CSSL50 1 grains display higher chalkiness with less translucence, when compared with its parental line Aso minori. Scanning electron microscopy showed that the chalky endosperm is comprised of round and loosely packed starch granules with large air spaces, in contrast to the translucent Asominori grains that are filled with densely packed granules. CSSL50 Tubacin clinical trial 1 grains have a higher content of short chain amylopectins, but less medium or long amylopectin chains. This observation is consistent with the Rapid Visco Analyzer profile which provides a comprehensive evaluation of the grain quality. The relatively lower ratio of medium or long chain amylopectin in CSSL50 1 is correlated with the higher breakdown frequency of its starch granule when heated, indicating that the importance of the fine structure of amylopectin in normal starch granule appearance and degree of grain chalkiness.

Overall, CSSL50 1 has a higher percentage of grain with chalki ness, chalkiness percentages, degree of endo sperm chalkiness, starch content, amylose content, sucrose content and protein content when compared with Asominori. These results collectively indicate that the occurrence of grain chalki ness is associated with changes in starch granule shape, amylopectin chain length profiles, amylose and protein content, and RVA profile characteristics. Increased grain filling rate and enhanced activities of starch enzymes in CSSL50 1 The observation that CSSL50 1 grains are high in short chain amylopectin, but low in medium and long ones suggest that the grain filling rate at early stage of grain development may be faster in CSSL50 1 than in Asomi nori.

We thus measured the fresh and dry grain weight at several grain filling stages for CSSL50 1 and Aso minori. The results indeed showed that grain filling rate at 15, 20, and 25 DAF is faster in CSSL50 1 than that in Asominori. In contrast, Asominori exhibits a smooth and steady grain filling course. These results suggest that the faster grain filling pace before 15 DAF in CSSL50 1 could be an important contribut ing factor for the formation of chalkiness at the later stage of endosperm development. This notion is consis tent with a previous study showing that a steady grain filling rate is required for prevention of chalkiness in rice endosperm.

Unexpectedly, no significant changes were detected in Brefeldin_A photosynthesis efficiency in CSSL50 1 rice leaves during 10 15 DAF of the grain fill ing stage, suggesting that photosynthesis efficiency is not tightly linked with chalkiness formation in rice grains. Since grains of CSSL50 1 contain higher starch, amy lose and sucrose contents compared with Asominori, we speculated that enzymes involved in starch synthesis might be more robust in CSSL50 1 than in Asominori. To confirm this, the enzymatic activities of four major enzymes involved in grain starch synthesis were mea sured during the first 30 days after flowering.