The Microbial Origin of the Allelopathic Potential of Adenostoma fasciculatum H & A

The release of phytotoxic compounds has been proposed to be involved in the inhibition of seed germination under the canopy of Adenostoma fasciculatum (McPherson and Muller 1969). McPherson et al. (1971) tentatively identified the phytotoxins as phenolic compounds; however, the soil extraction data presented in this study indicate that insufficient quantities of available phenolics are present in the soil to account for the lack of growth observed in the field. Soil from burned and unburned Adenostoma—dominated sites were exchanged and planted with herb seeds. While toxicity developed in soil from the unburned site, the resulting pattern of germination suggested that the toxicity originated from the soil and not from the shrub canopy. Treatments to disrupt the metabolism of soil microbes, and to isolate selectively the microbial fraction from the soil, as well as other experiments, lead to the conclusion that the soil toxicity was the result of phytotoxins of microbial origin. It is suggested that the role of Adenostoma in the suppression of herb growth may lie in its association with soil microbes that produce and release substances capable of inhibiting the germination and growth of plants.