Effects Of High And Low Temperature Stress On Photosynthesis And Photoprotection Mechanism Of Medicago Sativa At Seedlings Period

Alfalfa(Medicago sativa L.)is the largest cultivated pasture in Xinjiang,and it plays an important role in the sustainable development of Xinjiang’s grass and animal husbandry and the construction of the ecological environment.The temperature in Xinjiang is low and unstable in spring and autumn,while hot in summer.Therefore,temperature stress severely restricts the development of Xinjiang alfalfa industry.Photosynthesis is the basis of plant growth and development,and it is also the basis of agricultural and pastoral crop yields,but higher plant photosynthesis is very sensitive to environmental stress.The high temperature or low temperature stress encountered during the growth and development of alfalfa seriously affects the photosynthesis of alfalfa leaves,thereby affecting the growth and development of alfalfa and its yield composition.In this study,Xinmu No.4(Medicago sativa L.cv.Xin Mu No.4,XM4),the main Xinjiang alfalfa variety,was used as the research object,Gannong No5(Medicago sativa L.cv.Gan Nong No.5,GN5)was used as a control,and the high(40℃)and low temperature(10℃)were studied respectively.Effects of Stress on Germination Characteristics,Photosynthetic Activity and Photoprotection Mechanism of Alfalfa Seeds.The main findings are as follows:(1)Both high and low temperature stresses delay the germination rate of alfalfa seeds,but have no significant effect on the final germination rate of seeds;in the initial stage of seed germination Mitochondrial respiration is the only source of energy for seed germination.In this study,we found that in the initial stage of seed germination,the alternative oxidase(AOX)pathway makes an obvious contribution to reducing the oxidative stress of high temperature on seed germination.Cytochrome oxidase(COX)pathway contributes greatly under low temperature conditions,mainly to provide sufficient ATP for the seed germination under low temperature.In the growth stage of seedlings,the contribution of AOX pathway to root growth at different temperatures is greater than that of COX pathway;In addition,we also found that the effects of different temperatures on seed germination and the contribution of AOX and COX pathways to seed germination were not significantly different between the two alfalfa varieties with different fall dormancy levels.(2)Both high and low temperature stress significantly reduced the expression of Rubisco protein,thereby reducing the carbon assimilation rate of alfalfa,and finally leading to slow growth and reduced yield of alfalfa.In addition,both high and low temperature stress significantly damaged the chloroplast structure of the leaves,resulting in a decrease in the chlorophyll content of the leaves and a significant change in the composition.Both high and low temperature stress will reduce the photochemical activity of the leaves,causing different degrees of photoinhibition in the leaves,and the low temperature stress damages the photosynthetic machinery more seriously.In addition,we also found that the low temperature stress not only damages the leaf photosystem II(PS II)the donor side,the acceptor side and the reaction center include the turnover of the D1 protein,which reduces the stability of the PS II complex and also inhibits the PSI activity;High temperature stress had little effect on photosynthetic apparatus and photosynthetic electron transport.(3)High temperature stress increases the heat dissipation of plants,and heat dissipation could be inactivated under low temperature stress;high temperature stress will not cause an increase in cyclic electron transfer(CEF)activity.However,low temperature stress will increase the activity of CEF,and the activity of CEF increases more significantly in “GN5” to reduce the oxidative damage of photosystem I(PS I).Both high and low temperature stresses can cause ROS accumulation,and low temperature stress aggravate the accumulation of ROS,and at the same time active oxygen scavenging enzymes(CAT,SOD,APX)could be activated.Among them,APX seems to have a greater role in low temperature to reduce activity Oxidative stress on photosynthetic machinery by oxygen;this result is slightly different between the two varieties.The amount of AOX protein increases significantly under high temperature stress,and the amount of AOX protein decreases significantly after low temperature stress.This indicates that the AOX pathway has a high photoprotection under high temperature,while low temperature stress can significantly inhibit the photoprotection of the AOX pathway.(4)After comparing two kinds of alfalfa,it was found that the oxygen-evolving complex(OEC)in the leaves of “XM4” was inactivated under high temperature stress,but the OEC activity of “GN5” was greater under high temperature stress;Compared with “GN5”,the PS II light-harvesting pigment binding protein(Lhcb2)in “XM4” is more sensitive to temperature changes;under low temperature stress,“XM4” mainly protects itself by increasing the activity of antioxidant enzymes To avoid injury,“GN5” enhances cold resistance by increasing the activity of CEF.