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 negat ive) derived by that plant. 'Benefit' is defined as the percent change in a plant performance parameter of a mycorrhizal individual relative to the me an of a number of mycorrhizal plants, grown in identical conditions. The mo del predicts a general curvilinear relation between colonization density an d 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 coloni zation is often non-linear. Four examples of empirical data which provide a good fit to the model, with third-order polynomial regression are given. I t is suggested that if the curvilinear relation is of general occurrence th en it can provide an explanation for many of the apparently anomalous resul ts seen in the mycorrhizal literature.