Effect Of Potassium Application On Seedling Alfalfa Resistance To Thrips (Thysanoptera: Thripidae)

In order to explore the effect of potassium application on the resistance to thrips of alfalfa at the seedling stage,two varieties of the Medicago sativa,Gannong No.3 and Gannong No.9were 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 oviposition selection,growth and development,adult longevity and fecundity of O.loti were investigated which fed with alfalfa leaves applied with different amounts of potassium,meanwhile the soluble sugar,free amino acid and potassium contents in alfalfa leaves were measured in lab conditions.The damage degree of alfalfa was evaluated,the growth of alfalfa plants,and the content of nutrients in roots,stems and leaves,and the content of endogenous hormone of functional leaves and root crowns were measured under different potassium treatments after alfalfa were infested by O.loti for 7 and 14 days in net room.The results showed that:1.The number of eggs laid on alfalfa leaves by O.loti decreased first and then increased with increasing amounts of potassium application,and that the number of eggs on leaves of Gannong No.9 were less than eggs on leaves of Gannong No.3 at the same amount of potassium application.The hatching rate of eggs and the survival rate of 1st-2nd instar nymphs did not significantly vary in comparison with the control,the survival rate of 3rd-4th instar nymphs and the total immature survival rate went down significantly.Both development duration of eggs and1st-2nd instar nymphs did not change significantly,while the 3rd-4th instar nymphs and the total immature development duration were prolonged.The fecundity and longevity of the second generational adult decreased on the leaves of alfalfa applied potassium.The relationship analysis showed that potassium contents in alfalfa leaves of Gannong No.3 was no correlated significantly with the total immature survival rate and the fecundity of adult of O.loti.However,the soluble sugar content and the sugar to nitrogen ratio in both variety leaves were negatively correlated significantly with the survival rate of the 3rd-4th instars nymphs,the total immature survival rate,and the fecundity of adult.Potassium contents in alfalfa leaves of Gannong No.9was negatively correlated significantly with the survival rate of the 3rd-4th instars nymphs,the fecundity and longevity of the second generational adult.And the soluble sugar content and the sugar to nitrogen ratio in alfalfa leaves were negatively correlated significantly with the survival rate of the 3rd-4th instars nymphs,and the fecundity of adult.2.The damage index of Gannong No.3 and Gannong No.9 were all decreased significantly infested by O.loti for 7 and 14 days with the amount of potassium increasing in net room.The plant height and branches of alfalfa increased firstly and then decreased with the amount of potassium increasing,the biomass of roots,stems and leaves increased significantly,and the root to shoot ratio increased after potassium applied.The proportion of leaves and stems were larger than that shoots,as the damage time continues,the biomass of each organ increased significantly,and the proportion of leaves and roots were larger than that of shoots.3.The content of soluble sugar,starch and free amino acid in the roots,stems and leaves of alfalfa increased firstly and then decreased in net room,and the root to shoot ratio of soluble sugar and starch increased after potassium application.The soluble sugar of potassium alfalfa is more distributed to the leaves and roots as the damage time continues,and the content of starch and free amino acids in each organ are increased,and alfalfa assigned more resources for the construction of photosynthetic organs and storage organs.4.The content of IAA and GA₃in alfalfa leaves and root crowns increased,whereas the content of ABA decreased after potassium application in net room.At the initial stage of damage infested by O.loti,the content of GA₃in alfalfa leaves was higher than that of root crowns,meanwhile the content of ABA was lower than that of root crowns.The content of IAA in the two positions were no obvious differences,which indicated that the GA₃distributed in the root crowns was more than in leaves,and stimulated the growth point and promoted the growth of branch under mild thrips stress.The content of IAA in the root crowns of alfalfa,GA₃content in leaves and root crowns increased significantly as the damage time continues,while the ABA content in the root crowns increased significantly.In conclusion,potassium-applied alfalfa has significant antixenosis to the oviposition of O.loti adults.The application of potassium increases the soluble sugar and the sugar to nitrogen ratio in alfalfa leaves,which exertes significant antibiosis to O.loti by affecting the development of nymphs,shortening the longevity and reducing fecundity of adults.Potassium is mainly used to enhance the growth performance of alfalfa and to reasonably and effectively distribute the biomass of various organs,carbohydrates and endogenous hormones,thereby improving the tolerance of alfalfa to O.loti.Furthermore,excessive of potassium will weaken the resistance of alfalfa to O.loti.In this study,the optimal fertilization rate was 60（K₂O）mg/kg soil.