REGULATION OF SOIL WATER DEFICITS ON STEM-STORED CARBOHYDRATE REMOBILIZATION TO GRAINS OF RICE

Grain yields of rice in China are often low and vary substantially due to the shortages of available water. Assimilates stored prior to grain filling have been identified as important contributors to grain yield in drought environments, but evaluating their benefit has been hampered by an inadequate understanding of the physiological mechanism of remobilization to grains of stored carbon reserves during grain filling. A moderate soil drying applied at grain filling period can enhance plant senescence and may improve the yield through remobilizing more pre-stored food to grains. The study was designed to test the following hypothesis: if soil drying is controlled properly at the 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 carbon reserves may increase yields where photosynthetic assimilation is decreased by soil drying. Further evaluation of enzyme regulation in the remobilization process will help to understand the physiological mechanisms. In the current study, two rice combinations (cvs `Shanyou63' and `Pc311/Zao') were grown in cement containers and pots at Yangzhou University. Controlled soil water deficit were imposed at 9 d after anthesis in consideration that the division of endosperm cell is sensitive to water deficit. We found that, compared to well-watered treatments, the remobilization of stored carbohydrates in the rice stem was significantly enhanced under water-deficit conditions, and the increased contribution to grain yields was 1.9-3.0 times as that of rice grown under well-watered conditions. Pronounced effects of water-deficit treatment on carbohydrate metabolism were observed in rice stems. Sugar concentrations and carbon distribution among sugar components were altered under water-deficit conditions. Starch breakdown in rice stems under soil water deficit occurred through action via the hydrolytic pathway: α-amylase, β-amylase, α-glucosidase and D-enzyme were induced and enhanced by soil drying. Starch phosphorylase activity was decreased under water-deficit conditions, indicating that phosphorylase was not involved in the processes of starch remobilization. Water deficit markedly altered the pattern of sucrose metabolism by shutting down the bypass of carbon flow through the sucrose synthase (SuSy) catalyzed system and enhanced the unidirectional flow through the irreversible sucrose-phosphate synthase (SPS) and acid invertase (AINV) catalyzed pathway. SPS was induced and activated by water deficit, and played a pivotal role in enhancing synthesis of sucrose through the conversion of stored carbon into sucrose. During rapid remobilization of stored sugar, the sucrose content was increased by 10.3% in `Shanyou63' and 20.45% in `Pc/Zao' as compared to the well-watered treatments. It is concluded that the enhanced remobilization of stored non-structural carbohydrates (NSC) in rice stems under soil water deficit was due to induced hydrolytic enzyme activities, increased SPS activity and activation state, but decreased invertase activity.