Free sugars in relation to starch accumulation in developing rice grain.

The changes in sugars (water-soluble carbohydrates) were studied in the developing grain of rice (Oryza sativa L., variety IR28 and IR29) in relation to the role of these sugars as precursors of ADP glucose in starch accumulation. The levels of total sugars, total reducing sugars and free glucose, sucrose and other nonreducing sugars, maltooligosaccharides, and total and nonsucrosyl fructose followed closely the changes in the rate of starch accumulation, in both IR28 and 29; the peak value occurred 9 days after flowering. The level of soluble carbohydrates remained high in the caryopsis and also in milled rice after starch accumulation, suggesting that the supply of sugar precursors does not limit starch accumulation in the rice grain. Because of a higher level of reducing sugars, the level of free sugars in the grain of waxy rice IR29 was higher than that of nonwaxy IR28.