Age-dependent effects of jasmonic acid treatment and wind exposure on foliar oxidase activity and insect resistance in tomato

Jasmonic acid (JA) treatment of tomato plants induces several defense-relat ed oxidative enzymes and increases pest resistance in a manner thought to s imulate natural insect wounding. In a full-factorial greenhouse experiment, we examined the independent and interactive effects of plant age and expos ure to wind-induced mechanical stress (MS), on the ability of JA to induce defense in tomato. In general, treatment of 4-, 6-, and 8-week-old tomato p lants with 1 mM JA resulted in the induction of peroxidase and polyphenol o xidase activity and reduced the relative growth rate of first-instar Manduc a sexta larvae fed treated leaves, in accordance with other studies. Peroxi dase activity increased with plant age and was induced by JA most strongly in older plants. In contrast, polyphenol oxidase activity did not change wi th plant age and was induced by JA most strongly in young plants. While rel ative growth rates of M. sexta were lower on older plants overall, JA reduc ed growth rates most strongly in young plants, in which JA treatment enhanc ed polyphenol oxidase activity by more than 70%. MS enhanced the activity o f peroxidase, but substantially reduced the activity of polyphenol oxidase; the latter most intensely on older plants. M. sexta tended to grow more sl owly on MS-treated plants, although this effect wins not significant. Thus, reduced polyphenol oxidase activity in MS-treated plants did not lead to a n increase in growth rate of M. sexta, possibly because peroxidase activity was still elevated in MS-treated plants. Significant interactions between JA and MS and three-way interactions were not detected for any variable, al though the inductive effects of both JA and MS interacted in complex ways w ith plant age. Our results indicate that resistance traits in tomato are di fferentially affected by JA and wind exposure and differ in their relative contribution to defense as plants age.