DIFFERENTIAL ACCUMULATION OF PHYTOALEXINS IN LEAVES OF SUSCEPTIBLE AND RESISTANT GENOTYPES OF GROUNDNUT (ARACHIS-HYPOGAEA L) INOCULATED WITH PUCCINIA-ARACHIDIS SPEG

The accumulation of phytoalexins in a rust-susceptible (KH 241-D) and a rust-resistant (PI 259747) genotype of groundnut 2, 6, 15 and 25 day s after inoculation with the rust pathogen, Puccinia arachidis Speg. w as investigated. Within 2 days of inoculation of the resistant genotyp e, total phytoalexins reached 35 mu g/g fresh weight, remained close t o this level for a further 13 days but reached 104 mu g/g at 25 days a fter inoculation. In contrast, the susceptible genotype accumulated on ly 4 mu g/g fresh weight 2 days after inoculation and this increased g radually to 13, 23 and 52 mu g/g fresh weight at 6, 15 and 25 days, re spectively. Extracts from uninoculated leaves of both genotypes showed the presence of an antifungal compound which appeared as a diffuse sp ot in the Thin Layer Chromatography (TLC) bioassay, but this was essen tially absent in inoculated leaves and did not correspond to any peak detected on High Performance Liquid Chromatography (HPLC). Compounds w ith UV spectra corresponding to isoflavanones were the major component of the phytoalexin response whereas other phytoalexins, formononetin, daidzein, and medicarpin comprised a minor component. A further compo und with a retention time of 23.0 min on HPLC and a lambda max of 280 nm only accumulated in inoculated leaves of the resistant genotype.