Senescence‐related translocation of nonstructural carbohydrate in rice leaf sheaths under different nitrogen supply

The translocation of nonstructural carbohydrates (NSC) from leaf sheaths to filling grains after anthesis contributed greatly to the grain yield of cereal crops. In this study, the effect of nitrogen (N) supply levels on the accumulation and translocation of NSC in leaf sheath tissues and its relationship with the initiation and progression of leaf senescence during grain filling was investigated using two rice (Oryza sativa L.) genotypes, namely, premature flag leaf senescence mutant (psf) and its wild-type. Three N treatment levels were used to examine N-supply induced alteration in the activities of several key enzymes involved in NSC translocation and N assimilation in different leaf sheaths. The results show that the NSC translocation rate in leaf sheaths under low nitrogen (LN) treatment was significantly higher than those under normal nitrogen (NN) and high nitrogen (HN) treatments. However, the positive effect of LN on the NSC translocation in leaf sheath was closely associated with its negative effect on grain yield, due to accelerated leaf senescence and shortened leaf longevity. Comparatively, the upper-positional sheath had a lower NSC amount and higher NSC translocation rate than the lower-leaf sheaths after heading. High N suppressed sucrose-phosphate synthase (SPS) activity in leaf sheaths, but enhanced the activity of key enzymes involving in N assimilation in leaf sheaths. The upper sheath had higher activity of sucrose-metabolizing enzymes and lower activity of N-assimilating enzymes. Hence, the upper-leaf sheath had a relatively weak N assimilation and stronger NSC translocation than the lower-leaf sheaths.