Abstract: Little is known the roles and mechanisms of hormones and key enzymes in the formation of the quality of grains at different positions within a rice panicle. This study investigated if the hormones in the grains are related with rice quality. Two rice cultivars, Yangdao 6 (indica) and Yangjing 9538 (japonica), were field-grown. All primary branches on a panicle were evenly divided into three parts of the upper, middle and basal. The grains were numbered as 1 to 6 on a primary branch and 1 to 3 or 4 on a secondary branch from the top to base. The results showed the contents of zeatin (Z) + zeatin riboside (ZR), indole -3- acetic acid (IAA) and abscisic acid (ABA) and activities of starch synthase (SSase), starch branching enzyme (Q-enzyme) in earlier-flowered grains were obviously greater than those in later-flowered grains at early filling stage, it was reversed at late filling stage. The earlier the spikelets flowered, the shorter the time reaching the maximum grain-filling rate (Vmax) was, the higher the contents and activities were. The contents of Z + ZR, IAA and ABA, the ratio of ABA to IAA+Z+ZR, and activities of SSase and Q-enzyme at early grain filling stage were positively and significantly or very significantly correlated with 1000-grain weight, amylose accumulation rate and amylose content, but negatively and significantly or very significantly correlated with chalkiness degree, the results were reversed at late grain filling stage. Activities of SSase and Q-enzyme at early grain filling stage were negatively correlated with Gel consistency (GC), whereas positively correlated with GC at mid and late grain-filling stages. The correlation of the hormones and enzymatic activities with crude protein content varied with cultivars, and the correlation with head milled rice rate was insignificant.

Key words: Rice, Grain position, Hormone, Enzyme activity, Quality