Effect Of Potassium Application On Alfalfa Nutrients,Secondary Metabolites And Resistance To Thrips(Thysanoptera: Thripidae)

In order to investigate the effects of potassium on alfalfa related mechanism and its resistance to thrips,two strains of the alfalfa,the resistant strain to thrips（Gannong No.9）and the susceptible strain to thrips（Gannong No.3）were chosed in this study,different potassium levels（0,6,9,12,15 g/m2）were arranged to research and measure the changes of damage index,thrips density,potassium content,height,yield,nutrient and secondary metabolites content.1.Effect of different potassium levels on damage index,thrips density,potassium cotent and growth index of Gannong No.9 and Gannong No.3The damage index,height,yield of two alfalfa varieties increased significantly with increasing potassium levels.The potassium content in old and heart leaves increased as the increase of potassium concentration.The thrips density had no siginificant difference in 2nd and 3rd cuttings of alfalfa.Correlation analysis indicated that the potassium content and thrips density of alfalfa leaves had no correlation with the damage index.Potassium may mainly improve the tolerance of alfalfa to thrips.The most economical fertilizer application rate was 9 g/m2（K₂O）in this experiment.2.Changes of nutrient content of Gannong No.9 and Gannong No.3 alfalfa after treatment to different potassium levelsThe free amino acids content in Gannong No.9 and Gannong No.3 alfalfa heart leaves was significantly decreased and the decrease rate was increased,the starch content and change rate were increased,the soluble sugar content and the C/N ratio both increased,nitrogen content decreased and reached the maximum at the K₁5 level in the second in 2nd cutting.The free amino acids content and change rate of two alfalfa varieties reached the minimum at the K₁2 and K₉ levels,the starch content and change rate at the K₁5 and K₉ levels reached the maximum,the C/N ratio reached the maximum at the K₁2 and K₉ levels respectively,the soluble sugar content was the highest at the K₁2 and K₉ levels,and the nitrogen content was the lowest at the K₁5 and K₆ levels in 3rd cuttings.The free amino acids content of Gannong No.9 and Gannong No.3 old leaves reached the minimum at the K₁2 level while those decrease rate reached the maximum,the starch content and change rate reached the maximum at the K₁2 level,the soluble sugar content of the old leaves increased,the nitrogen content of Gannong No.3 decreased while those in Gannong 9 increased,and the ratio of carbon to nitrogen increased with the potassium level increasing in the 2nd cutting.In the 3rd cutting of old leaves,the free amino acids content of two alfalfa varieties reached the minimum at the K₆ level while those decrease rate reached the.Compared with the control,the starch content increased significantly while the change rate reached the maximum at the K₉ and K₁2 levels,the soluble sugar content and the C/N ratio increased,the nitrogen content decreased.The free amino acids content of two alfalfa varieties leaves in 3rd cutting was lower than that of the 2nd cutting.The effect of potassium on the soluble sugar and starch content of alfalfa leaves in 2nd cutting was higher than that of the 3rd cutting.3.Changes of secondary metabolites content of Gannong No.9 and Gannong No.3 alfalfa after treatment to different potassium levelsThe total phenolics content in the two alfalfa varieties heart and old leaves were higher than that of the untreated control with the potassium application increasing.The flavonoids content in the alfalfa heart and old leaves increased firstly,and then decreased in the 2nd cutting.The flavonoids content in the old leaves was more than 1.5 times higher than that in the heart leaves.The flavonoids and lignin content in alfalfa heart and old leaves increased as the increase of potassium application in the 3rd cutting,but there was no significant difference on lignin content under the same potassium level.