Physiological Response Characteristics Of Medicago Sativa Under Freezing-thawing And Salinity-alkalinity Compound Stress

Freezing-thawing stress is a common natural phenomenon in Northeast China,grass subjected to freezing-thawing stress due to cold snap at spring and autumn season,and appeared phenomenon of poor growth and low yield,finally reduce the utilization value of forage.At present,the study on the cold tolerance and salt tolerance ability of forage is mainly focused on single factor stress,but there is little research on the response mechanism of grass under the combined stress of freezing-thawing and saline-alkali.Alfalfa?Medicago sativa?is perennial herb,which has the characteristics of cold resistance,alkali resistance and strong resistance,is one of the most widely cultivated grass in the world and was used for improve the saline-alkali soil.In this study,Dongmu 1 Alfalfa as the research object,through the experiment of the plant under artificial freezing-thawing with Na₂SO₄,NaCl,NaHCO₃ and Na₂CO₃?four kinds of stress is common salt components in northeast China?respectively composite stress to study the changes of MDA protein,soluble sugar,proline content and SOD,POD activity in Alfalfa seedlings.The physiological response characteristics of Alfalfa seedlings to freezing-thawing and saline alkali stress were studied systematically,which provided the theoretical basis for the evaluation of cold tolerance and salt tolerance and the improvement of saline alkali soil in cold regions.The main results are as follows:In the simulation experiment,the contents of osmotic adjustment substances were increased first and then decreased in Alfalfa seedlings during the freezing-thawing stress.In the thawing-freezing stage?10^-3??,with the decrease of temperature,the contents of proline,MDA and soluble sugar were increased both in single freezing-thawing stress group and in freezing-thawing and saline-alkali?Na₂SO₄,NaCl,NaHCO₃,Na₂CO₃?combined stress group,which reached the peak at-3?,significant difference was observed?P<0.05?compared with that of 10?,5?and 0?;The change tendency of the protein is the same,the peak value appears at5?and-3?,it is more sensitive at-3 ?,and has significant difference compared with that of 10??P<0.05?.In the freezing-thawing stages?-3¹0??,with the decrease of temperature,in the freezing-thawing group and freezing-thawing andneutral salt?Na₂SO₄?NaCl?combined stress group,the contents of MDA,proline and soluble sugar were decreased slightly but still very high,and no significant difference was observed in the different temperature treatment group?P>0.05?,but the content of protein decreased significantly?P< 0.05?;The contents of protein,proline,MDA and soluble sugar were significantly decreased both in freezing-thawing group and freezing-thawing and alkaline salt?NaHCO₃ ? Na₂CO₃?combined stress group?P<0.05?.The activities of antioxidant enzymes increased with the decrease of temperature in Alfalfa seedlings.however,there was a slight difference in different treatment groups.During the whole experiment,SOD and POD activity showed no significant change?P>0.05?in CK group,the changes of SOD and POD activity is consistent in freezing-thawing stress group and freezing-thawing and saline-alkali combined stress group,which showed a trend of increased first and then decreased after the stable during freezing-thawing stress and the SOD,POD activity were higher in all the temperature treatment groups than that of CK group.During freezing-thawing treatment,the activities of SOD and POD were increased gradually with the decrease of temperature,and the peak at-3 ? in the freezing-thawing stress group and freezing-thawing and saline-alkali combined stress group,compared with that of 10?,5? and 0?,there are significant differences was observed?P<0.05?;With the increase of temperature,the activities of SOD and POD were decreased in freezing-thawing and neutral salt combined stress group,however,the activity of SOD decreased slightly and the activity of POD decreased firstly and then increased in freezing-thawing and alkaline salt combined stress group.In the freezing-thawing and saline-alkali stress,cellar membrane lipid peroxidation damage was increased in Alfalfa seedlings,the content of MDA increased,and forcing the Alfalfa Seedlings to produce a series of physiological defense mechanism of saline-alkali tolerance and cold resistance that can induce the activity of SOD and POD increased to rapidly eliminate reactive oxygen species which can protective effects of superoxide anion free radicals on Alfalfa Seedlings.At the same time,the contents of proline and soluble sugar increased which can adjust the osmotic potential of cell membrane to maintain the normal physiological activityof Alfalfa seedlings.