Density-dependent phytotoxicity: distinguishing resource competition and allelopathic interference in plants.
Bahiagrass (Paspalum notatum) plants were grown in 5 × 5 cm pots at densities of 2, 4, 8 and 16 plants/pot and treated with 0, 100, 200 and 400 µg gallic acid and hydroquinone per g soil, and tomatoes (Lycopersicon esculentum) were grown in 0.1 m2 × 10 cm trays at 25, 72 or 196 seedlings per tray in soils from <2 m from the base of black walnut (Juglans nigra) trees, or from >10 m from the base. Plant DW was recorded after 5.8 and 11 weeks (Bahiagrass) and after 5 and 10 weeks (tomatoes). Phytotoxicity decreased as plant density increased. Phytotoxins caused characteristic deviations from expected yield-density relationships. Low to moderate phytotoxin concn caused a decrease in the slope of the log yield-log density relationship. 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 the soil. Ecological implications of these findings are discussed.