On the relation between arbuscular mycorrhizal colonization and plant 'benefit'

A simple model is proposed which describes the relation between the extent of mycorrhizal colonization of a plant and the 'benefit' (positive or negative) derived by that plant. 'Benefit' is defined as the percent change in a plant performance parameter of a mycorrhizal individual relative to the mean of a number of mycorrhizal plants, grown in identical conditions. The model predicts a general curvilinear relation between colonization density and benefit, where benefit is maximized at some value of colonization. It is based on the fact that the relation of plant P uptake to mycorrhizal colonization is often non-linear. Four examples of empirical data which provide a good fit to the model, with third-order polynomial regression are given. It is suggested that if the curvilinear relation is of general occurrence then it can provide an explanation for many of the apparently anomalous results seen in the mycorrhizal literature.