THE EFFECT OF PHYTOTOXINS ON COMPETITIVE OUTCOME IN A MODEL SYSTEM

The difficulty of distinguishing allelopathy from resource competition among plants has hindered investigations of the role of phytotoxic al lelochemicals in plant communities. The effects of allelopathic substa nces on competitive outcome when two species differ in their sensitivi ty to an inhibitor were modelled by applying atrazine, a commonly used herbicide for broadleaf weeds, to corn-soybean mixtures. A target-nei ghbor design was used, in which differing densities of a neighbor spec ies are planted around one individual of the target species. This desi gn is particularly appropriate to investigations of allelopathy, due t o the density-dependent nature of phytotoxic effects. Neighbor density greatly influenced the response to the toxin. At corn densities of 0, 3, 6, 9, and 12 plants per pot and atrazine treatment of 3.0 mg/kg, t he 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 g rowth of soybean at higher corn densities is contrary to the predicted effects of resource competition and is due to uptake of atrazine by t he corn plants, which decreased the amount available to the soybean ta rget. Detoxification of soil by neighbors may explain in part the conf licting assessments of some putatively allelopathic species, such as b lack walnut (Juglans nigra).