THE DEVELOPMENT OF ENDOMYCORRHIZAL ROOT SYSTEMS .6. THE RELATIONSHIP BETWEEN DEVELOPMENT OF INFECTION, AND INTENSITY OF INFECTION IN YOUNG LEEK ROOTS

Leek plants (Allium porrum L.) were grown on gamma-irradiated soil in tall pots with two layers of inoculum of the vesicular-arbuscular mycorrhizal fungus Glomus mosseae (Nicolson and Gerdemann) Gerdemann and Trappe, at depths of 6 and 9 cm, respectively. The main axes of primary and adventitious roots extended linearly with time. However, the total root length and infected root length both increased exponentially, as did total root numbers. The fractional infection had a lag phase, a phase of rapid increase, and then a steady value. The density of internal hyphae rapidly increased in the main axis of infected primary roots while the infected segments extended at a constant rate, which sharply demarcated fronts each progressing at approx. 0.2 cm d-1. Where internal hyphae from the two infected segments met there was no resulting increase in hyphal density. This coalescence formed a single infection segment of uniform hyphal density. On these main axes, the total number of entry points increased exponentially with time after initial infection, but after 20 d the rate of increase declined sharply. At sites of initial infection there were high concentrations of unusually small entry points, but away from these the numbers per unit length of root diminished. At a higher concentration of soil phosphorus, results were similar for hyphal density and numbers of entry points, but fractional infection was less, because of increased root growth. We conclude that the length of infected root closely reflects hyphal biomass at moderate levels of soil phosphorus.