Abstract: The grain fresh weight and grain-filling rate at early grain developing stage in rice ( Oryza sativa L. ) were higher in the upper than in the lower part of panicle during grain filling. The grains in the upper and/or the lower part of panicle were analyzed for the endogenous plant hormones. The results indicated that the peak content of ABA and/or GA1 + 3 in the developing grains appeared at about the same stage when the highest increasing rate of grain appeared in the upper and in the lower part of the panicle respectively. After heading, the IAA content in developing grains steadily decreased in both the upper and the lower part of the panicle, GAn + 7 content decreased at first and then increased, and again decreased. At the early grain-filling stage, the ABA and GA1 + 3 contents in the developing grains were higher in the upper than in the lower part of the panicle. The role that the hormones play in the developing grains and the reason why the grains seed set better in the upper than in the lower part of the panicle were discussed.

Key words: Rice, Hormones, Grains in different part of panicle, Seed set