain appearance quality. The presence or absence of chalk appearance is an important quality characteristic in rice. It is widely known that the occurrence of chalky grains of rice including white-core, white-belly, white-base and white-back kernels is increased by high temperature stress during grain filling. 1) Several research groups have investigated morphological properties of chalky grains, showing that abnormal starch granules were loosely packed in the chalky grains. 2-7) The surface of round-shaped starch granules in the chalky grains caused by high temperature stress had so many small pits. 3) Interestingly, the feature of the change of granule surface was similar to that occurring during germination. 8) The examination of how the chalkiness of rice grains affects the cooking quality is also an important issue. The chalky grains exhibited their characteristic physicochemical properties of the cooked rice different from those of the perfect grains. 4-6,9-11) From the obtained results, they mentioned that the cooked/steamed rice of chalky grains had undesirable higher hard texture. 12) The mechanism of grain chalkiness under high temperature stress is considerably complicated: disorder of photosynthesis, translocation efficiency, source-sink relationship, and protein expression in ripening seeds may involve in such chalking mechanism. Interestingly however, it has been shown that the environmental temperature at the grain filling stage influenced the starch composition in rice grains. 5,11-17) High temperature stress decreased the amylose contents and altered the fine structure of amylopectin, 13,16) suggesting that the unusual expression of Abstract: We studied the translucent and opaque characters of high temperature stressed chalky grains harvested in Niigata of 2010. Appearance of chalky grains of 2009 and 2010 (ripening temperatures: 24.4 and 28.0 C, respectively) showed some resemblance, and the high temperature stress of 2010 remarkably extended the chalking of grain. Scanning electron microscopic observation showed that round-shaped starch granules with numerous small pits were loosely packed in the opaque part of chalky grain. Reduction of mechanical resistance of cooked rice of chalky grain was observed in a breaking test (apparent strain, 0-75%), suggesting that the loose packing of starch granules probably led to the softness of chalky grain. In addition, slight differences in chain-length distributions of starches were observed in the grains of 2009-to-2010, however, no significant differences in the chain-length distributions between the translucent and opaque parts of perfect and chalky grains were found in both 2009 and 2010. Furthermore, the glucose contents in opaque parts were increased by the high temperature stress of 2010, though those in perfect grains were not different regardless of the environmental changes of 2009-to-2010. Together with previous findings on the increased expression of α-amylases in endosperm, these results suggested that unusual starch degradation rather than starch synthesis is involved in occurring of chalky grains of rice under the high temperature stress during grain filling period.