Density-dependent chemical interference--an extension of the biological response model.

The response of plants to many phytochemicals changes from stimulatory to inhibitory as the concentration of the phytochemical increases. In this paper, a previous biological response model is extended to yield estimates of plant responses to changes in phytochemical concentrations in the case of density-dependent phytotoxicity. This requires a knowledge of plant densities, phytochemical concentrations in soil, and the relationship between the two. According to this model extension, inhibition is a probable outcome in density-dependent chemical interference, but phytotoxic effects may become stimulatory as plant density increases. In addition, low phytochemical concentrations in soil may cause an increase in the slope of the biomass-density relationship compared to the slope of control treatments. Experimental data from the literature support this model extension, and in several cases density-dependent chemical interference can be estimated mathematically.