Study On Physiological Mechanism Of Hight Light Induces Leaf Senescence In Wheat (Trticum Aestivum L.)

III Wheat（Trticum aestivum L.）is one of the main food crops in the world,and the growth of wheat yield is lower than that of the world population.Therefore,radiation use efficiency（RUE）is considered an important approach for wheat genetic yield improvement.In the field,the important limiting factor to improve the RUE of wheat and other cereal crops is the photoinhibition or photooxidation stress induced by high light,which promotes leaf premature senescence,and thus grain yield loss of cereal crops including bread wheat.However,the direct evidence of HL induced senescence lacked up till now.Here,seedlings of two winter wheat cultivars Xiaoyan 54（XY54,HL tolerant）and Jing 411（J411,HL sensitive）were transferred from low light（LL,300 μmol m–2 s–1）to HL（800 μmol m–2 s–1）for 2–8 d to explore the association of long-term HL stress with leaf senescence in wheat.The main results of this study are as follows:Firstly,long-term high light stress causes photooxidative to induce senescence.In the process of transferring XY54 and J411 from low light to high light incubator for continuous 8-day culture,it was found that the content of total chlorophyll,chlorophyll a and chlorophyll b decreased sharply at 6–8 days of high light treatment,while the control group LL8 d had little change compared with 0 days.The results showed that the long-term high light stress accelerated the degradation of chlorophyll,but the carotenoid content increased,indicating that plants in response to the high light stress activated defensive mechanism to reduce photooxidation damage of light.Fv/ Fm,P I,ETo/ CS and RC/ CSm continue to decrease with the time of high light treatment.In contrast to the changing trend of DIo/ CS,the light suppression is caused by long-term high light stress,the photochemical conversion efficiency of the PSII reaction center is reduced,and the heat dissipation is increased.After 6–8 days of high light treatment,the Rubisco activity of XY54 and J411 continued to decrease,and was significantly lower than that in the control group,indicating that long-term high light stress inhibited the photosynthesis of the two cultivars.Under long-term high light stress,the activity of PEPC began to decrease on 4 days after the high light treatment of J411,and decreased significantly on the 8 day of XY54,and J411 was significantly lower than that of XY54,indicating that the long-term high light stress inhibited the CO₂ assimilation rate,and the ability of the plant to respond to the high light stress was related to the genotype.The excessive production of the oxidation substances such as O2ˉand MDA,the decrease of the antioxidant enzymes such as SOD,CAT,APX and POD,leads to the imbalance of active oxygen metabolism in the wheat leaves under high light,which aggravated the damage of cell membrane and advanced the senescence process of leaves.Compared with 0 days,MDA content increased significantly from low light to high light for 2 days.And the content of MDA in the two cultivars increased continuously with the persistence of high light stress,and J411 is significantly higher than XY54.This indicates that continuous high light stress increases the photooxidative stress of the leaves and,in turn,promotes the excessive production of ROS,reduces the activity of the antioxidant enzymes,thereby accelerating the senescence of the plant,and the ability to tolerance high light is also related to the genotype.The expression of Ta SAG1,Ta SAG5 and Ta SAG8 continuously increased,and were significantly higher than 0 days and LL8 d,indicating that long-term high light induced up-regulation of SAGs expression and senescence occurred.Collectively,reduction of antioxidant enzymes activity may result in long-term HL stress induced leaf senescence.Secondly,long-term high light stress causes imbalance of sugar metabolism and induces senescence.XY54 and J411 were transferred from low light to high light incubator for 2 days,and the A,gs,Ci and E of the two cultivars all decreased sharply,indicating that high light inhibited the opening of stomatal conductance,directly affected the intercellular CO₂ concentration and transpiration rate of leaves,and led to the decrease of photosynthetic ability of leaves.Lhca1,Lhca3,Lhcb4 and D1 continued to decrease with the duration of high light treatment.Although Lhcb2 and Rubisco were induced by high light at 2 days,the expression level decreased from 4 days as the duration of high light treatment continued.It indicates that long-term high light leads to photooxidative stress and chlorophyll-binding protein degradation,which directly affects photosynthesis of plants.In the high light treatment for 0–8 days,the expression of Tarbc S and Ta PEPC continued to decrease with the prolongation of high light treatment time,and was significantly lower than that of the control group LL8 d.It indicated that the photooxidative stress caused by high light inhibited the activity of Rubisco and PEPC,which led to the decline of photosynthesis ability of the two cultivars under high light.Continuous high light treatment resulted in a continuous accumulation of soluble sugar and starch content in XY54 and J411 leaves,where in the content of the sucrose is the highest,the starch is the second,the accumulation of the sugar content in the J411 is obviously higher than the XY54 in the time point of each high light treatment.During the long-term high light treatment,the sugar content was significantly higher than 0 days and LL8 d,and the activities of SPS,SUS and invertase（A/N-INV and SAI）were increased,but the expression of Ta SUT and Ta SUS were inhibited.The accumulation of sucrose did not inhibit the synthesis of sucrose in turn,and the enzyme activity in the decomposition direction increased,but the expression level was significantly inhibited compared with 0 days and LL8 d,indicating that the photooxidative stress induced by strong light disrupts the metabolic balance between the sugar content and the enzyme associated with the sugar metabolism,The transport of sucrose is inhibited,and the accumulation of sugar,in turn,increases cell damage,leading to leaf senescence.In addition,the sugar content in the leaves of J411 was significantly higher than that of XY54,while the activities of enzymes related to the direction of sucrose decomposition,such as SUS,A/N-INV and SAI,were significantly lower than XY54 at 8 days.It is indicated that the accumulation of sugar in J411 leaves under high light is also related to genotype.Correlation analysis showed that in thelong-term high light induced leaf senescence,the accumulation of sugar and the inhibition of most sugar metabolism related genes were positively correlated with chlorophyll content.Compared with the high light sensitive variety J411,the tolerant cultivar XY54 accumulates less sugar,indicating that sugar metabolism may be the regulatory target of wheat for high light stress.