Abstract: 【Objective】This study was designed to elucidate the grain filling characteristics and the causes of poor grain plumpness of some two-line hybrid rice combinations and their hormonal mechanism. 【Method】With two-line hybrid rice combinations which show difference in seed-setting and grain filling as materials, starch , sucrose, zeatin (Z) + zeatin riboside (ZR), indole-3-acetic acid (IAA), abscisic acid (ABA), ethephon(ethylene-releasing agent), SuSase and StSase in grains and the seed-setting and grain filling were investigated. The correlations among them were analyzed.【Result】The poor grain filling of two-line hybrids was mainly attributed to higher unfilled grain rate and lower filling degree of inferior grains. At the early and mid grain filling stages, the sucrose content in inferior grains was greater than that in superior grains for the combinations with poor grain filling, indicating that the substrate concentration was not the principal factor for their slow grain filling and poor grain plumpness of the inferior grains of two-line hybrids. Z+ZR, IAA and ABA in superior grains were obviously greater than those in inferior grains at early grain filling stage. The maximum and mean contents of Z+ZR, IAA and ABA were positively and very significantly correlated with the maximum and mean grain-filling rate, filling degree, and grain weight. The evolution rate of ethylene was greater in inferior grains than in superior grains, and greater for the combinations with poor grain plumpness than those with good grain plumpness at early or mid filling stages. The evolution rate of ethylene was negatively and significantly correlated with grain filling rate, grain filling degree, and grain weight. Spraying ethephon at early grain filling stage increased the evolution rate of ethylene, reduced ABA content and activities of SuSase and StSase, and decreased grain filling degree and grain weight. The results were reversed when cobatous nitrate (an inhibitor of ethylene synthesis) was applied. The results suggested that the hormones and their balance play a role in the regulation of grain filling and enzymatic activities, and the poor grain filling is attributed to the low contents of Z+ZR, IAA, and ABA, and high evolution rate of ethylene in the inferior grains of some two-line hybrid rice combinations.【Conclusion】The results suggested that hormones play important roles in the grain filling of some two-line hybrid rice combinations, and their filling degree could be improved through regulating hormonal contents.

Key words: Two-line hybrid rice, Grain-filling, Hormonal regulation, Physiological mechanism