Inducible phytoalexins in juvenile soybean genotypes predict soybean looper resistance in the fully developed plants.

The hypocotyl of different soybean genotypes was tested for its inducible phytoalexin (i.e. glyceollin or coumestrol) accumulation and its inducible soybean looper resistance in response to chemical elicitation. A very highly insect-resistant soybean genotype (PI 227687) produced significantly more phytoalexins than a relatively insect-susceptible one (Davis) in response to the same chemical elicitation. The resultant standardized hypocotyl assay allowed quick categorization of unknown soybean genotypes regarding the level of insect resistance in the fully developed plants. Glyceollin was a better indicator of inducible resistance than coumestrol. Elicitor concentration influenced the amount of glyceollin and coumestrol accumulated. Younger seedlings (4-5 d old) responded stronger to chemical elicitation than did older ones (7-10 d old). The elicited accumulation of glyceollin showed a temporal pattern that peaked at 72 h. Accumulation of coumestrol showed a gradual increase. Elicitation of phytoalexins in juvenile soybean plants by sulfhydryl-binding reagents was found to be useful for the prediction of genotypic differences in the level of insect resistance in the fully developed plants.