Effects Of Salt Stress On Phosphorus Utilization Of Different Alfalfa Varieties

Phosphorus（P）plays an important role in plant growth and development.It is a component of many key compounds in plants and participates in various metabolic processes of plants.Lack of phosphorus in soil restricts plant growth and yield,P applied to soil often exists in the form of insoluble phosphate with low availability,thus constraining the absorption and utilization of P by plants.The widespread saline soil in our country affects the growth and nutrient absorption of plants,causing problems such as low yield and poor quality.In arid and semi-arid regions,soil P deficiency and salinization are factors that simultaneously restrict crop productivity.Salinity may weaken the positive effect of P-fertilizers on plant growth.In order to explore the mechanisms of soil salinity on P-utilization efficiency of different varieties of alfalfa,we investigated the effects of soil P availability and salinity on plant growth,P nutrition and salt tolerance of three alfalfa（Medicago sativa L.）varieties.A pot experiment was carried out to grow three varieties of alfalfa,i.e.Golden Empress,Salt-tolerant Star and GannonⅦ,in a loess soil,external P was added as an aqueous KH₂PO₄ solution at four rates,i.e.0,40,80,160 mg P kg^–1 dry soil;Na Cl was added at four rates,i.e.0,0.4,0.8,1.6 g kg^–1 dry soil.Shoot dry mass,root dry mass,concentrations of P（[P]）,sodium（[Na]）,and potassium（[K]）in leaves,stems and roots were determined;rhizosheath carboxylates,alkaline phosphatase activity in the bulk soil,rhizosphere soil p H and bulk soil p H,and leaf soluble sugar concent were analyzed.The key results were as follows:（1）For all varieties,adding P significantly increased shoot dry mass and root dry mass,but adding Na Cl reduced shoot dry mass and root dry mass;When P was added,P uptake by plants increased.When P was added at160 mg kg^–1,P concentrations in roots,stems,and leaves all reached the maximum.However,increasing salinity inhibited the P uptake.When Na Cl was added at 1.6 g kg^–1,P concentrations in roots,stems,and leaves all significantly decreased,when compared with that at 0Na Cl.The increase of salinity significantly reduced the K/Na in roots,stems,and leaves;root K/Na of Golden Empress,Salt-tolerant Star and GannonⅦdecreased by 74%,84%and 75%,respectively,at 1.6 g Na Cl kg^–1 compared with that at 0Na Cl.K/Na in stems and leaves of all three varieties were higher than that in roots;（2）Rhizosheath tartrate amount was only markedly affected by soil P for all varieties,it decreased with the increase of P level.However,salinity only affected the tartrate amount of GannonⅦ.The availability of P and salinity both significantly affected soil alkaline phosphatase activity of Golden Empress and Salt-tolerant Star,but the effect on phosphatase activity of GannonⅦwas not significant.When Na Cl was added at 1.6 g kg^–1,compared with that at 0Na Cl,soil alkaline phosphatase activity of Golden Empress and Salt-tolerant Star significantly decreased;（3）The rhizosphere soil p H of most treatments was lower than that of the bulk soil p H,the decrease of soil p H under salt stress might increase the availability of soil P and P uptake by plants,and the decrease in p H was possibly a mechanism to cope with salt stress;（4）A high-dose(160 mg kg^–1)of P significantly reduced the P-uptake efficiency of all three varieties,and the P-utilization efficiency of GannonⅦwas also significantly reduced.Salinity significantly reduced the P-uptake efficiency of all three varieties.When Na Cl was added at 1.6 g kg^–1,P-uptake efficiency of Golden Empress,Salt-tolerant Star and GannonⅦdecreased by 64%,66%and 79%respectively compared with that at 0Na Cl;In summary,the results of this experiment showed a significant interaction between soil P availability and salinity,and the interaction depended on the variety.The effects of phosphorus and salinty alone were significant on shoot dry mass,root dry mass,[P],[Na]and P-uptake efficiency of all three alfalfa varieties;they also markedly affected soil alkaline phosphatase activity of Golden Empress and Salt-tolerant Star.The interactionbetween P and salinity had a significant effect on root[Na].The increase of salinity exacerbated P deficiency in the soil.Appropriate application of P-fertilizers can improve the salt tolerance of alfalfa and increase its productivity in saline soils.