Relationship Of Leaf Senescence With ROS Generation And NSC Translocation In Rice (Oryza Sativa L.) Leaf Sheath As Affected By N Supply

Premature senescence of rice leaf results in the shortage supply of carbohydrate to grains,yield,and quality.Nitrogenous fertilizers are often applied with the sole aim to achieve high grain yields,but heavy nitrogen?N?application can lead to negative consequences,such as environmental pollution,low nitrogen-use efficiency,poor grains quality,reduced grain yields,pest damage and diminishing economic returns on the number of fertilizers applied.Therefore,study on premature senescence of rice plant during grain filling,the mechanism of occurrence and acceleration of premature leaf senescence in rice,the method of inhibiting and delaying the occurrence of leaf senescence is very important for the selection of rice for breeding.With regard to these,premature senescence flag leaf mutant rice?psf?at filling stage derived by radiation mutagenesis and its corresponding wild type 7954?WT?were employed as the experimental materials imposed hydroponically to different N levels,mainly to study;Relationship of leaf senescence with ROS generation and NSC translocation in rice leaf sheath as affected by n supply from the following aspects:Relationship of Nitrogen Deficiency Induced Leaf Senescence with ROS Generation and ABA Concentration in Rice LeavesNitrogen?N?deficiency is one of the critical environmental factors that induce leaf senescence,and its occurrence may cause a marked decline of leaf photosynthetic ability and grain yield.However,the physiological metabolism underlying N deficiency-induced/accelerate premature leaf senescence of rice plants and its relationship with the abscisic acid?ABA?and reactive oxygen species?ROS?burst in leaf tissues are not well understood.In this paper,the effects of N supply on several senescence-related physiological parameters and also N deficiency induced-alteration in the temporal pattern of ABA and ROS during leaf senescence were investigated by using the premature flag leaf senescence mutant rice?psf?and its Wildtype under three N treatments.Detached leaf segments from different N treatments were further employed to examine the induction of leaf senescence by exogenous ABA and its relation to N metabolism and ROS generation.The results showed that N deficiency hastened the initiation and progression of leaf senescence.Low N?LN?treatment enhanced the expression of 9-cis-epoxycarotenoiddioxygenase genes(NCEDs,but it suppressed the expression of the transcript of two ABA 8'-hydroxylase isoform genes?ABA8ox2 and ABA8ox3?.High N?HN?supply delayed the initiation and progression of leaf senescence,synchronously with the low ABA concentration.Exogenous ABA incubation enhanced H₂O₂ generation and MDA accumulation in a dose-dependent manner,but it decreased the activities of glutamine synthetase?GS:E.C.6.3.1.2?,glutamate dehydrogenase?GDH:E.C 1.4.1.2-4?and soluble protein content in detached leaf.These results provided reinforcing evidence that the participation of ABA in the regulation of ROS generation and N assimilating/remobilizing metabolism in rice leaves was among the regulatory switch steps for induction of leaf senescence by N deficiency.Senescence-Related Translocation of Nonstructural Carbohydrate?NSC?in Rice Leaf Sheathes Under Different Nitrogen Supply.The translocation of non-structural carbohydrate?NSC?from leaf sheaths to filling grains after anthesis contributed greatly to the grain yield of cereal crops.In this study,the effect of N supply levels on the accumulation and translocation of NSC in different positional rice leaf sheaths and its relationship with the initiation and progression of leaf senescence during grain filling was investigated using two rice genotypes,namely,premature flag leaf senescence mutant?psf?rice and its Wildtype?WT?.The results showed that re-mobilization and translocation of pre-anthesis NSC in leaf sheath were closely associated with the initiation and progression of leaf senescence.N deficiency?LN?enhanced the NSC accumulation,and translocation in leaf sheaths,in contrast to high N?HN?treatment.The upper positional leaf sheath had significantly lower NSC and transferring rate of total soluble sugar and starch contents compared with the downer-positional leaf sheaths.The key enzymes activities involving NSC translocation in sheath tissues were further determined,which enable the clarification of the regulatory mechanism of N deficiency/heavy N application on the accumulation and translocation of NSC in the rice leaf sheath.N deficiency suppressed the activities of glutamine synthetase?GS?,glutamic-oxaloacetic transaminase?GOT?,glutamic-pyruvic transaminase?GPT?,Sucrose synthase?Susy?,and Fructose-1,6-bisphosphatase?FBPase?,but it had an inhibitory impact on the rapid decline of sucrose phosphate synthase?SPS?activity in the leaf sheaths.The upper leaf sheath had a relatively weaker ability of N assimilation but stronger C metabolism than the downer-leaf sheath.Correspondingly,high nitrogen?HN?treatment had relatively higher grain yield than the medium nitrogen?MN?level for the psf mutant;the reverse was true for wildtype.Senescence Related Alteration in Nitrogen Metabolism and Sugar Accumulation in Rice Flag leaf Under Different Nitrogen SupplyIn this paper,the effects of different N supply on several C/N related physiological parameters and the metabolic regulation of these processes in flag leaves was investigated by using the premature flag leaf senescence mutant rice?psf?and it's Wildtype?WT?.The results showed that N deficiency hastened the initiation and progression of leaf senescence,which was closely associated with the significant decreases in leaf N,soluble protein,soluble sugar,hexose and starch content in rice leaves.HN supply resulted in the suppression of sucrose,sucrose/starch ratio,and C/N after anthesis,while LN enhanced N translocation efficiency and NSC translocation rate.LN caused a marked increase in C metabolizing enzymes;contrarily,sufficient N enhanced N metabolizing enzymes activities.Furthermore,the transcriptional profile of key genes involving in C/N metabolism result showed that HN significantly enhanced the transcripts level of OsGS2,OsNR1,Osfd-GOGAT,and OsNADH-GOGAT genes and gradually decreased along with leaf senescence,but the transcriptional expression of OsGDHs was rhythmic.N deficiency induces expression on OsSPS,OsSusy,and?amylase isoforms but down-regulated OsChFBP,OsCyFBP,and OsSUTs isoforms.