INDUCED RESISTANCE IN SOYBEAN TO HELICOVERPA-ZEA - ROLE OF PLANT PROTEIN-QUALITY

Resistance in soybean to Helicoverpa tea is comprised of both constitu tive and inducible factors. In this study, we investigated the inducti on of resistance by H. iea in both greenhouse and field studies. In a greenhouse experiment, fourth-instar H. tea growth rates were reduced by 39% after 24 hr feeding and by 27% after 48 hr when larvae fed on p reviously wounded V3 foliage (cv. Forrest) compared with undamaged fol iage. In a field study, the weight gain by larvae was more than 52% gr eater when larvae fed for 72 hr on undamaged R2/R3 soybean plants (cv. Braxton) compared to those that fed on previously wounded plants. A s ignificant component of the induced resistance is due to a decline in the nutritional quality of foliar protein following foliar damage by H . tea. Foliar protein was extracted from damaged and undamaged foliage and incorporated into artificial diets. Larval growth was reduced 26% after four days and 49% after seven days on diets containing protein from damaged plants compared to larvae feeding on foliar protein from undamaged plants. Chemical analyses of protein quality also indicated a decline in quality in damaged plants compared to unwounded plants. I ncreases in lipoxygenase activity (53%), lipid peroxidation products ( 20%), and trypsin inhibitor content (34%) were observed in protein fro m wounded plants. Moreover, a 5.9% loss in free amines and 19% loss in total thiols occurred in protein from wounded plants. Larval feeding causes a significant increase in foliar lipoxygenase activity that var ied among genotypes. Lipoxygenase isozymes were measured at pH 5.5, pH 7.0, and pH 8.5 in V3 stage plants of Forrest, Hark, D75-1069, and PI 417061 genotypes. Lipoxygenase activity in each genotype was signific antly increased after 72 hr of larval feeding at each pH level tested, with the exception of lipoxygenase isozymes at pH 5.5 in genotype PI 417061. Larval feeding on R2/R3 stage plants (field-grown cv. Braxton) for six days also increased foliar lipoxygenase activity.