| Remobilization and transfer of the stored food in vegetative tissues to the grain in monocarpic plants such as rice and wheat require the initiation of whole plant senescence. Delayed senescence caused by either too much nitrogen fertilizer or an adoption of lodging-resistant cultivars that stay "green" too long results in much non-structural carbohydrate in the straw and leads to low harvest index. Heavy use of nitrogen fertilization, 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 grams, and results in a decrease in grain weight and yield. A moderate soil drying applied at grain filling period can enhance the plant senescence and may improve the yield through remobilizing more pre-stored food to grains. In current study, the concrete container and pot experiments were conducted at Yangzhou University and Hong Kong Baptist University, from 2000 to 2003. The wheat and rice selected here was considered as different forms of stored carbohydrate in plant straw. Two cultivars of wheat or rice were grown with different N levels at initial heading in order to have variable development of senescence. A moderate soil drying was imposed from 9 d after anthesis inconsideration 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 by enhanced relocation of carbon stored reserves, and improved use of pre-stored C reserves may help yield where the senescence is unfavorably delayed. This research investigated the effect of drought stress on the grain filling and remobilization of stored carbohydrate in wheat and rice crops grown at heavy use of nitrogen, and evaluated that the physiological and biochemical reaction in both processes of carbon and nitrogen metabolism response to water deficit, and examined how assimilate distribution in whole plant reacted to water shortage, and determined what ABA play the role in grain filling and remobilized storage under moderate soil drying conditions. The results were shown:1. Plant senescence was unfavorably delayed by heavy use of nitrogen, which resulted in a slow grain filling and low grain yield and more water soluble sugar (WSC) was left in straw. Water deficits treatment significantly decreased mid-day leaf potential, chlorophyll content and photosynthetic rate, and enhanced plant senescence at mid-later stage of grain filling. However, an advantaged effect that such drought stress significantly promoted remobilization of stored carbohydrate and increased grain-filling rate. When compared to well-watered treatments, the contribution ratio of remobilized carbohydrate to grain yield under water deficit conditions were respectively increased by 14.0-15.7 (NN) and 19.6-19.8(HN) percentage points in wheat, 6.4-10.7(NN) and 9.9-17.2(HN) percentage points in rice. The grain weight and grain yield under water deficit treatment were significantly decreased at NN, but significantly increased at HN. The results indicated that, at NN level, an early senescence induced by drought stress increased grain-filling rate, but severely shortened the active period of grain filling. The gain from accelerated grain-filling rate did not compensate fully for the loss caused by shortened duration, which resulted in a decrease in grain weight and yield. At HN, grain yield in wheat and rice were remarkably increased by 17.5-20.2% and 24.4-26.6%, respectively. The results showed that the gain in grain from accelerated rate of grain growth, as a result of a moderate soil drying during grain filling, could compensate fully for the loss caused by a decreased period of grain filling andresulted in an increase in grain weight and yield in cases where plant senescence was unfavorably delayed. However, it should be empha... |
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