Remobilization of carbon reserves in response to water deficit during grain filling of rice

Delayed senescence in rice plants caused by heavy use of nitrogen results in much non-structural carbohydrate (NSC) left in the straw and leads to low harvest index. This study was designed to determine if a moderate water deficit imposed during grain-filling could enhance plant senescence and thus, lead to more remobilization of NSC stored in vegetative tissues to grain. Two lodging-resistant rice cultivars, Wuyujing 3 (japonica) and Yangdao 4 (indica), which usually show delayed senescence, were grown in the field. Two levels of nitrogen, either normal (5 g N m^-2, NN) or high amounts (10 g N m^-2, HN) were applied at heading. Two levels of soil-water potential (ψ_soil), well-watered (WW, ψ_soil = 0) and water deficit stressed (WS, ψ_soil = -0.05 MPa), were imposed 9 days after anthesis (DAA) until maturity. Leaf water potential of both cultivars markedly decreased at mid-day under WS treatments but completely recovered by early morning. Chlorophyll (Ch1) concentrations and photosynthetic rate (Pr) of the flag leaves declined faster under WS than under WW treatments, indicating that the water deficit enhanced senescence, while HN greatly delayed it. The reduction of NSC in the culm and sheath was substantially enhanced by the WS treatment, NSC remobilization being increased by 23.8-27.1% at NN and 19.6-36.7% at HN, respectively, compared to the WW treatment. The contribution of remobilized NSC to grain, partitioning of fed ¹⁴C from the flag leaves into grain, and harvest index were all significantly increased by WS at either NN or HN. WS also shortened the grain-filling period by 2.9-5.5 days at NN and 5.7-7.4 days at HN, and increased grain-filling rate at NH and NN by 0.18-0.29, and 0.31-0.37 mg per day per grain, respectively, when compared to WW treatments. The percentage of ripened grains, grain weight and grain yield under WS treatments were not significantly reduced at NN, but increased significantly at HN. Water applied to WS during the water-withholding period was only 64-70% of that to WW treatments. We conclude that the early senescence induced by a moderate water deficit during grain-filling period can enhance the remobilization of stored assimilates and accelerate grain-filling of rice. Such enhancement may improve grain yield in cases where plant senescence is unfavorably delayed by heavy use of nitrogen. This practice would also contribute to water-saving in rice production.