| Alfalfa(Medicago sativa L.)has the advantages of strong resistance,high yield,good quality and high economic value.It is widely cultivated in northwestern regions such as Gansu,Shanxi and Xinjiang in China.Summers in southern China are hot and rainy,and alfalfa is particularly sensitive to high-temperature environments and is difficult to plant in southern China.Therefore,under the background of vigorously developing grass husbandry in south China,how to improve the heat tolerance of alfalfa and explore alfalfa varieties suitable for planting in south China is an urgent problem to be solved.Therefore,this study used 10 different varieties of alfalfa as materials to study their morphological and physiological changes under heat stress(35℃/30℃(day temperature/night temperature)),and the membership function method was used to comprehensively evaluate the heat resistance of alfalfa to select the thermosensitive varieties and explore the methods to improve the heat resistance of alfalfa by adding exogenous potassium silicate and urea.The main research results are as follows:1.After 14 days of heat treatment(35℃/30℃(day temperature/night temperature)),the growth of 10 varieties of alfalfa was severely inhibited.Specifically,heat stress restricted the reduction of plant height,root length and root,stem,weight of fresh leaves.Chlorophyll a content,chlorophyll b content and total chlorophyll content decreased while MDA and soluble sugar content increased.The evaluation results show that under heat stress,the three varieties of alfalfa(alfalfa alfalfa king(0.84),WL-656HQ(0.64),and PinxiGL(0.57)have strong high-temperature tolerance,while alfalfa Sandeli(0.41),Victoria(0.38)and WL-712(0.36)are heat sensitive varieties.2.Under heat stress(35℃/30℃(day temperature/night temperature)),2 mmol/L potassium silicate sprayed on the leaf surface can promote the growth of alfalfa,significantly increase the content of chlorophyll a,chlorophyll b and total chlorophyll,and reduce the conductivity Value and MDA content,significantly reduce SOD activity and soluble sugar content,and alleviate the oxidative stress suffered by alfalfa.At the same time,the maximum photosynthetic efficiency(Fv/Fm)of the photosystem Ⅱ and the relative electron transfer rate(ETR)of the photosystem Ⅱ at this concentration were significantly increased,and the photosynthetic efficiency was enhanced,and the contents of Ca,Na and Mg in the leaves were significantly increased.Compared with the concentration of 1 mmol/L,2 mmol/L was the better spraying concentration.3.Spraying 2.5 mmol/L urea under heat stress(35℃/30℃(day temperature/night temperature))can enhance the photosynthetic performance of plants and promote the accumulation of biomass by reducing the value of electric conductivity,lowering the content of MDA,reducing the activity of SOD,reducing the content of soluble sugar and increasing the content of chlorophyll.In addition,the addition of 2.5 mmol/L under heat stress can promote the absorption of three trace elements Mn,Cu and Zn by plants,indirectly improve the scavenging rate of antioxidant enzyme system for reactive oxygen species and enhance the respiration of roots,while the effect of spraying 5 mmol/L urea on the alleviation of heat stress is not significant.In summary,heat stress has a toxic effect on the growth and physiology of alfalfa plants,and the exogenous addition of certain concentration of potassium silicate and urea can effectively enhance the heat resistance of alfalfa under heat stress.This conclusion provides a theoretical basis for using potassium silicate and urea to reduce the toxic effect of alfalfa under heat. |
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