Phytoalexin accumulation in the roots of chickpea (Cicer arietinum L) seedlings associated with resistance to fusarium wilt (Fusarium oxysporum f.sp. ciceri)

Four cultivars of chickpea which differed in their reaction to Fusarium oxysporum f.sp. ciceri, the pathogen causing wilt, were grown in wilt infested soil. The root xylem of plants showing wilt symptoms was heavily occluded by hyphae. Hyphal occlusion of stem xylem was also recorded up to the fifth internode. The roots of resistant cultivars were penetrated but hyphal growth was very slow. No localized cell death (hypersensitivity) or gross structural changes (lignification) were observed in the vicinity of invading hyphae in resistant cultivars, suggesting that the resistance was dependent upon chemical rather than physical mechanisms. The concentration of the pterocarpans medicarpin and maackiain in the roots increased in the presence of two races of the pathogen. Both pre- and post-induction concentrations of pterocarpans were significantly greater in wilt-resistant cultivars indicating an association between phytoalexin induction and resistance. Medicarpin and maackiain showed antifungal activity to F. oxysporum f.sp. ciceri at similar concentrations to those recorded in wilt-resistant chickpea roots. No significant difference in the sensitivity of races 1 and 2 to the antifungal activity of the pterocarpans was detected, bur. the accumulation of phytoalexins in response to the more virulent pathotype race 2 was lower in ail cultivars than those produced in response to race 1. We conclude that they are fundamental components of the resistance mechanism of chickpeas to wilt. (C) 1997 Academic Press Limited.