The effect of potassium application on induced defense of alfalfa by Odontothrips loti feeding

Objectives To explore the effct of potassium application on induced defence in alfalfa leaves by thrips feeding.

Methods Medicago sativa Gannong 3 was chosen as experimental material and Odontothrips loti was selected as research insect to carry out pot experiment in net greenhouse. The four treatments were without and with potassium application under insect damage condition (K0, K2), and under no insect damage (K0CK, K2CK), respectively. At the 5-leaf and 1-center period of alfalfa, O. loti adults were immigrated on alfalfa plant with 3 heads per plant in K0 and K2 treatment. At the 1, 3, 5, 7 and 9 days of O. loti feeding, the damage index of alfalfa in K0 and K2 treatment were investigated, the contents of jasmonic acid (JA) and salicylic acid (SA), and the activities of phenylalnine ammonialyase (PAL) and lipoxygenase (LOX) in the upper leaves (top three leaves and the center tip) and lower leaves (the four leaves from the bottom, excluding the leaves of lateral branches) of the four treatments were determined.

Results Both K and thrips feeding induced the signaling pathways of JA and SA. Compared with K0, K2 decreased the damage index of alfalfa by 10.61%, 21.40%, 23.80%, 27.79%, and 33.06% at 1, 3, 5, 7 and 9 days (P<0.01). The JA and SA contents, and the LOX and PAL activities in upper and lower leaves were all in order of K2>K0>K2CK>K0CK in all the sampling day, except the upper leaf LOX activity in K2 treatment at the 5 day. The peaks of LOX and PAL activity in upper leaves were tested the earliest in K2 treatment. In the same time and treatment, the JA and SA contents and the LOX and PAL activities in upper leaves were higher than those in lower leaves.

Conclusions O. loti feeding induces the synthesis and increases of the activities of defense enzymes LOX and PAL in alfalfa leaves, promoting the synthesis and accumulation of SA and JA, and the secondary metabolic activity of upper leaves (the feeding site of thrips) reaches peak earlier and stronger than that of the lower leaves. Potassium application speeds up the secondary metabolism in form of earlier and higher LOX and PAL activities and the resulted earlier and higher JA and SA acid contents, thereby further decreases the damage of alfalfa when exposure to O. loti feeding.