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.