DISTINGUISHING RESOURCE COMPETITION AND CHEMICAL INTERFERENCE - OVERCOMING THE METHODOLOGICAL IMPASSE

Understanding allelopathy may hold the key to new weed management stra tegies. However, the difficulty of distinguishing chemical interferenc e from competition has hindered studies of allelopathy in natural and cultivated plant communities. Experimental rigor has increased, but ha s yet to provide unambiguous proof of allelopathy. The complexities of allelopathic interactions, as illustrated by ongoing investigations i n the Florida scrub, make it unlikely that clear examples will be fort hcoming. While conclusive proof of chemical interference may not be at tainable, the challenge of obtaining strong supportive evidence remain s. Progress is needed in bioassay methods that distinguish allelopathy from other interference mechanisms. Phytotoxic effects are density-de pendent in a manner inconsistent with resource competition, suggesting that allelopathy can be distinguished by characteristic growth respon ses across planting densities. In monoculture, greater phytotoxicity a t low plant densities causes deviations from expected yield-density re lationships. In mixed culture, the target-neighbor method, in which di ffering densities of a neighbor species are planted around a target pl ant, 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 hyp othesis of resource competition and provides compelling, though not co nclusive, evidence for chemical interference. Once evidence of allelop athy 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 originall y applied to the analysis of organic pollutants should allow the measu rement of allelochemical flux rates in addition to static concentratio ns.