Relationships Of Changes In Cytokinin Concentrations In The Roots And Grains During Grain Filling Of Rice With The Filling Of Grains And Their Regulation

The changes in zeatin (Z) and zeatin riboside (ZR) concentrations in the roots and grains during grain filling of rice (Oryza saliva L.) and their relationships with the proliferation of endosperm cells and grain plumpness were investigated, with different rice genotypes (including hybrid combinations) as materials and the plants being grown in either water culture or pots. The main results were as follows:1. Z + ZR concentrations in both the roots and grains were high at early grain filling stage. The maximum Z + ZR concentrations appeared at 3 to 9 d after anthesis (DAA) in the roots and 6 to 12 DAA in the grains, and Z + ZR concentrations were decreased thereafter. The proliferation dynamics of endosperm cells were closely associated with the changes in Z + ZR concentrations in roots and grains. The proliferation rate of endosperm cells, number of the cells, grain filling rate and 1000-grain weight were very significantly and positively correlated with Z + ZR concentrations in both roots and grains at the early filling stage, but not significantly correlated Z +ZR concentrations at the mid (13-27 DAA) and late (28-42 DAA) stages, indicating that cytokinins in the roots and grains at early grain filling stage play an important role in regulating the proliferation rate of endosperm cells and grain filling in rice.2. The differences in Z + ZR concentrations in the roots and grains were existed among the genotypes or treatments. The genotypes with better grain plumpness generally showed higher Z + ZR concentrations in the roots and grains at early grain filling stage. Z+ ZR concentrations in the roots/ root exudates and grains, endosperm cells, filled-grain percentage and grain weight were obviously increased when roots were treated with 10-7 mol zeatin, or the nutrient solution concentration in water culture was increased (two times as high as in normal water culture) during grain filling period, or nitrogen fertilizer (5-15 kg/667m2) was applied to pots at heading. The results werereversed when roots treated with 10-4 mol Na2S at initial heading, or the nutrient solution concentration in water culture was reduced (half of the normal solution). The results suggest that cytokinin concentrations in rice roots and grains are determined by both genotypes and plant nutritional status. It would be expected to create a cultivar or hybrid combination with high cytokinin concentrations in the roots and grains and good grain plumpness if great attention is paid to choose the parents with high cytokinin concentrations in the roots and grains during grain filling period, especially at early filling stage, in rice breeding. Cytokinin concentrations in the roots and grains during grain filling, especially at early filling stage, would also be increased, grain filling be facilitated, and grain weight and grain yield be raised if spraying a promoting-type of plant growth regulators such as cytokinins at initial grain filling stage or applying nitrogen fertilizer at heading to rice plants.