Effects Of Root Exudates From Alfalfa With Different Root System Architecture On Phosphorus Uptake Under Low Phosphorus Stress

Alfalfa（Medicago sativa.L）is a leguminous forage widely distributed and long-cultivated in semi-arid areas of the Loess Plateau.It has enormous economic value and positive ecological effects,playing an irreplaceable role in livestock development and ecological environment construction.However,the lack of available phosphorus in soil is a key factor that limits alfalfa yield and quality in semi-arid regions.As the main organ for plants to absorb water and nutrients,the root system can release root exudates that act on the rhizosphere microenvironment in a low-phosphorus environment,and its activation of insoluble phosphorus in the rhizosphere is the key for plants to adapt to low-phosphorus stress way.In addition to being affected by environmental factors,the secretion of root exudates is closely related to root architecture.Therefore,it is of great significance to find out the relationship between root exudates and root architecture and the mechanism of their effects on phosphorus uptake for improving the phosphorus uptake capacity of alfalfa and breeding excellent local cultivars with high phosphorus efficiency.In this study,two alfalfa varieties with different root architectures were used as materials:the introduced cultivars WL354HQ（WL354）and the Chinese native cultivars Longdong alfalfa（LD）,and the visualized root box cultivation system was used to systematically study the effect of alfalfa root exudates on phosphorus supply.Levels[3 phosphorus levels:control condition CK(500 mmol L^-1·P),moderate low phosphorus stress MP(125 mmol L^-1·P),severe low phosphorus stress LP(50 mmol L^-1·P)]To compare the response characteristics of root exudates of alfalfa varieties with different root architectures,and to reveal the effect of root exudates on alfalfa phosphorus uptake.The main results are as follows:（1）At both MP and LP levels,LD and WL354 show a steady increase in the content of tartaric,lactic,and acetic,which gradually stabilize after 60 days.However,the content of citric acid and malic acid exhibits a distinct inverted V-shaped pattern,significantly increasing in the initial stages of stress and gradually decreasing between days 30 and 60 of the stress period.Compared to WL354,LD releases more tartaric acid,citric acid,malic acid,and acid phosphatase,resulting in a lower p H within the rhizosphere.Furthermore,compared to the 0-30 cm soil layer,the variation of root exudates in soil layers below 30cm demonstrates an opposite trend.（2）As phosphorus stress intensifies,WL354 exhibits a significant decrease in both coarse and fine root surface area and root length.On the other hand,LD shows a V-shaped trend.These characteristics are more pronounced in the 0-30 cm soil layer compared to the soil layers below 30 cm.（3）WL354 shows a stronger correlation between metrics such as coarse root length,coarse root surface area,root length,and root diameter with phosphorus uptake efficiency.On the other hand,LD exhibits a stronger correlation with acid phosphatase and p H.The results indicate that purple clover can enhance phosphorus uptake efficiency and plant yield by increasing fine root length and enhancing acid phosphatase activity.（4）WL354 and LD exhibit different root morphology and secretion response strategies under phosphorus stress.WL354 primarily adapts to low-phosphorus environments by increasing root length and specific root length,as well as reducing root tissue density.On the other hand,LD mainly promotes phosphorus absorption in the root system by increasing the secretion of tartaric,citric,malic,and acid phosphatase.In summary,to adapt to low-phosphorus environments,LD adopts conservative characteristics in root morphology and shifts towards an acquisition strategy in root exudates,mainly manifested by an increase in organic acid content and an increase in acid phosphatase activity,which helps to improve phosphorus absorption efficiency and promote plant growth.On the other hand,WL354 has a high ratio of root length to low root tissue density,indicating that its roots have stronger outward expansion ability,which can better help alfalfa to absorb nutrients from the external environment and improve phosphorus absorption efficiency while ensuring the nutrients required for normal growth.There is an obvious trade-off relationship between the root architecture and exudate content of the two cultivars.The results of this study enrich our understanding of the mechanism of alfalfa root adaptation to low-phosphorus environments and provide a theoretical basis for the selection of high-efficiency and excellent native alfalfa varieties and the promotion of weight loss and efficiency increase in alfalfa cultivation.