NOVEL PHYTOALEXINS INCLUDING ELEMENTAL SULFUR IN THE RESISTANCE OF COCOA (THEOBROMA-CACAO L) TO VERTICILLIUM WILT (VERTICILLIUM-DAHLIAE KLEB)

Four phytoalexins were identified from inoculated stems of cocoa genot ypes resistant to Verticillium dahliae. Following purification by repe ated flash chromatography and TLC, they were identified by NMR and GC- MS. The most abundant and polar compound was the triterpenoid arjunoli c acid; two related phenolics were 3,4 dihydroxyacetophenone and 4-hyd roxyacetophenone. The least polar was unambiguously identified as the most stable form of elemental sulphur, cyclo-octasulphur S-8, by GC-MS and X-ray crystallography, Respective toxicities to V. dahliae conidi a (ED(50) germination in mu g ml(-1)) were 12.8, 92.5, 7.2, 3.6. Sulph ur and arjunolic acid first appeared after 10 and 3 days respectively, were present in the wood of stems at levels well above (greater than or equal to x 13) those required for toxicity and they persisted for l ong (greater than or equal to 50 days) periods; they were found only a fter infection whereas the phenolics were detected in control stems an d were enhanced to similar extents by infection or wounding. Sulphur a ccumulation was localized to xylem cells. In contrast, condensed tanni ns, although increasing approx. two-fold after infection, were preform ed, of low toxicity (ED(50) greater than or equal to 383 mu g ml(-1)) and concentrations in two resistant and one susceptible genotypes were not significantly different. This is the first report of phytoalexins in T. cacao and of sulphur accumulation in plants linked with active defense. (C) 1996 Academic Press Limited