Distribution of anacardic acids associated with small pest resistance among cultivars of Pelargonium xhortorum

Composition of anacardic acids (phenolic acids known to be associated with small pest resistance in Pelargonium xhortorum) was examined in 13 diploid and 25 tetraploid cultivars by high-performance liquid chromatography (HPLC). The presence of an unusual desaturation (omega (omega)-5) in the alkyl tail of anacardic acids present only in glandular trichome exudate of pest-resistant diploid inbred lines had previously been associated with a sticky-trap pest-resistance phenomenon. In this study, we examine Pelargonium cultivars for variability in anacardic acid composition to assess the distribution of omega 5 desaturation among commercial cultivars, to determine possible interactions between omega 5 desaturation and other plant desaturation mechanisms, and to examine the possible impact of ploidy on omega 5 desaturation. An unsaturation index (UI) is derived to compare exudates differing widely in composition yet which may provide a similarly effective sticky-trap pest-resistance mechanism based on exudate viscosity. omega-5 Anacardic acids were observed in the glandular trichome exudate of all 38 commercial cultivars examined. No diploid cultivar produced omega 5- and omega 9-anacardic acids, although the simultaneous production of omega 5 and omega 9-anacardic acids was observed in three tetraploid cultivars. Total omega 5-anacardic acids comprised from 42.4% (tetraploid cultivar Perlenkette-syn. Snowhite, Weiss) to 86.8% (tetraploid cultivar Amanda). Commercial P. xdomesticum cultivars had no omega 5 anacardic acids. UIs ranged from 60.9 (tetraploid cultivar Dixieland) to 103.4 (diploid cultivar Pinto White). In contrast, anacardic acids collected from a pest-susceptible inbred line contained no omega 5-anacardic acids end had a UI of 38.7. No significant differences among ploidy levels were observed for UIs or for most specific anacardic acid components, with the exception of 24:1 omega 5-anacardic acid, in which the mean diploid value (32.1%) was significantly higher than that of the mean tetraploid value (27.6%). We conclude that omega 5-anacardic acid production occurs in all Pelargonium cultivars observed and that these cultivars are predicted to exhibit resistance to small arthropod pests. Significant genetic variability in specific anacardic acid composition appears to exist among Pelargonium cultivars, suggesting that breeding for pest resistance can be readily monitored by HPLC of anacardic acids.