Responses Of Alfalfa To Different Water Availabilities And Its Regulation Mechanisms Of Water Use Efficiency

Alfalfa(Medicago sativa L.,lucerne) is widely planted and used all over the world because of the advantages of good quality and drougt resistance,but it is also a water-consuming species.So it is an urgent issue to study the response of its water use to different water availabilities and the mechanism underbeneath its different water use efficiency as more and more stands have been grown in the semiarid and arid areas of China.In this study,the responses of three alfalfa varieties to different water availabilities and water use efficiency were investigated in a greenhouse pot experiment trying to uncover the related mechanism.The changes in leaf net photosynthetic rate(Pn),transpiration rate(Tr), intercellular CO₂ concentration(Ci),stomatal conductance(gs),stomatal density and carbon isotope composition(δ¹³C) of three alfalfa varieties were measured at three soil water levels in a greenhouse pot experiment.WUE of Longdong and Xinjiangdaye varieties was the greatest at 50%field water capacity(FWC) and the lowest at 75%FWC,while contrary result was observed for Algonquin variety.At 25%FWC,WUE of all varieties was the greatest at a daytime scale,while for 75%FWC and 50%FWC,no consistent results were shown.Pn/Ci declined as water stress became more serious,while stomatal density increased andδ¹³C as well.It suggested that water stress could to some extent enhance WUE of alfalfa,but different responses could be observed among varieties.In addition,it also suggested that stomatal regulation and leaf carboxylation efficiency would play roles together in modulating WUE of alfalfa.The changes in leaf relative water content(RWC) and proline content of three alfalfa varieties were measured at three soil water levels in this study.Under drought stress,leaf RWC decreased in Algonquin and Xinjiangdaye and maintained steady in Longdong,but the difference of RWC among water treatments for three alfalfa varieties was not significant,suggesting that three varieties hold the ability to maintain enough water within plant body in response to some water stresses. Longdong variety owns the steadiest ability.Proline content in the leaves of Longdong was the least under 75%and 25%FWC and was the greatest under 50%FWC,suggesting that with the severity of water stress,Longdong may depend on not only proline change to adjust osmotic potential. The changes in leaf relative electric conductivity(REC),MDA content,the activities of some antioxidases of three alfalfa varieties were measured at three soil water levels in this study.Under drought stress,MDA content and REC increased, suggesting that cell integrity has been damaged.Under drought stress,the activity of SOD,POD and CAT of three alfalfa varieties varied and the same species under different water conditions showed different enzyme activity.In response to different extents of drought stress,different enzymes may take into effects synergically to resist water deficit protectecting the plant from being injured.The biomass of three varieties were measured at three soil water levels in this study.Both fresh and dry weight of three alfalfa varieties decreased as water availability turned more serious.At the same water level,the biomass of Longdong was the greatest and that of Algonquin,the least.In conclusion,the maintanence of osmotic potential and integrity of cell promise the normal functions of cell and tissue,i.e.stomata and chloroplast in the mesophyll cell.Thus the plant can survive with relatively enough water use efficiency,which will finally be observed as plant biomass,i.e.fresh weight and dry matter.