Genotypic Difference Of Salt Tolerance And The Effect Of Rhizobium Inoculation In Alfalfa

Alfalfa(Medicago sativa L.)is widely cultivated forage crop with high protein and nutrition in northern China.However,soil salinization seriously affects the alfalfa production in Xinjiang.Therefore,improving salt tolerance in alfalfa has an important significance for development of alfalfa industry in Xinjiang.It is reported that there is genotypic difference in salt tolerance in alfalfa,and the symbiosis with rhizobia can improve salt tolerance in alfalfa.In addition,Na+ and Cl-stresses have the different effects on plants growth.However,little is still known about the mechanism of salt tolerance in alfalfa.In the present study,firstly,the salt tolerance of GJ,ADN,DY and ZM was identified by medium culture and hydroponics;secondly,the optimal rhizobia were isolated,purified and identified from alfalfa cultivated soils in Xinjiang,salt tolerance of which was preliminarily evaluated;furthermore,alfalfa genotypes with different salt tolerance in Na+,Cl-or Na Cl treatments were employed to explore the mechanism in improvement of salt tolerance by rhizobium inoculation.The results were shown as follows:(1)The results from seed germination experiment showed that salt treatment affected significantly taproot growth in alfalfa,and the taproot length in four alfalfa genotypes was severely inhibited with increasing salt concentration.ADN had the best salt tolerance,while GJ and ZM stood the opposite place.However,in the hydroponics,ZM had higher salt tolerance.These results indicate that salt tolerance during seed germination was different from that in vegetative growth stage.Thereby,selection of salt-tolerant genotypes could not be evaluated only in seed germination stage.(2)The results from hydroponics showed that salt treatment significantly affected alfalfa growth,and there were obvious differences in salt tolerance among four alfalfa genotypes.Under salt treatment condition,the fresh weight and dry weight of ZM plants increased distinctly,total root length and root surface area increased,and root potassium content increased,while the shoot sodium content reduced compared to the other three genotypes.It indicated that higher salt tolerance in ZM is realized mainly through inhibiting the sodium transport from root shoot.In contrast,the fresh weight and dry weight of GJ plants reduced distinctly,chlorophyll and carotenoid contents were also largely reduced,but the root malondialdehydecontent was increased compared to ZM.It indicated that GJ suffers greater oxidative stress,and thereby is poorly salt tolerant.(3)The results from abundance of indigenous rhizobia in alfalfa planting area of Xinjiang showed that the number of indigenous rhizobia varied in the range of 7.6×1010per gram of dry soil.On this basis,salt tolerance test of rhizobia in the media were proceeded,and the result indicated the rhizobium strain 180 had the highest salt-tolerant,and was able to normally grow in the 8% Na Cl medium,but rhizobium standard strain USDA1002 grew only in 0% Na Cl,and the selected rhizobium strain H6 from this experiment only in 2% Na Cl medium.(4)The results from rhizobium inoculation test showed that the inoculation with rhizobia USDA1002,H6 and 180 alleviated the effect of salt stress on the growth of ZM and GJ,and increased the nitrogen contents of the shoots and roots.Under salt stress,rhizobium strain H6 had better inoculation effect,indicating salt tolerance of rhizobia in the media was different from that in inoculation experiment.Moreover,inoculation with rhizobia more obviously alleviated salt salinity in salt-sensitive genotype GJ.(5)The results from rhizobium inoculation at different salt treatments showed that rhizobium inoculation was able to greatly increase salt tolerance as well as plant nitrogen content in alfalfa.Na Cl and Na+stresses significantly inhibited alfalfa growth,while inoculation with rhizobia could significantly relieve the inhibitory effect of Na Cl and Na+stresses on plant growth.In summary,ZM had the better tolerance to salt stress,but GJ was sensitive to salt salinity.Inoculation with rhizobia was able to improve salt tolerance in alfalfa by increasing plant nitrogen content.Inoculation with rhizobia more obviously alleviated salt salinity in the salt-sensitive genotype than the salt-tolerant genotype.Salt salinity was mainly attributed to Na+stress,but not Cl-stress in alfalfa.