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Contrary to constitutive resistance, inducible resistance to Mexican bean beetle (MBB) (Epilachna varivestis) herbivory in PI 227687 soybean leaves was positively correlated with total phenolic content and temporally unique, increasedL-phenylalanine ammonia-lyase (PAL) andL-tyrosine ammonia-lyase (TAL) activities. Initial expression of the induced resistance was localized at or near the site of herbivory. Systemic parameters of the induced resistance also were observed. Inducible MBB resistance in PI 227687 soybeans apparently involves increased phenylpropanoid metabolism.

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Constitutive (i.e., preformed) parameters of Mexican bean beetle (MBB) resistance were demonstrated in PI 227687 soybeans grown under highly controlled and reproducible biotron environmental conditions which favored optimal plant growth and development. Constitutive antifeedants in PI 227687 against the beetle were variously soluble in hexane, ethyl acetate and 100% methanol. Antifeedants also remained in the aqueous residue after the sequential organic extractions. PI 227687 and MBB-susceptible 'Davis' soybean lines each showed characteristic temporal patterns of L-phenylalanine ammonia-lyase activity. Healthy PI 227687, but not healthy 'Davis', also showed a characteristic temporal pattern of L-tyrosine ammonia-lyase activity. Activities of the above two enzymes are positively correlated with plant phenylpropanoid metabolism. PI 227687 and 'Davis' leaves did not differ in content of total phenols. Products from phenylpropanoid metabolism apparently contribute to PI 227687 resistance to MBB feeding.