Phytoalexin production by high- and low-coumarin cultivars of Melilotus alba and Melilotus officinalis

The isoflavonoid phytoalexin medicarpin has been found to accumulate in leaflets of high- and low-coumarin cultivars of Melilotus alba and Melilotus officinalis following inoculation with the fungus Helminthosporium carbonum. Little or no medicarpin was isolated from leaflets treated with deionised water. Medicarpin was also produced when leaflets were irradiated with short-wavelength ultraviolet light. Elimination of the high-coumarin character from sweet clover cultivars does not appear to affect phytoalexin production, and in consequence, low-coumarin varieties should not be more susceptible to fungal colonisation than their high-coumarin counterparts.