Abstract:

Heavy application of nitrogen fertilizer, especially in highly productive areas, is well known to lead a delayed senescence. Unfavorably-delayed senescence retards the active and ordered process that partitions the photosynthetic assimilates into grains, and results in a decrease in grain-filling rate and grain weight. A moderate soil drying applied at grain filling period can enhance the plant senescence and may improve grain filling. In present study, the concrete container experiments were conducted at Yangzhou University. Two rice combinations were grown at the rate of 5 g N·m^-2 (NN) and 20 g·m^-2(HN) at initial heading in order to have variable development of senescence. A moderate soil drying was imposed from 9 d after anthesis in consideration that the division of endosperm cell is sensitive to water deficit. The study was designed to test the hypothesis: if a soil drying is controlled properly at mid-late stage of grain filling, an early senescence induced by drought stress would accelerate the rate of grain filling and increase grain weight and yield where the senescence is unfavorably delayed. If so, it would be further evaluated whether the enzymes regulating grain starch synthesis were changed by water stress.
In present study, we found that, plant senescence was unfavorably delayed by heavy application of nitrogen, which resulted in a slow grain filling and low grain yield (Table 1). Water deficit treatment significantly reduced mid-day leaf water potential and enhanced plant senescence at mid-later stage of grain filling (Fig.1, Fig.2). Such drought stress significantly increased grain-filling rate by 8.5%–13.6% at NN and 25.3%–28.0% at HN, and shortened grain-filling period by 2.8–3.9 d at NN and 4.2–4.9 d at HN, respectively, when compared to well-watered treatments(Table 1). The grain weight and grain yield in water stress treatments were significantly decreased at NN, but significantly increased at HN. A slow rate of starch accumulation caused by HN application was attributed to enzyme activity decreases in the metabolism of sucrose to starch (Fig.3, Fig.5 and Fig.6). The treatment of water stress increased sink activity by increasing enzyme activities, including sucrose synthase (SuSy), ADP-glucose pyrophosphorylase (ADP-Gppase), soluble starch synthase(SSS) and starch branching enzyme (SBE), and resulted in enhanced starch accumulation in grains (Fig.5, Fig.6). In NN treatments, the shortage of assimilation supply and the enzyme activity decreases in the metabolism of sucrose to starch were responsible for the decreases in grain weight and yield under water stress conditions(Fig.4, Fig.5 and Fig.6). It is concluded that a soil drying is properly applied in conditions where plant senescence was unfavorably delayed caused by heavy application of nitrogen fertilizer, can improve grain filling through increasing starch accumulation rate, as the result of inducing and enhancing activities of SuSy, ADP-Gppase, SSS and SBE.

Key words: Water stress, Grain-filling rate, Sucrose synthase, Key enzymes for starch synthesis, Rice