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 days after inoculation with the rust pathogen, Puccinia arachidis Speg. was investigated. Within 2 days of inoculation of the resistant genotype, total phytoalexins reached 35 mu g/g fresh weight, remained close to this level for a further 13 days but reached 104 mu g/g at 25 days after inoculation. In contrast, the susceptible genotype accumulated only 4 mu g/g fresh weight 2 days after inoculation and this increased gradually to 13, 23 and 52 mu g/g fresh weight at 6, 15 and 25 days, respectively. Extracts from uninoculated leaves of both genotypes showed the presence of an antifungal compound which appeared as a diffuse spot in the Thin Layer Chromatography (TLC) bioassay, but this was essentially absent in inoculated leaves and did not correspond to any peak detected on High Performance Liquid Chromatography (HPLC). Compounds with 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 compound 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.