PRECAUTIONS IN THE USE OF YEAST EXTRACT MANNITOL MEDIUM FOR EVALUATION OF LEGUME SEED TOXICITY TO RHIZOBIUM

Toxicity of clover seeds to rhizobia has been reported. Yeast extract medium and a defined medium were used to test whether the the growth medium was involved in the toxic response. Complete inhibition of four Rhizobium trifolii strains by subterranean clover (Trifolium subterraneum L.), arrowleaf clover (T. visiculosum Savi.), and red clover (T. pratense L.) seeds consistently occurred on yeast extract amended agar but not on a defined medium. The toxic response probably involved a reaction between tannic acid from seeds and iron in the medium. Addition of 200 .mu.M FeEDTA to yeast extract medium alleviated the growth inhibition response of rhizobia to all seeds tested.