Density-Dependent Phytotoxicity: Distinguishing Resource Competition and Allelopathic Interference in Plants

(1) Experimental plant populations were grown at varying densities and varying levels of phytotoxins in the soil to investigate the potential interacting influences of allelopathy and resource competition on plant response and yield-density relationships. (2) Experiments were performed with (a) bahiagrass, Paspalum notatum, grown in soil treated with gallic acid and hydroquinone, putative inhibitors produced by Polygonella myriophylla and (b) tomato, Lycopersicon esculentum, grown in soils from under and around black walnut, Juglans nigra. (3) Phytotoxicity decreased as plant density increased. This was attributed to plant 'dilution' of phytotoxins, i.e. the sharing of the available phytotoxin among many plants at high densities such that each received a small sub-lethal dose. (4) Phytotoxins caused characteristic deviations from expected yield-density relationships. Low to moderate phytotoxin concentrations caused a decrease in the slope of the log yield-log density relationship. Contrary to the expected consequences of increased density and resource competition, the presence of high phytotoxin concentrations may cause a reversal in the slope of predicted log yield-log density relationships at low plant densities, such that maximum individual plant weight occurs at an intermediate density. (5) Demonstration of either decreasing phytotoxicity with increasing plant density or a reversal in slope of the predicted log yield-log density relationship is proposed as an indication of the presence of toxic substances in soil. (6) The ecological implications of these results are discussed.