Seed companies are currently marketing varieties of glandular-haired alfalfa, Medicago sativa L., with resistance to the potato leafhopper, Empoasca fabae (Harris). Although it took 30 years to develop and release the varieties, little is known of alfalfa glandular trichome chemistry and its role in resistance. Our previous studies showed crude trichome extracts affect the leafhopper's settling behavior. To identify these active natural products, we used the resistant glandular genotype 'G98A' (Cal/West Seeds) and the susceptible nonglandular variety 'Ranger'. Trichomes were mechanically isolated from stem sections of G98A and Ranger, then were extracted and fractionated using flash chromatography. Fractions were assayed by applying aliquots to the surface of a parafilm-covered artificial diet sachet. Leafhoppers were offered a choice between G98A fractions vs. crude G98A extracts, and G98A fractions vs. an acetone control. This allowed for identification of a fraction with the highest degree of leafhopper deterrency. GC-MS analysis of the nonvolatile trichome fractions revealed six fatty acid derivatives with alkaloidal properties. Other large, nonvolatile compounds also comprise the trichome exudate. Volatile compounds were not detected in the trichome exudate. Additional bioassays have not shown volatile compounds from G98A to be repellent to the leafhopper. Neither have stem tissue extracts shown antifeedant activity when mixed into artificial diet. In combination, these results indicate that alfalfa glandular trichomes do not contain volatile repellant compounds, but do contain nonvolatile, fatty acid derivatives that deter potato leafhopper settling. Because these compounds are nonvolatile, the trichome resistance mechanism probably requires direct contact with the trichome exudate.
Species 1: Homoptera Cicadellidae Empoasca fabae (potato leafhopper)
Species 2: Medicago sativa (alfalfa)
Keywords: trichome chemistry, host preference
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