The Physiological Mechanism About Nitrogen To Mitigation Premature Under High Temperature After Flowering Of Spring Wheat

With global warming,the climate change will be one of the important limiting factor to agricultural development in the future.In recent years,the frequent occurrence of extreme weather,wheat yield can be affected premature senescence under the high temperature seriously especially in grouting period.In order to study the physiological mechanism of the effects of nitrogen on spring wheat under high temperature conditions after anthesis,and formulate nitrogen application measures to alleviate the damage of high temperature.The field trials was carried out in 2019 and 2020,and the split zone test design was adopted.The main zones were nitrogen include 0 kg·hm-2(N0),75 kg·hm-2(N1),150 kg·hm-2(N2),225 kg·hm-2(N3),300 kg.hm-2(N4),and the sub-zones has temperatures of 25℃±2℃(CK)and 35℃±2℃(HT).By analysis the interrelationships among photosynthetic characteristics,fluorescence characteristics,antioxidant characteristics,key enzymes of starch formation and carbon and nitrogen metabolism of spring wheat after anthesis with temperature stress under different nitrogen application,as well as the differential genes and metabolic pathways of spring wheat leaves and grains under nitrogen application and high temperature stress,thus revealing the internal mechanism of nitrogen alleviating the high temperature and premature senescence of spring wheat after anthesis.The main results are as follows:1.Increasing the amount of nitrogen fertilizer application can significantly increase the SPAD value,leaf area index,chlorophyll content and photosynthetic fluorescence parameters and other indicators of spring wheat;The SPAD value,leaf area index and chlorophyll content decreased in different degrees after high temperature stress,the N3 treatment decreased the least.N3 increased leaf area index,SPAD,PI and net photosynthetic rate by 48.16%,64.11%,36.90%and 57.65%,respectively,compared with NO treatment under high temperature stress of 25 d after anthesis in 2019.It can be seen that under high temperature stress,reasonable nitrogen application can delay the senescence of spring wheat flag leaves and prolong the duration of leaf function.2.Increasing the amount of nitrogen fertilizer application can significantly increase the activity of flag leaf protective enzyme in spring wheat,thus removing harmful substances such as MDA and O2-in time,while the activity of protective enzyme decreased significantly after high temperature stress,among which the N3 processing has the smallest decline.Under high temperature,N3 decreased by 25.45%and 33.09%compared with the MDA and O2-content of the NO treatment,SOD activity,POD activity,CAT activity and proline content increased by 31.78%,39.11%,32.68%and 61.21%under high temperature stress of 25 d after anthesis in 2019.The results showed that high temperature stress could accumulate harmful substances such as MDA and O2-in spring wheat and increase the degree of membrane lipid peroxidation,while the reasonable amount of nitrogen fertilizer could increase the content of protective enzyme activity and non-enzymatic protective substances.So as to remove harmful substances timely to enhance the stress resistance of plants.3.Increasing the amount of nitrogen fertilizer application can reduce protease and nuclease activity significantly,and keep the content of soluble protein,DNA and RNA content in the leaves at a high level.Under high temperature stress,the activity of protease and nuclease will increase,resulting in a decrease in soluble protein and nucleic acid content.The results showed that the soluble protein content,DNA content and RNA content of N3 treatment were significantly higher than that of NO treatment under high temperature stress,while protease activity and nuclease activity were significantly lower than those of NO treatment.Therefore,an appropriate amount of nitrogen application can effectively resist the stress of high temperature,reduce protease and nuclease activity,so that nucleic acid and protein in plants can maintain a high level,and the aging process will be slowed down.4.Increasing the application rate of nitrogen fertilizer can significantly improve the activities of key starch forming enzymes in spring wheat such as ADPG-PPase,UDPG-PPase,GBSS,SSS,and SBE.The stress of high temperature can significantly reduce the activity of key enzyme of starch formation,the content of amylose,amylopectin and total starch in N3 treatment were 6.85%,2.32%and 5.98%higher than those in NO treatment,respectively.It can be seen that a reasonable application of nitrogen fertilizer can resist the harm of high temperature,improve the activity of key enzyme in starch formation,thereby enhancing the efficiency of starch synthesis and promote starch accumulation.5.According to the results of transcriptome analysis of grains and leaves of spring wheat,the stress of high temperature can cause that different genes in multiple metabolic pathways such as DNA replication,carotenoid biosynthesis,starch metabolism,photosynthesis and other metabolic pathways,and most of them are down-regulated.However,the application of nitrogen fertilizer under the stress of high temperature will greatly up-regulate the expression of genes related to photosynthesis,starch formation and carbon-nitrogen metabolism.It can be seen that high temperature stress will hinder photosynthesis and slow down the formation of starch,while the appropriate amount of nitrogen fertilizer will up-regulate the expression of most genes,alleviate the down-regulation of genes caused by high temperature stress,and make photosynthesis and starch synthesis proceed smoothly.6.High temperature stress after anthesis can significantly reduce the number of grains per spike,thousand-grain weight,and seed setting percentage of spring wheat,however,with the increase of nitrogen application,the thousand-grain weight and yield both increased significantly.Based on NO treatment,each increase of 75 kg·hm-2 of nitrogen fertilizer will increase the yield by 2.36 t.hm-2(CK)and 2.82 t·hm-2(HT)on average.and the average increase rate is 30.55%(CK)and 39.67%(HT),It can be seen that under high temperature stress,the appropriate nitrogen application is conducive to the increase of yield.To sum up,the appropriate amount of nitrogen fertilizer can effectively improve photosynthetic efficiency and antioxidant ability of plants,promote metabolism of plant protein and nucleic acid,alleviate the harm of high temperature to wheat,and benefit grain starch accumulation and yield formation.Therefore,in actual production,an appropriate amount of nitrogen fertilizer application can effectively resist high temperature stress,alleviate premature aging phenomenon,and thus reduce yield loss.