Cadmium alters the growth of the ectomycorrhizal fungus Paxillus involutes: a new growth model accounts for changes in branching

Cadmium reduced the growth rate of Paxillus involutus in pure cultures either on agar or liquid medium. On Cd-containing agar most of the mycelium grew submerged rather than on the surface as occurs on Cd-free agar. Cadmium increased hyphal density by both increasing the number of laterals at a branch point and decreasing the distance between branch points. These variables were included in a new model to determine the specific growth rate on the basis of mycelial length for the fungus growing on agar. The degree to which Cd reduced the specific growth rate was the same whether based on mycelial length from agar plates according to the new model or on mycelial mass from a liquid medium. The new model for specific growth rate (length) derived from agar cultures is particularly suited to those situations where a growth-modifying agent alters the branching frequency and the distance between branch points.