Varietal Difference in the Effects of Applied Gibberellic Acid (GA₃) on the Number of Spikelets per Panicle in Rice Plants

Abstract

Eight rice varieties were grown in pots and the varietal difference in the effects of applied gibberellic acid (GA₃) on the number of spikelets per panicle were examined. GA₃ (5 ppm) was sprayed onto rice plants once a week from the vegetative growth stage to heading. The number of spikelets per panicle was highly related to the number of spikelets per primary rachis branch and the first internode diameter in control plants, and this relation was the same in GA₃-applied plants. The effects of GA₃ on the number of spikelets per panicle differed among the varieties. In the varieties that showed an increase in the number of spikelets per panicle in response to GA₃, the first internode diameter was also increased and the number of primary rachis branches or the number of spikelets per primary rachis branch was increased. Therefore, in these varieties, the factors which caused varietal differences in the number of spikelets per panicle in control plants may be promoted by GA₃. The differences in sensitivity to GA₃ among the varieties suggest that the roles of endogenous GAs in determining the number of spikelets are also different among varieties.