Effects Of Phosphorus Application On Seedling Stage Of Medicago Sativa Nutrients And Resistance To Odontothrips Loti(Thysanoptera:Thripidae)

In order to investigate the mechanisms of phosphorus on resistance of alfalfa to thrips,two varieties of the Medicago sativa,Gannong No.3(which was widely planted in northwest china but susceptible to thrips)and Gannong No.9(which was relatively resistant to thrips)were selected in this study.Odontothrips loti was selected as research subject which was the dominant pest of thrips in alfalfa planting area of Northern China.The egg laying selection and development duration of O.loti on alfalfa leaves under different phosphorus levels were observed in lab.The alfalfa leaves damage index,growth characteristics,photosynthesis gas exchange parameters,and nutrient and mineral contents in alfalfa stems and roots were determined through damaged by pests for 7,14 and 21 days.The results showed that:1.The egg laying amounts on alfalfa leaves of thrips increased with the increasing of phosphorus levels for the same variety.The 1-2 instar nymph survival ratios of O.loti increased obviously after phosphorus application,but their development periods showed no significant changes.In addition,there was no significant different on survival ratios and development periods for 3-4 instar nymph.The abovementioned results indicated that two alfalfa varieties had no significant antixenosis and antibiosis influence to thrips under phosphorus application.2.The damage index of Gannong No.3 and Gannong No.9 were all decreased after phosphorus application.In particular,it decreased significantly after damaged for 14 days and 21 days.P3 treatment exhibited the lowest level of two varieties,such as Ganong No.3 decreased 18.25% and 19.86%,and Ganong No.9 decreased 9.58% and 12.33%,respectively.Notably,the resistance of alfalfa to thrips increased effectively with phosphorus application,obvious greatly for Gannong No.3 variety.The biomass of alfalfa leaves,stems and roots increased significantly after phosphorus application.The most significant periods were damaged for 14 days.After damaged for 21 days,the proportion of leaves increased,the proportion of roots decreased and the proportion of stems had no significant changes with the increase of phosphorus levels.Therefore,phosphorus application accelerated compensatory growth of alfalfa leaves or leaf crow under high pest pressure,but excessive phosphorus application(P4 level)could reduce the resistance of alfalfa to thrips.3.After phosphorus application,the net photosynthetic rate of Gannong No.3 and Gannong No.9 were all increased significantly.The net photosynthetic rate and water utilization efficiency of two varieties were all decreased with the long damage duration time.Proper amount of phosphorus application could effectively improve the photosynthesis ability,and promote the compensatory growth in alfalfa,and ultimately enhance the tolerance of alfalfa to thrips.The content of amino acids decreased significantly in leaves,but the content of free amino acids was increased significantly in stems and roots for two varieties.The content of soluble sugar and starch were also significantly increased in whole plant with phosphorus application.The amino acids contents in alfalfa roots,stems and leaves were all increased,while the starch contents decreased with the long damage duration times.Thus,we suspected that alfalfa organs can use more substance resources for construction of photosynthetic and storage organs after damaged by pests.Their objective was promoted greater compensatory growth abilities and the tolerance to thrips.However,excessive phosphorus application(such P4 level)could inhibit assimilate synthesis in alfalfa.4.With the increase of phosphorus levels,the nitrogen and phosphorus contents in alfalfa roots,stems and leaves increased significantly,and the potassium contents in leaves and stems increased significantly,but the potassium contents in roots decreased significantly.With the long damage duration times,the mineral elements accumulated to the root gradually for promoting the roots development and assimilating more nutrients.In conclusion,there was no significant changes of antibiosis and antixenosis in alfalfa to thrips using phosphorus.Substantially,Phosphorus application mainly enhanced the photosynthesis and promoted the rational distribution of the photosynthetic products and the mineral elements in order to increase the tolerance of alfalfa to thrips.Furthermore,excessive phosphorus application can lead to induce the tolerance of alfalfa to thrips.In this study,P3 level(0.81 g/10 kg soil)was the best treatment.