Effects Of Nodules On Cold Tolerance In Alfalfa (Medicago Sativa L.)

Alfalfa was divided into three different groups by rhizobia treatment: alfalfa inoculatednodule bacteria, alfalfa inoculated inactive nodule bacteria and alfalfa without nodules.Alfalfa in each group was exposed to0℃for0h,2h,4h,6h,8h. We measured thesephysiological and chemical indexes: leaf relative electrical conductivity, Malondialdehyde(MDA), peroxidase (POD)activity, superoxide dismutase (SOD) activity, catalase (CAT)activity, soluble sugar content, soluble protein content, free proline content in order toinvestigate the physiological change of alfalfa with symbiotic rhizobium under coldstress．The analytical methods of Principal component analysis, gray correlation andsubordination function were used to comprehensively analyze the effect of differenttreatments on the cold resistance of alfalfa by comparing their physiological and chemicalindexes. And the mechanism of alfalfa inoculated nodule bacteria showing a high level coldtolerance was studied. The main findings and conclusions are showed as follows:1、Leaf electrical relative conductivity increased with the severity of the cold stress,MDA content decreases with severity of the cold stress. Alfalfa with activated inoculatednodule bacteria show a higher leaf relative conductivity and MDA content compared to thealfalfa with deactivated inoculated nodule bacteria and non-inoculated nodule bacteria. ColdStress undermines the permeability of plant cell membranes, but not excessively increasedperoxidation of cell membranes. Alfalfa with activated inoculated nodule bacteria shows ahigher degree of membranes damage during cold stress.2、The antioxidant protective enzymes in the different treatments of alfalfa showedvarying degrees of increase with time except the CAT in the root of alfalfa, especially in lowtemperature stress of a final (8h), SOD, POD and CAT activity increased significantly in theaboveground part of the alfalfa. And the enzyme activity in alfalfa with the activatedinoculated nodule bacteria shows the greatest increase among all the treatment.3、Soluble sugar content, soluble protein content increased with the treatment time ofcold stress, but proline content showed the opposite. Also alfalfa with activated inoculatednodule bacteria maintains a higher content of soluble sugar, soluble protein and prolineduring cold stress. Under cold stress, alfalfa increased osmotic adjustment substance contentand levels of protective proteins to enhance their hardiness, alfalfa with activated inoculated nodule bacteria kept a higher level of the osmotic adjustment substance to improve its coldtolerance.4、The result of principal component analysis shows that MDA and soluble sugar arethe main indexes to evaluate the cold tolerance of alfalfa. And the soluble protein in the rootof alfalfa, POD and SOD activity may also have some beneficial effect on the evaluation ofthe cold tolerance of alfalfa. The leaf relative conductivity and MDA have the negativeeffect on the plant hardiness, but the osmotic adjustment substances(soluble sugars, solubleprotein and proline) and the Oxidation protective enzymes(CAT,SOD,POD) help plantsprotect themselves during cold stress.5、Gray correlation analysis results show that the CAT activity, SOD activity andsoluble sugar content in the roots and the soluble sugar content, proline content in theaboveground parts are most sensitive to cold stress. Compared with aboveground part, rootsare more sensitive to cold stress. Among all the indexes soluble sugars and SOD in alfalfaare the most sensitive indexes that can be used as the evaluation of cold toleranceexpression in different treatment of alfalfa.6、The membership function analysis results for three different treatments of alfalfawere:0.586,0.511,0.417separately. Proving that alfalfa with activated inoculated nodulebacteria have a stronger cold tolerance in the quantitative point of view.