The processes of infection of roots of Trifolium subterr aneum by vesicular–arbuscular mycorrhizas are analysed using a modification of a model presented previously. The modified model uses as dependent variables the fraction of the root length infected and the linear density of infection units. It allows the evaluation of the rate of initiation of new infection units and of the rate of growth of hyphae within the root as functions of time.
The model is applied to data for T. subterraneum grown in a 20-foid range of densities of randomly-mixed propagules in soil; we show the rate of initiation of new infection units in both main and lateral roots was proportional to the propagule density over short periods (5 to 11 day). However, the rate of initiation of infection units in main roots during this period was approximately half that in lateral roots. The rate of growth of hyphae in the root did not depend on the propagule density.
As the main roots ceased to grow, both rates of infection and of mycelial growth fell: these rates did not fall for lateral roots during the 26 days of the experiment. The results raise the question of the dependence of mycelial growth-rate on the age of the infection unit, which is discussed.
The model predicts that the fraction of the root length infected approaches asymptotically a value less than one, as has been repeatedly observed. Other models require arbitrary modification to accommodate such findings.