The development of endomycorrhizal root systems: VII. A detailed study of effects of soil phosphorus on colonization

Leek plants (Allium porrum L.) were grown in a controlled environment on a mixture (2: 1 w/w) γ-irradiated (1.0 Mrad) sandy loam and sand, at six concentrations of bicarbonate-soluble phosphorus (P) ranging from 22 to 344 mg P kg^-1 (soil basis). Inoculum of the vesicular-arbuscular (VA) mycorrhizal fungus Glomus mosseae (Nicolson & Gerdemann) Gerdemann and Trappe was placed (M) or not (NM) in a layer 3 cm below the soil surface. At intervals of 10 d, lengths of main axes of roots and their lateral branches, and of the segments of infection within them, were measured. From these data we calculated the mean (harvest interval method} rates of linear extension of root tips and of infection fronts for each member of root, averaged over the whole root system. The mean delay, d, between a root encountering the layer of inoculum and the subsequent formation of internal infection, was also derived. Addition of P to soil did not affect rates of extension of roots, but increased the initiation of main axes and laterals. Infection segments extended twice as fast in laterals as in main axes. At low rates of addition, P did not affect fungal behaviour but increased the length of root available for colonization. When bicarbonate-soluble P exceeded 140 mg kg^-1 , the rates of extension of infection fronts in both main axes and laterals were approximately halved, and d was considerably increased. The density (the area ratio of fungal to host tissue in a longitudinal squash) of the hyphae and arbuscules respectively, and the number of entry points per unit length of root, were greatly reduced by added P. However, the ratios of numbers of entry points/hyphal density and of arbuscule density/hyphal density were unaltered, and the morphology of the fungus was not noticeably affected. It is probable that formation of entry points was the rate-limiting step for colonization, and that this rate was reduced by added P.