HORMONAL FACTORS DETERMINING A PREFERENCE IN GROWTH AND FILLING OF GRAINS AT DIFFERENT SPIKELET POSITIONS WITHIN THE PANICLE OF RICE.

水稻（Oryza sativa L.）在生长初期，特别是在降低源库比的条件下，籽粒生长缓慢，这不是由于碳水化合物供应的限制，而是伴随着胚乳细胞数和最终克重的减少。在灌浆期通过水培溶液向根系施用细胞分裂素，促进了下部颖花的早期生长。在小穗下部的幼粒上直接施用脱落酸（阿坝）也促进了其早期生长。细胞分裂素或阿坝的这种生长促进作用不是由碳水化合物供应的增加引起的。此外，这些处理增加了最终的重量和胚乳细胞的数量在较低的小穗位置。这些效果非常类似的处理增加源/库比的影响。叶面喷施阿坝促进了下部颖花籽粒的早期生长和最终粒重。在生育前期，高穗位籽粒中阿坝含量显著高于低穗位籽粒，降解处理显著提高了低穗位籽粒中阿坝含量。但不同小穗位和不同处理间籽粒细胞分裂素水平差异不显著。这些结果表明，幼粒中的阿坝是从叶片中转运出来的，在籽粒发育过程中起着生长促进剂的作用，幼粒中的阿坝水平决定着不同穗位籽粒的生长和灌浆。

It is known that the grains at the lower spikelet positions within the panicle of rice (Oryza sativa L.) grew slowly during early growth, especially under reduced source/sink ratio conditions, and the lower growth rate of these grains was not caused by the limitation of carbohydrate supply, and was accompanied by reduction of endosperm cell number and final gram weight. Cytokinin application to roots through water culture solution during the grain filling enhanced the early growth of gram at the lower spikelet position. Direct application of abscisic acid (ABA) to the young grain at lower spikelet position enhanced its early growth, too. This growth promotion by cytokinin or ABA was not caused by the increment of carbohydrate supply. In addition, these treatments increased the final weight and endosperm cell number in grain at lower spikelet position. These effects strongly resemble the effects of the treatment increasing source/sink ratio. Foliar application of ABA enhanced the early growth and final weight of the grain at lower spikelet position. ABA levels in the grain at the higher spikelet position during early growth stage were higher than that in the grain at the lower spikelet position, significantly, and ABA levels in the grain at the lower spikelet position increased significantly by degraining treatment. But, there were no significant differences in grain cytokinin levels either between spikelet positions or treatments. These results suggest strongly that ABA contained in the young grain is transported from the leaf and acts as a growth promoter in natural grain development, and ABA levels in the young grains determine a preference in growth and filling of grains at different spikelet positions within the rice panicle.