Foliar application of l-phenylalanine and ferulic acids to pea plants: induced phenylalanine ammonia lyase activity and resistance againstErysiphe pisi

Phenylalanine ammonia lyase (PAL) activity was measured using HPLC in pea leaves following exogenous application of l-phenylalanine and ferulic acid. Treatment with different concentrations (50, 100 and 150 ppm) of l-phenylalanine caused increased activity of PAL in comparison to the control. In pea leaves treated with 50 ppm l-phenylalanine, maximum PAL activity was observed after 72 h of treatment. Application of ferulic acid first reduced PAL activity at lower concentration (50 ppm) but increased at higher concentrations of the compound (100 and 150 ppm) in pea leaves as compared to the control. Maximum PAL activity was 0.19 nM cinnamic acid/min/g fresh wt. after 24 h at 50 ppm and then increased with time. Treatment with both the compounds significantly reduced conidial germination of Erysiphe pisi on pea leaves. They were equally effective at 100 and 150 ppm in reducing conidial germination. The conidial germination on l-phenylalanine-treated leaves was 26% after 24 h and that on ferulic acid-treated leaves was 34% as compared to the control (46%). Foliar application of different concentrations of l-phenylalanine increased the level of ferulic acid in the leaves of pea plants. Maximum accumulation of ferulic acid (79.3 and 83.5 μg/g fresh wt.) was observed following the application of l-phenylalanine after 24 h and 48 h, respectively. At 50 ppm, ferulic acid accumulation in pea leaves was 35.6 and 39.4 μg/g fresh wt. and 74.3 and 86.5 μg/g fresh wt. at 100 ppm.