Abstract: In this paper the changes in grain filling, starch accumulation and activity of enzymes associated with starch synthesis in two different hybrid rice varieties were analyzed under different N supplying dates (earlier-date-emphasized, mean-date-emphasized and later-date-emphasized). The results showed that the N application of later-date-emphasized could promote grain filling rate, increase grain weight and amylopectin content. The time reaching a maximum activity in three enzymes of ADPglusoce pyriphosphorylase (ADPG), starch synthesis enzyme (SSS) and starch branching enzyme (SBE) in grains of two different rice varieties were not changed obviously, but the mean and maximum activity of these three enzymes changed，and the changes of SSS and ADPG were bigger than SBE as N supplying date changed. The N application of earlier-date-emphasized increased SSS activity and the N application of later-date-emphasized increased ADPG and SBE activities. The mean SSS activity in whole filling period, and ADPG and SBE activities at middle and late filling stage were significantly or very significantly correlated with grain filling rate and accumulating rate of amylose and amylopectin. Both of ADPG and SBE played an important role to the changes of amylose and amylopectin content. The N application of later-date-emphasized increased amylose and amylopectin accumulating rate.

Key words: Rice, N supplying dates, Starch, Starch synthesis enzymes, Grain filling