The Development of Endomycorrhizal Root Systems V. The Detailed Pattern of Development of Infection and the Control of Infection Level by Host in Young Leek Plants

Leek plants (Allium porrum L.) were grown on partly sterilized soil, in tall pots so that roots grew downwards unimpeded, with inoculum of the vesicular-arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe placed either under the seedling or dispersed uniformly throughout the soil. The age of each individual root, and the distribution of infection in single roots were both determined in each of a series of sequential harvests. The development of the root systems was unaffected by placement of inoculum. Formation of adventitious roots continued up to 42 d, when branching commenced. The rate of initiation of roots was approximately exponential but more accurately fitted a logistic function. The increase with time in total length of root for the root system (Lt) was approximately exponential but the length of the single roots (Ltr) increased linearly. The total length of infection in whole root systems (Li) also increased exponentially with time, but the rate of extension for the sum of the individual lengths of infected root (infection 'segments') in single roots (Lir) was linear, with an apparent delay of approx. 5 d before infection could be observed. The rate of increase in Lirwas very similar for placed (0.53 cm d-1) and dispersed (0.58 cm d-1) inocula, even though the number of infection segments per root differed widely. This suggests that the host controls the rate of growth of the fungus. On this basis, and assuming each root encountered a new propagule every 4 d, it was possible to predict the lengths and numbers of infection segments in single roots. The percentage infection in whole root systems and single roots, plotted against time, showed a delay, then a sharp rise to a final constant value. This pattern of development can be explained for single roots by simple arithmetical rules. Using these simple rules, and assuming that the rate of production of adventitious roots fitted either a logistic or exponential equation, it was possible to model the development of infection up to 42 d for a whole root system either algebraically or by numerical simulation.