The Effect of Phytotoxins on Competitive Outcome in a Model System

Abstract

The difficulty of distinguishing allelopathy from resource competition among plants has hindered investigations of the role of phytotoxic allelochemicals in plant communities. The effects of allelopathic substances on competitive outcome when two species differ in their sensitivity to an inhibitor were modelled by applying atrazine, a commonly used herbicide for broadleaf weeds, to corn—soybean mixtures. A target—neighbor design was used, in which differing densities of a neighbor species are planted around one individual of the target species. This design is particularly appropriate to investigations of allelopathy, due to the density—dependent nature of phytotoxic effects. Neighbor density greatly influenced the response to the toxin. At a corn densities of 0, 3, 6, 9, and 12 plants per pot and atrazine treatment of 3.0 mg/kg, the dry mass of the soybean (target) plant increased from 0.2 g with no neighbors to 0.5 g with 9—12 neighboring corn plants. The increased growth of soybean at higher corn densities is contrary to the predicted effects of resource competition and is due to uptake of atrazine by the corn plants, which decreased the amount available to the soybean target. Detoxification of soil by neighbors may explain in part the conflicting assessments of some putatively allelopathic species, such as black walnut (Juglans nigra).