Physiological Mechanisms Of Drought Priming And Magnesium In Alleviating High-temperature Stress During Grain Filling In Wheat(Triticum Aestivum L.)

The middle and lower reaches of the Yangtze River and the Huang Huai wheat regions are the main production areas of winter wheat in China.The wheat is often damaged by high temperature stress during grain filling stage,which leads to premature senescence of wheat plants and decrease of yield.More than 70%of the contribution to grain yield comes from the accumulation of photosynthetic substances during post-anthesis.It is of great significance to stabilize the wheat yield and grain filling under high temperature stress as well as to explore the mechanism of alleviating photosynthetic inhibition under high temperature stress at grain filling stage.In this study,pot experiments were conducted,and Yangmai 16 was used as the material.This study systematically studied the effects of drought priming(mild drought priming at 7-11 days after j ointing),magnesium application(20 kg·hm-2 magnesium fertilizer was applied at the booting stage)and interaction treatments on high temperature stress(the high temperature treatment was 32/22℃,and the control temperature was 26/16℃)during wheat grain filling.From the aspects of photosynthetic electron transfer capacity,carboxylation efficiency and leaf senescence,the alleviating mechanisms of drought priming,magnesium application and interaction on wheat yield formation and flag leaf photosynthetic ability under high temperature stress were analyzed.The main results are as follows:1.Drought priming and magnesium application alleviated the impacts of high temperature stress on grain yield and photosynthetic rate of wheat at grain filling stage.Under high temperature stress,the net photosynthetic rate and dry matter accumulation decreased,which inhibited the grain contribution rate and harvest index after anthesis,resulting in the decrease of grain weight and grain yield by 19.03%.Drought priming alleviated the decrease of dry matter accumulation and grain contribution rate after anthesis by reducing the loss of photosynthetic rate under short-term and long-term high temperature stress.The loss of grain weight and yield under high temperature stress alleviated by 7.04%and 7.94%,respectively.Drought priming also reduced the loss of harvest index compared with the untreated wheat plants.Compared with the plants without pretreatment,magnesium application and combined treatment significantly increased the net photosynthetic rate of leaves and dry matter accumulation after anthesis at the control temperature and maintained the net photosynthetic rate ata high level of under high temperature stress,thus maintaining the dry matter accumulation and post-anthesis grain contribution rate.The results showed that the formation of grain weight and grain yield under high temperature stress were stabilized by magnesium application and combined treatment.Compared with the control treatment,the grain weight and grain yield of magnesium application treatment were alleviated by 8.83%and 7.20%,respectively.The seed weight and grain yield of combined treatment under high temperature stress were alleviated by 10.56%and 9.29%,respectively.Therefore,drought priming,magnesium application and combined treatment can alleviate the loss of grain yield under high temperature stress at grain filling stage,the alleviating effect of combined treatment under high temperature stress was slightly greater than that of single magnesium treatment and single drought priming treatment.2.Drought priming and magnesium application alleviated the inhibition of Rubisco carboxylation efficiency under high temperature stress at grain filling stage in wheat.High temperature stress significantly inhibited the net photosynthetic rate,leaves maintained stomatal opening and higher transpiration rate,which promoted leaves cooling.While CO2 accumulated in intercellular and intracellular,Rubisco activation state and Rubisco activation enzyme activity significantly decreased under short-term high temperature stress,resulting in the reduction of CO2 carboxylation efficiency under high temperature stress.Under short-term high temperature stress,drought priming,magnesium application and interaction treatments could stabilize leaf water potential,maintain leaf transpiration and stomatal opening,and alleviate leaf temperature rise.Under short-term high temperature stress,the plants with mild drought priming in the early stage could stabilize Rubisco activity and reduce the loss of Rubisco activity and activation state,but it could not maintain Rubisco activity and activation state under long-term high temperature stress.Magnesium application and interaction treatments increased Rubisco activase activity significantly,which promoted Rubisco activity and activation state,and maintained a high level under high temperature stress.Magnesium and its interaction increased the content of Rubisco and Rubisco activase activity,which alleviated the decrease of Rubisco content and Rubisco activase content under high temperature stress.Therefore,drought priming,magnesium application and interaction treatments stabilized the Rubisco carboxylation efficiency which alleviated the inhibition of high temperature stress on photosynthetic carboxylation efficiency.3.Drought priming and magnesium application alleviated the inhibition of photosynthetic electron transfer efficiency under high temperature stress at grain filling stage.High temperature stress led to the decrease of photosynthetic quantum yield,the increase of heat dissipation and the decrease of electron transfer efficiency.Under short-term high temperature stress,wheat leaves could maintain the stability of photo reaction center through heat dissipation and other ways.After 5 days of high temperature stress,photosynthetic system was inhibited,and the integrity of photosynthetic system and thylakoid membrane was destroyed.Drought priming and interaction could slow down the decrease of photochemical quenching under short-term high temperature stress and reduce photoinhibition under high temperature stress through rapid response of enhanced heat dissipation and cyclic electron transfer.Electron transfer rate and photosynthetic quantum yield could be stabilized after 5 days of high temperature stress.Magnesium application and interaction treatment significantly improved the photosynthetic quantum yield and photochemical efficiency.Magnesium application and interaction treatment also maintained D1 protein content under high temperature stress,and accelerated the turnover ability of removing damaged D1 protein under high temperature stress,which promoted the stability of light reaction center under high temperature stress;Magnesium application and interaction treatment reduced PSI donor side restriction,promoted ATP synthase synthesis under proton transmembrane gradient,and reduced thylakoid membrane electron leakage.Therefore,under high temperature stress,drought priming,magnesium application and interaction treatments could stabilize light energy utilization and electron transfer,which reduced redundant electron damage.4.Drought priming and magnesium application alleviated leaf senescence under high temperature stress at grain filling stage in wheat.High temperature stress accelerated the reduction of leaf nitrogen content,the accumulation of oxidative substances,the improvement of antioxidant system activity,as well as the aggravation of leaf oxidative damage.High temperature stress resulted in functional chlorosis of wheat leaves and shortened the duration of leaf function.In the early stage,mild drought priming alleviated the accumulation of oxidative substances under high temperature stress,enhanced the activities of antioxidant enzymes and alleviated the oxidative damage of leaves under high temperature stress.The results showed that the application of magnesium and interaction treatment increased the nitrogen content of leaves and reduced the loss of nitrogen content under high temperature stress.Magnesium application and interaction treatment improved antioxidant enzyme activity,which alleviated the accumulation of oxidative substances in leaves under high temperature stress.Magnesium application and interaction treatment alleviated the chlorosis of wheat leaves under high temperature stress by stabilizing chlorophyll degrading enzyme activity.In conclusion,drought priming,magnesium application and interaction treatmentstabilized the accumulation of photosynthetic dry matter in leaves under high temperature stress after anthesis,alleviated the decrease of grain yield under high temperature stress at grain filling stage,and alleviated the loss of Rubisco carboxylation efficiency by alleviating the decrease of Rubisco activating enzyme activity under high temperature stress at grain filling stage.The electron transfer efficiency was stabilized by improving the light protection ability and D1 protein turnover,and then the leaf oxidative stress was reduced.Magnesium application also maintained the leaf functional staying green.Thus,the accumulation of photosynthetic dry matter in leaves under high temperature stress after anthesis was stabilized,and the decrease of grain yield under high temperature stress during grain filling stage was alleviated.