The presence of methylated coumestans in annual Medicago species: Response to a fungal pathogen

The methylated coumestans 3'-methoxycoumestrol and 4'-methoxycoumestrol previously found in perennial medics were identified in annual Medicago species. Only 4'-methoxycoumestrol was present in sufficient quantity to affect the oestrogenic activity of the medic pastures. The total coumestan content varied with species, and was higher in plants grown on gravelly soils than on sands and higher in stems than pods, but in each case there was little change in the relative proportions of coumestrol and the methylated coumestans. Infection of leaves of a common burr medic with a leaf rust (Uromyces striatus) caused a sixfold increase (from 15 to 80 p.p.m.) in the total coumestan content. The increase, measured in leaves of equal age, was clearly related to the degree of rust infection.