Differences In Acquisition Strategies Between Different Phosphorus Efficient Alfalfa Genotypes

Alfalfa（Medicago sativa L.）is an important legume forage in China,which plays an important role in the development of herbivorous animal husbandry and ecological restoration.Northwest China is the main area of alfalfa grassland distribution.Soil low phosphorus stress and insufficient germplasm resources innovation are the main reasons for limiting the productivity of alfalfa grassland.On the other hand,alfalfa artificial grassland is a kind of farmland system with low nutrient input,which is subjected to more severe low phosphorus stress than food crops,and the research on the mechanism of low phosphorus stress of alfalfa is not comprehensive.Although many studies have focused on the differentiation of plant root functional traits under low phosphorus stress,there are few comparative studies on different genotypes of forage.In this study,the differentiation of root functional traits related to low phosphorus adaptation of alfalfa genotypes with different phosphorus efficiency was compared,including root morphology,rhizosphere activation and symbiotic traits of mycorrhizal fungi（AMF）related to phosphorus acquisition.Based on the identification of 182alfalfa varieties in the previous field experiment,this study compared the low phosphorus adaptation mechanism of 22 alfalfa varieties（Experiment 1）.The first experiment was a two-factor randomized block experiment,including 22 genotypes（10P efficient genotypes,10 P inefficient genotypes and 2 control varieties）and three phosphorus fertilizer gradients(10,60 and 120 mg Pkg^-1 dry soil),of which the control varieties were Longzhong alfalfa and Longdong alfalfa.After 60 days of sowing,the changes of root morphology,rhizosphere activation and AMF symbiotic traits under different treatments were analyzed.On this basis,four alfalfa genotypes with different phosphorus efficiency were selected to study the differentiation patterns of root morphology and rhizosphere activation strategies of different genotypes after blocking the AMF symbiosis pathway（Experiment 2）.Experiment 2 was a three-factor randomized block experiment,including four genotypes（two P-efficient genotypes and two P-inefficient genotypes）,two P fertilizer gradients(10 and 120 mg Pkg^-1 dry soil)and two AMF treatments（+AMF and-AMF）.Plant growth and root pathway changes were analyzed after 80 days of sowing.Through the above pot experiments,we studied（1）the differentiation of root phosphorus acquisition traits of alfalfa genotypes with different phosphorus efficiency under low phosphorus stress;and（2）Effects of AMF symbiosis on low phosphorus adaptation mechanism and phosphorus efficiency differentiation of alfalfa.The main results are as follows:1.Soil low phosphorus stress(10 mg Pkg^-1 dry soil)significantly reduced the aboveground and underground biomass of all alfalfa varieties.Under the three phosphorus gradients,the aboveground and underground biomass of the phosphorus-efficient genotype was significantly greater than that of the phosphorus-inefficient genotype（p<0.05）,and the control variety was between the two.The root mass ratio（the ratio of root biomass to total plant biomass）of alfalfa under three phosphate fertilizer gradients was about 0.29,and there was no differentiation between different phosphorus efficiency genotypes at the same phosphate fertilizer level.2.Low phosphorus stress caused differential responses of root traits in different genotypes of alfalfa.High phosphorus efficiency genotypes had larger root surface area（or total root length）,AMF colonization rate,rhizosphere organic acid content and lower rhizosphere p H.The P-inefficient genotypes had larger specific root length（or smaller root diameter）and larger alkaline phosphatase activity.3.Pearson correlation analysis showed that the total biomass of alfalfa in low phosphorus environment was significantly positively correlated with total root length,total root surface area,root diameter,rhizosphere organic acid and AMF infection rate（p<0.05）,but significantly negatively correlated with rhizosphere p H,alkaline phosphatase activity and specific root length（p<0.05）.It can be seen that the differentiation of different root functional traits in low phosphorus environment is the main reason for the difference in biomass of alfalfa.The results of principal component analysis of total biomass and root functional traits showed that P-efficient genotypes focused on the following variables in low phosphorus environment:total root length,root surface area,root diameter,organic acid and AMF infection rate,while P-inefficient genotypes focused on root mass ratio,specific root length,phosphatase and p H.With the increase of soil phosphorus content,the differentiation trend of root traits of phosphorus efficient and phosphorus inefficient genotypes decreased significantly.It can be seen that AMF symbiosis and root morphological pathways are more dependent traits of phosphorus-efficient genotypes in low phosphorus environment,while the differentiation of rhizosphere activation pathways among different phosphorus-efficient genotypes is not obvious.4.After blocking AMF symbiosis,the aboveground and underground biomass of alfalfa decreased significantly.The biomass decreased by 94%in low phosphorus environment(10 mg Pkg^-1 dry soil),and decreased by 11.8%in high phosphorus environment(120 mg Pkg^-1 dry soil).Under the condition of AMF symbiosis,the biomass of phosphorus-efficient genotypes was greater than that of phosphorus-inefficient genotypes,and there was no difference in alfalfa biomass between phosphorus-efficient and phosphorus-inefficient genotypes after blocking AMF symbiosis.It can be seen that AMF symbiosis is particularly important for maintaining the growth of alfalfa plants in low phosphorus environment and the growth advantage of phosphorus efficient genotypes.5.Under low phosphorus stress,after blocking the AMF symbiotic pathway,the strategies of phosphorus efficient and phosphorus inefficient genotypes in response to low phosphorus stress were differentiated,which was manifested as a significant positive correlation between phosphorus efficient genotype and root morphology（specific root length,root surface area,total root length）,and phosphorus inefficient genotype was positively correlated with rhizosphere activation（organic acids,p H）.Under the symbiotic condition of AMF,the infection rate of AMF synergized with the activities of rhizosphere organic acids and alkaline phosphatase to promote the adaptation of phosphorus-efficient genotype to low phosphorus.6.The principal component analysis of root traits in the two pot experiments showed that the AMF infection rate in the low phosphorus environment was negatively correlated with the specific root length,but positively correlated with the root diameter,that is,the differentiation of phosphorus efficient genotype and phosphorus inefficient genotype conformed to the collaborative gradient of root economic space,and the phosphorus acquisition strategy of alfalfa partially supported root economic space falsehood.In summary,the low-phosphorus adaptation strategies of different genotypes in alfalfa species partially support the root-root economic space hypothesis,that is,there is differentiation in the collaborative gradient of the root economic space.Under low phosphorus conditions,alfalfa phosphorus efficient genotypes adapted to low phosphorus environment by adjusting root morphology and AMF symbiosis,while phosphorus high efficiency and phosphorus low efficiency genotypes.