FOLIAR NUTRIENT CONCENTRATIONS OF INSECT SUSCEPTIBLE AND RESISTANT SOYBEAN GERMPLASM

Differences in host plant quality to insect herbivores may be explaine d by variations in secondary plant compounds and plant nutrients. Soyb ean [Glycine max (L.) Merr.] germplasm resistant to foliar feeding ins ect pests have been identified and developed. Investigations into the chemical basis of this resistance have met with limited success. The o bjectives of this study were to determine the foliar concentrations of 12 nutrients in 18 field-grown soybean lines with known quantitative levels of resistance or susceptibility to the Mexican bean beetle (MBB ) (Epilachna varivestis Mulsant) and to determine if a relationship ex ists between nutrient concentrations and resistance. Eighteen soybean genotypes were evaluated. The six MBB-resistant lines of Group 1 inclu ded PI 171451 or lines derived therefrom. The six MBB-resistant lines of Group 2 included PI 229358 or lines derived therefrom. The six Grou p 3 lines were MBB-susceptible commercial cultivars. The MBB feeding d amage and foliar concentrations of P, Ca, and Fe in resistant Groups 1 and 2 were significantly less (P < 0.01) than in susceptible Group 3. Significant positive linear contrasts indicated that as damage increa sed, foliar concentrations of P, Ca, and Fe increased. Results indicat e that none of the foliar nutrients analyzed in this study have a dire ct cause-effect relationship with MBB resistance. However, the foliar concentrations of three nutrients (P, Ca, and Fe) are significantly lo wer in MBB-resistant soybean genotypes using PI 171451 or PI 229358 as the resistant parent, which suggests that these nutrients are associa ted or linked to the true cause of resistance.