Circumstantial Evidence for Phytoalexin Involvement in the Resistance of Peanuts to Aspergillus flavus

Three stilbene phytoalexins, elicited by slicing and incubating imbibed peanut kernels under aerobic conditions, inhibited spore germination and hyphal extension of Aspergillusjavus with EDS, values in the range 4-9-124 pg ml-I.Phytoalexin yield was dependent on cultivar, conditions and duration of incubation after slicing, and crop history.The yield of phytoalexin from ten cultivars studied, after slicing and incubating at 25 "C for 24 h, ranged from 28 to 935 pg per g fresh weight and was negatively correlated with dry kernel colonization by A.flauus [r = -0.868when plotted as In (phytoalexin concn) against In (percentage peanut colonization)].When the incubation period was extended to 96 h there was no such correlation.Reduced phytoalexin yields were obtained when sliced kernels of one cultivar studied were incubated in water or at 37"C, and no phytoalexin was obtained when the slices were incubated under nitrogen gas or frozen before aerobic incubation.Drought stress during pod development in four cultivars studied reduced phytoalexin yields of sliced kernels incubated at 25 "C for 24 h by 17-65% compared with non-stressed controls.I N T R O D U C T I O N Strains of Aspergillusjavus Link and the closely related species A. parasiticus Speare produce a group of secondary metabolites termed aflatoxins.Most work has been done with aflatoxin B, , which is the most poisonous (Wogan & Pong, 1970).This compound, in addition to its toxicity to a wide range of animals, is also teratogenic and carcinogenic (Heathcote & Hibbert, 1978b;Wogan, 1966).These studies suggest that aflatoxin contaminated food may be a health hazard to man.Indeed, a positive association between aflatoxin ingestion and liver cancer in man has been found in many population studies in tropical regions of Africa, India, South-East Asia and the Philippines (Heathcote & Hibbert, 1978b).Aflatoxin contamination of food has also been associated with kwashiorkor (Hendrickse et al., 1982).Aflatoxins can occur in many food materials but peanut meal from tropical countries is one of the commodities most frequently contaminated (Hiscocks, 1965).Originally it was thought that invasion of peanuts and aflatoxin prouction occurred mainly in storage, but it is now known that infection may occur before or during harvest (Ashworth & Langley, 1964;Williams & McDonald, 1983). Kernels of different cultivars of peanuts vary widely in their susceptibility to invasion by A.flauus when inoculated with a spore suspension, and this has been correlated with their susceptibility to A.juvus invasion in the field (Zambettakis, 1983).Variation in susceptibility to A. flavus invasion is also influenced by both pre-and post-harvest environmental factors (Mixon, 1981).These include the type of plant residue in the soil (Griffin & Garren, 1976), the Ahhreriution: EDso, effective dose (50%).