Effect of temperature on glandular trichome exudate composition and pest resistance in geraniums

One inbred geranium line (71-18-6) which exhibits host plant resistance to the twospotted spider mite and foxglove aphid, becomes susceptible to these pests under some temperature conditions, while another resistant line (71-17-7) maintains its resistance. Resistance to these pests is conferred by the presence of glandular trichomes and the exudate they produce. The tall glandular trichome exudate of resistant geraniums has been identified as chiefly C₂₂ and C₂₄ unsaturated anacardic acids. Plants of a resistant line, a susceptible line and the temperature sensitive resistant line were exposed to three different temperature regimes and were analyzed chemically and for resistance using a foxglove aphid bioassay. Results indicate that the loss of resistance in the temperature sensitie line is not due to reduced production of the anacardic acids which comprise the exudate. Similarly, analysis of the exudate present on the leaf exterior indicate the loss of resistance in line 18-6 with increasing temperature is not the result of reduced translocation of the exudate to the trichome tip. Evidence is presented to support the hypothesis that loss of resistance in line 18–6 with increasing temperature is the result of an increased proportion of the shorter chain (C₂₂) anacardic acid in the trichome exudate, which reduces the viscosity of the material and causes it to flow from the trichome tips as the temperature increases. Presentation of the material as an exudate droplet at the trichome tip is apparently required for resistance.