Abstract:

Improvement of grain filling is very important in rice production. Two rice cultivars with high lodging resistance grown in pots and tanks were used to investigate the possibility that a moderate water stress imposed during the grain-filling period of rice might accelerate grain-filling rate, and hormones might mediate this process. Two levels of nitrogen, normal amount (NN) or high amount (HN), were applied at heading. Well-watered (WW) and water-stressed (WS) treatments were imposed from 9 d after anthesis till maturity. Leaf water potentials, grain filling rate, and cytokinins and abscisic acid (ABA) contents in the leaves, stems and grains were determined. The results showed that leaf water potentials of both cultivars were markedly decreased at midday in WS treatments but completely recovered by early morning. In WW treatments, grain-filling rate was decreased by HN, while the grain-filling rate was increased and the grain filling period shortened under both HN and NN by WS treatments. At the HN level, filled grain percentage, grain weight and yield were significantly higher than those under WW, implying that the effect of accelerated grain filling rate outweighed the possible loss of photosynthesis due to the shortened grain filling period when subjected to water stress during grain filling. Zeatin (Z) + zeatin riboside (ZR) contents in the stems and grains were transiently increased at early grain filling stage, reached a maximum at 9－12 d after anthesis, and decreased thereafter. Apposite to the performance of Z + ZR, ABA content in the stems and grains was low and slowly increased at early grain filling stage, then reached the peak value simultaneously with the grain-filling rate. The Z + ZR content was decreased, but ABA content significantly increased in plants, especially in the grains by WS. ABA content in grains was positively and significantly correlated, whereas Z + ZR content in grains was not correlated, with grain filling rate. The activities of sucrose synthase, adenosine diphosphoglucose pyrophosphorylase, starch synthase and starch-branching enzyme were significantly enhanced, and at last grain weight was increased by spraying ABA at low concentration on HN-WW plants at early grain filling stage (9－13 d after anthesis). The results were reversed when fluridone (an inhibitor of ABA synthesis) was applied. In conclusions, grain-filling rate is accelerated by a moderate water stress during the grain-filling period of rice and ABA plays an important regulatory role in the process.

Key words: Rice (Oryza sativa), Soil moisture, Nitrogen nutrient, Abscisic acid, Cytokinin, Grain filling