THE PHYTOALEXIN RESPONSE OF GROUNDNUT AND ITS ROLE IN DISEASE RESISTANCE

Phytoalexins are low molecular weight antimicrobial compounds that are synthesized by many plants in response to challenge by microorganisms . They are thought to play an important role in resistance and in some instances have been shown to do so. So far 19 compounds have been rep orted from groundnuts which have been classified as phytoalexins. In g eneral, cotyledons produce stilbenes while leaves produce isoflavonoid s and some miscellaneous long chain compounds. The speed at which phyt oalexins accumulate after challenge and the concentrations reached are influenced by genotype of the plant and the challenge organism as wel l as environmental conditions. Thus the susceptible cultivar TMV2 accu mulated fewer and lower concentrations of stilbene phytoalexins than t he more resistant cultivar J11 in response to inoculation with Aspergi llus flavus. Moreover, drought stress, which predisposes groundnuts to invasion by A. flavus and consequent contamination by aflatoxins, imp aired the ability of cotyledons to accumulate phytoalexins. In leaves infected by rust, Puccinia arachidis, and Cercospora arachidicola, the cause of early leaf spot, the compound medicarpin was a prominent com ponent of the phytoalexin response. However. its demethylated product which is less inhibitory in fungal bioassays was also present in C. ar achidicola infected leaves. Since the fungus demethylates the compound in vitro it is probable that it also does so in vivo as a mechanism f or overcoming the plant's resistance.