Effect Of Nitrogen Application On Alfalfa Resistance To Thrips

In order to explore the effect of nitrogen application on alfalfa resistance to thrips,two alfalfa varieties,Gannong No.3 and Gannong No.9 were used as materials and Odontothrips loti was taken as the research object in the experiment.The oviposition selection and nymph development of thrips were observed under different nitrogen application conditions under indoor conditions.The damage degree of alfalfa,the growth of alfalfa plants,photosynthesis of leaves and the content of nutrients and mineral elements in roots,stems and leaves were measured after alfalfa were infested by O.loti for 7 and 14 days under the condition of pot culture in net room.The results showed that:1.The number of eggs laid on the leaves of two alfalfa varieties increased with nitrogen application level increasing,and that the eggs thrips laid on the leaves of Gannong No.3 were less than Gannong No.9 under the higher nitrogen level.The eggs hatchability of the two alfalfa leaves increased significantly.The eggs hatchability on the leaves of Gannong No.3 and Gannong No.9 increased by 16.88%and 19.38%respectively under N3 level.The survival rate of thrips nymphs of 1-2 star,3-4 star and the total survival rate of nymphs increased with nitrogen applicating.There was no significant change in nymph development duration at different stages.The fecundity,hatchability and survival rate of the O.loti were positively correlated with the content of nitrogen,free amino acid and soluble protein,and negatively correlated with the content of soluble sugar and total phenol in alfalfa leaves.Two alfalfa varieties had no antixenosis and antibiosis to thrips with nitrogen application.2.The damage index of two alfalfa varieties increased with nitrogen applicating,and the damage index of two alfalfa varieties first increased and then decreased with nitrogen level increasing.The plant height,stem diameter and aboveground biomass were the largest at N2level infested by O.loti for 7 days,and plant height and aboveground biomass were the largest at N3 level infested by O.loti for 14 days,meanwhile the ratio of root to shoot decreased.The content of chlorophyll in alfalfa leaves increased with nitrogen applicating,but the chlorophyll content decreased with the damaged index increasing.The net photosynthetic rate,transpiration rate,stomatal conductance and water use efficiency of alfalfa leaves with nitrogen level increasing,and the intercellular CO₂ concentration decreased infested by O.loti for 7 days.The net photosynthetic rate,transpiration rate,stomatal conductance and intercellular CO₂concentration decreased,and water use efficiency increased infested by O.loti for 14 days.Nitrogen application reduced the resistance of alfalfa to thrips.Nitrogen application promoted the growth of two alfalfa varieties,but reduced the ratio of root to shoot of alfalfa,which resulted to alfalfa regeneration capacity declined and alfalfa tolerance to thrips decreased.With the damage degree aggravating,the source of photosynthesis substrate of alfalfa was insufficient,which resulted to the resistance of alfalfa to thrips decreased.3.The content of soluble sugar,starch and total phenol decreased,and the content of free amino acid and soluble protein increased with nitrogen applicating.Meanwhile,the absorption of nitrogen,phosphorus and potassium by alfalfa roots,stems and leaves increased.With feeding time and damage degree increasing,the more soluble sugar was transferred to stem,but the less to leaves and roots,the content of starch and free amino acids in leaves decreased,the content of soluble protein,nitrogen and phosphorus was changed from‘leaves>stems>roots’to‘leaves>roots>stems’,and the potassium content did not change significantly.Nitrogen application made alfalfa lack of carbon source of synthetic defense substances,which reduced the resistance of alfalfa to thrips and aggravated the damage degree of alfalfa.