2013 Volume 60 Issue 1 Pages 61-67
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.