Philip. C. STEVENSON.
Royal Botanie Gardens, Kew, Surrey, TW9 3AB, UK and Natural Resources
International, Chatham Maritime Kent, ME4 4TB UK.
The chickpea, Cicer arietinum L., is a major source of human and domestic animal food panieularly in South Asia where its production is concentrated (Jodha and Subba Rao, 1987) The major fungal pathogen constraints of chickpeas in South Asia are Fusarium wilt (Fusarium oxysporum f sp. ciceri) and Botrytis Grey Mould (Botrytis cinerea) Both pathogens are diffcult to control with cultural methods or chemical applications. Natural resistance, however, provides an environmentally and eeonomieally appropriate alternative for small scale farmers in thc countries of this resources poor region Phytoehemieal studies of wild species of Cicer have shown that both roots and foliage express a phytoalexin response dominated by the pterocarpan isoflavonoid maackiain. Medicarpin is also produced in low concentrations in some species. Maackiain and medicarpin were shown to exhibit potent anti-fungal activity towards Fusarium spores at natural concentrations by inhibiting their germination and the hyphai growth of those spores which did germinate. In addition high constitutive levels of maackiain in the roots and increased production in the presence of the pathogen were both strongly associated with resistance Furthermore, maaekiain oeeurs at very high concentrations as the glucoside and malonylglucoside in the roots of some wild species of Cicer (Stevenson and Veitch, 1996) but these substituted derivatives are not antifungal. Fungal invasion of the roots of cultivated species (Cicer arietinim L.) elicits the production of maackiain (Stevenson et al., 1996). This is a three step reduction process from the isoflavonoid formononetin (Herbert 1989). Formononetin, the indirect source of maackiain in cultivated chickpeas, occurs at high concentrations as the aglycone, glucoside and malonyl glueoside. In wild species, however, the source of maackiah appears to be its glucoside and malonylglucoside. These derivatives are more readily available to the plant through a single glycosylation and the storage of maaekiain glycosides may be a valuable character in the development of resistant varieties.