Irrigation regimes modulate non-structural carbohydrates remobilization and improve grain filling in rice (Oryza sativa L.) by regulating starch metabolisms

Recently developed ‘super’ rice cultivars with greater yield potentials often encountered with problems of poor grain filling, especially in inferior spikelets. Here, we studied the enzyme activities related to starch metabolisms in rice stems and grains, and microstructures related to carbohydrates accumulation and transportation to study the effects of water regimes on grain-filling. Two ‘super’ rice cultivars were grown under two irrigation regimes of well-watered (WW) and alternate wetting and moderate soil drying (AWMD). Compared with WW treatment, the activities of ADP glucose pyrophosphorylas (AGPase), starch synthase (StSase) and starch branching enzyme (SBE), and the accumulation of non-structural carbohydrates (NSCs) in the stems before heading were significantly improved, and more starch granules stored in the stems in the AWMD treatment. After heading, the activities of α-amylase, β-amylase, sucrose phosphate synthase (SPS) and sucrose synthase in synthetic direction (SSs) were increased in the stems to promote the remobilization of NSCs for grain filling under AWMD. During grain filling, the enzymatic activities of sucrose synthase in cleavage direction (SSc), AGPase, StSase and SBE of the inferior spikelets were increased, which promoted the grain filling, especially for the inferior spikelets under AWMD. However, there were no significant differences in vascular microstructures. The grain yield and grain weight could be improved by 13.1 and 7.5%, respectively, by optimizing of the irrigation regime. Here, we concluded that the low activities of key enzymes in carbon metabolism is the key limitation for the poor grain-filling other than vascular microstructures, and AWMD can increase the amount of NSCs accumulation in the stems before heading, improve the utilization rate of NSCs after heading, and increase the grain-filling, especially the inferior spikelets, by manipulating the activities of key enzymes in carbon metabolism.