In order to explore the mechanisms of hormones in the regulation of grain filling, the changes in contents and activities of enzymes which catalyze sucrose metabolism and endogenous hormones in grains during grain filling were analyzed by using inter-subspecies hybrid rice combination Liangyoupeijiu and intervarietal hybrid rice combination Shanyou 63 as materials. Results showed that superior grains had higher contents of endogenous indole-3-acetic acid (IAA), zeatin riboside (ZR) and abscisic acid (ABA) compared with inferior grains at early grain filling stage, and the contents of IAA, ZR and ABA were positively and significantly correlated with the maximum and mean grain filling rate, grain filling percentage, and grain weight. The sucrose synthase (SuSase) activity in superior grains was higher and peaked earlier than that in inferior grains. The grain filling rate, grain filling percentage, grain weight and starch accumulation rate were positively and significantly correlated with the SuSase activity in grains. Application of low concentration of ABA at 8 d after flowering significantly increased endogenous ABA content and SuSase activity in grains, grain filling percentage, and final grain weight. On the contrary, when fluridone (an inhibitor of ABA synthesis) was applied, the ABA content and the activity of SuSase decreased. These results suggest that ABA plays a significant regulatory role in grain filling largely through affecting sucrose synthase activity.
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