Distinguishing Resource Competition and Chemical Interference: Overcoming the Methodological Impasse

Understanding allelopathy may hold the key to new weed management strategies. However, the difficulty of distinguishing chemical interference from competition has hindered studies of allelopathy in natural and cultivated plant communities. Experimental rigor has increased, but has yet to provide unambiguous proof of allelopathy. The complexities of allelopathic interactions, as illustrated by ongoing investigations in the Florida scrub, make it unlikely that clear examples will be forthcoming. While conclusive proof of chemical interference may not be attainable, the challenge of obtaining strong supportive evidence remains. Progress is needed in bioassay methods that distinguish allelopathy from other interference mechanisms. Phytotoxic effects are density-dependent in a manner inconsistent with resource competition, suggesting that allelopathy can be distinguished by characteristic growth responses across planting densities. In monoculture, greater phytotoxicity at low plant densities causes deviations from expected yield-density relationships. In mixed culture, the targetneighbor method, in which differing densities of a neighbor species are planted around a target plant, has been used to study phytotoxic effects. In the presence of an applied phytotoxin, increased growth of sensitive target plants as the density of insensitive neighbors increases is inconsistent with a hypothesis of resource competition and provides compelling, though not conclusive, evidence for chemical interference. Once evidence of allelopathy is obtained from plant growth studies, supportive analytical data must be obtained from analyses of toxin concentrations and flux rates in the soil and rhizosphere. The use of adsorbent materials originally applied to the analysis of organic pollutants should allow the measurement of allelochemical flux rates in addition to static concentrations. Research support provided by grants from the Coop. State Res. Service, USDA, under agreement No. 88-33520-4077 of the Competitive Res. Grants Program for Forest and Rangeland Renewable Resources, from the Conservation and Research Foundation, and from Research Corporation. Presented at a symposium, Allelopathy in Cropping Systems (jointly sponsored by Div. C-3, C-2, S-6, and S-8), at the ASA-CSSA-SSSA annual meetings in Seattle, WA, 14 Nov. 1994.