Mechanism Analysis Of Nitrogen Regulation On Senescence Of Flag Leaf After Anthesis And Nitrogen Transport During Grain Filling In Wheat

The effect of optimized nitrogen supply on N reutilization of wheat plant in the process from flag leaves to spike was evaluated in a long-term field experiment with wheat-maize rotation,which was first established in 2007.In 2018-2020,the wheat was supplied with five N levels at 0,112,160,208and 300 kg/ha(hereafter referred to as N₀,N₁₁₂,N₁₆₀,N₂₀₈ and N₃₀₀,respectively).The N160 was the optimized treatment and N₃₀₀ was the farmer’s practice while N₁₁₂ and N₂₀₈ were 30%less or more than the optimized rate,respectively.In 2019,wheat plants were sampled in returning green,jointing,flowering and maturity stages.At maturity stage,wheat plants were separated into spikes,leaves,stem and grain.Meanwhile,flag leaves were sampled at 1,10,15,24 and 30 days,and spikes were sampled at15,24 and 30 days after flowering（DAF）,respectively.Yield increased significantly with the increasing N rate up to N160 and no longer responded to the higher N rates,which resulted in the highest agronomy use efficiency（31.9 kg/kg）in the optimized management.Dry matter,N concentration and N content of flag leaves increased from flowering until DAF15,and turned to decrease afterwards.The N export from flag leaves（to spikes or grains）occurred most remarkably during DAF15-DAF24（decreased by 51%-69%N content）and decreased totally by69%-84%at maturity as compared to its highest level at DAF15.Nitrogrn export in the flag leaves of N0 treatment appeared to finish much earlier（during DAF15-DAF24）and in other N treatments continued until the maturity.Nitrogen content of spikes increased by 5.5-12.5 mg/ear（accounting for 50%-83%of total grain filling）during DAF15-DAF24 and increased by another 1.6-12.1 mg/ear during DAF24-DAF30（maturity）.It appears that the N export from flag leaves and the N import into spikes are well synchronized.Flag leaves at DAF15 were subjected to metabolomics analysis and RNA sequencing.The stage was selected for the N export to spikes in this stage turned higher,which probably suggesting a turnover of physiological change.Totally 8075 DEGs were screened and enriched by RNA-seq.After Map Man screening,29 differentially related genes with nitrogen gradients were obtained.Totally 894 differential metabolites were screened and enriched in metabolic pathways including amino acid metabolism,lipid metabolism,starch and sucrose metabolism,GSH metabolism and flavonoids metabolism.Most saccharides and alcohols,phenolic acids,flavonoid and flavonols and lipids were reduced whereas amino acids were generally increased by N application rates increased.However,the concentrations of several kinds of amino acids（glutamine,lysine,and arginine）in N300 were same with N0 and were significantly lower than that in N_160.The content of GSH increased in the plants with optimized N rate but decreased in N₃₀₀ whereas the content of oxidized glutathione increased with the increase of N rates.The changes of GSG-GSSH metabolites coincided with gene expression.Using the samples taken in the above experiment,zinc uptake and distribution of wheat plants were investigated to explore the effect of optimized N management.Zinc concentration and accumulation of plants during whole growth periods were significantly increased by N application and was highest in N₃₀₀.Zn concentration of flag leaves decreased by 50%during the period between flowering stage and maturity stage.The decrease patterns of Zn content in flag leaves were different among N gradients,which starting earlier in N₃₀₀ and N₂₀₈（at DAF15）than N₁₁₂and N₁₆₀（DAF24）.The results showed that increasing N rate significantly improved Zn uptake and accumulation in wheat plant and grain.However,N rate had no effect on the proportion of Zn accumulation before and after flowering,as well as the estimated share of Zn in grain which was re-translocated from other organs after flowering.Consequently,the optimized N management improved the N use efficiency and also maintained a desirable Zn concentration in wheat grain to meet the health requirement.