Root Traits Diversity In Alfalfa And Their Responses To Water And Phosphorus Supply

Alfalfa（Medicago sativa.L）plays an irreplaceable role in the development of animal husbandry and ecological environment construction in the semi-arid area of the Loess Plateau.However,drought and phosphorus（P）deficiency limited the growth of alfalfa in this area severely.Root system is the main organ for plants to absorb water and nutrients,and plays an irreplaceable role in the process of plant growth and adaptation to the environment.In this study,the diversity characteristics of root traits in 56 alfalfa cultivars from home and abroad were studied via nutrient solution sand culture method with the help of a visual rhizobox.And2 representative cultivars with different root architecture were selected:the introduced cultivars WL354HQ（WL354）and Chinese native cultivars Longdong alfalfa（LD）.The differential responses of root traits in two alfalfa cultivars to different water and P supply[(2water levels:80±5%of field water conditioning（normal water,HW）,40±5%of field water conditioning（drought stress,LW）;3 P levels:P₀（no P application）,P₅(5 mg P kg^-1 dry soil),P₂₀(20 mg P kg^-1 dry soil)]were investigated to reveal the strategies of root traits on water and P uptake in alfalfa cultivars with different root architecture.The main results were as follows:（1）Root traits of different alfalfa cultivars at the seedling stage differed significantly.One whole root trait（root length intensity）and nine local root traits（root length-topsoil,root surface area-topsoil,root volume-topsoil,root length-subsoil,root surface area-subsoil,root volume-subsoil,root tips number-subsoil,root length in diameter thin and root length in diameter thick）had high coefficients of variation（CVs≥0.25）.Based on these 10 root traits,13 alfalfa cultivars were selected,and then two alfalfa cultivars with different root architecture:WL354 and LD were selected as experimental materials for the subsequent research,taking into account the situation of alfalfa cultivation in semi-arid area of the Loess Plateau.（2）In terms of whole root traits,except for specific root length,specific root surface area and root tissue density,the other whole root traits of WL354 and LD all reduced,but the reduction degree of two cultivars differently.The proportion of rest whole root traits with WL354 decreased gradually,while LD showed a decreasing trend.Under normal water conditions,with the increase of P application,the rest of the whole root traits of WL354 and LD increased and increased followed by decreased,respectively.Furthermore,with the increase of P application,the rest of the whole root traits of two alfalfa cultivars both showed a trend of first increasing and then decreasing under drought stress conditions.（3）The change trend of root morphology in different soil layers of two alfalfa cultivars under different water and P conditions is basically consistent with the whole root characteristics.Compared with normal water conditions,total root length,total root surface area and root length in diameter thin distribution ratio of WL354 in the soil layer below 30 cm increased 12.3%,10.2%and 12.0%respectively under drought stress conditions,and the increase ratio was significantly higher than that of LD.Under normal water conditions,total root length,total root surface area and root length in diameter thin distribution ratio of the two alfalfa cultivars in the 0-30 cm soil layer were the highest.And with the increase of P application,the proportion of root distribution of LD in the 0-30 cm soil layer increased gradually.（4）Total root length,maximum root depth,root growth rate and root tips number of WL354 during the entire drought process decreased.With the increase of P application,the reduction ratio of the above root morphological parameters increased under drought stress conditions,and root tips number decreased significantly on the 30th day of drought.Drought mainly reduced the total root length,root growth rate and root tips number of LD at P₀,but the reduction ratio did not change significantly during drought.（5）Root water uptake efficiency,P uptake efficiency,shoot dry mass and root dry mass decreased of the two alfalfa cultivars under drought stress conditions.Among them,P uptake efficiency of the two alfalfa cultivars reached the highest at P₅ and the decrease ratio was the largest at P₂₀.With the increase of P application,the degree of inhibition on the shoot and root dry mass of WL354 by drought gradually increased,while the degree of inhibition of LD gradually decreased.Under normal water conditions,P uptake efficiency of the two alfalfa cultivars increasing gradually with the increase of P application.Under normal water conditions,Shoot and root dry mass of WL354 gradually increased,but shoot dry mass and root dry mass of LD reached the highest at P₅ with the increase of P application.Pearson’s correlation analysis showed that the whole root traits were not strongly correlated with the root water uptake efficiency,but were closely related to the P uptake efficiency and shoot dry mass of the two alfalfa cultivars.In conclusion,the root traits of different alfalfa cultivars at the seedling stage were significantly various,and the degree of local root traits variation was higher.The root distribution and dynamic changes of the two alfalfa cultivars had different response strategies to water and P supply.WL354 can increase the absorption of deep soil moisture by increasing the root distribution ratio in the soil layer below 30 cm.Under normal water conditions,both two alfalfa cultivars can increase the uptake of P in the topsoil by increasing the root distribution ratio of the 0-30 cm soil layer.With the increase of P application,root growth of WL354 became more sensitive to water,while LD showed the opposite trend.P application of5 mg P kg^-1 had the greatest effect on the growth of the two alfalfa cultivars under drought stress.