Regulation of Controlled Soil Drying on Remobilization of Stem-stored Carbohydrate to Grain in Wheat Grown under Unfavorably-delayed Senescence

Delayed senescence caused by either too much nitrogen application or an adoption of lodging-resistant cultivars that stay “green” too long results in much non-structural carbohydrate in 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 grains, 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. Two wheat cultivars were grown with different N levels(high N, HN and normal N, NN) at initial heading in order to have variable development of senescence. A moderate soil drying was imposed from 9 d after anthesis in consideration of 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, and therefore increase the yield. If so, it would be further evaluated that whose enzymes were up/down-regulated in the remobilized processes of stem-stored carbohydrate response to water deficit. This result showed that pronounced effects of nitrogen application and water-stressed treatment on carbohydrate metabolism were observed in wheat stems. Sugar concentration and carbon distribution among sugar components were altered under soil drying (Fig.2, Fig.3). Degradation of fructan in stems under water deficit was enhanced in wheat through inducing and increasing fructan exohydrolase(FEH) activity (Fig.2, Fig.4, Fig.6). Water stress markedly altered the pattern of sucrose metabolism by shutting down the bypass of carbon flow through the sucrose synthase catalyzed system and enhanced the unidirectional flow through the irreversible sucrose-phosphate synthase(SPS) and sucrose invertase(INV) catalyzed pathway (Fig.2, Fig.3, Fig.5). SPS was induced and activated by water deficit, and played a pivotal role in enhancing synthesis of sucrose through converting stored carbon into sucrose (Fig.5). During rapidly remobilized stage of stored food (10-30 DAA), sucrose contents of wheat stem under water deficit conditions were increased by 15.9%-19.0% (NN) and 34.7%-40.5%(HN)(Fig.3), respectively compared to well-watered treatments. At maturity, the greater proportion of pre-fed 14 C in the stems and sheaths was repartitioned into grain by soil drying under both nitrogen treatments (Fig.7). The grain weight and grain yield under water deficit treatment were significantly decreased at NN, but significantly increased at HN (Table 1). It is concluded that the enhanced remobilization of stored sugar in wheat stem under soil drying conditions was attributed to inducing FEH, increasing SPS activity and activation state, and but decreasing INV activity.