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

Abstract

Summary 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 (Lir) 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 (Ltr) was linear, with an apparent delay of approx. 5 d before infection could be observed. The rate of increase in Lir was very similar for placed (0.53 cm d1) and dispersed (0.58 cm d1) 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.

Phosphate Treatment Strongly Inhibits New Arbuscule Development But Not the Maintenance of Arbuscule in Mycorrhizal Rice Roots

“…According to this criterion, we investigated the changes in arbuscule life cycles after P treatment concerning the developmental stages of the youngest 10 intracellular colonizations in each infection front. Gradual development of arbuscules can be observed in this region (Buwalda et al, 1984) allowing the precise observation of P inhibition during colonization stages. The numbers of young arbuscules in infection fronts of P-treated roots tended to be decreased compared to those of watertreated roots.…”

“…The term "colonized region" was used in reference to hyphal-colonized root regions and single colonized regions were bound by two infection fronts comprising intercellular hyphae (Supplemental Fig. S1; Buwalda et al, 1984). Numerous infection units can coalesce to form a colonized region, where the border of each infection unit is not identified at least within a few days from the beginning of colonization (Sanders and Sheikh, 1983).…”

The Development of Endomycorrhizal Root Systems Vi. The Relationship Between Development of Infection, and Intensity of Infection in Young Leek Roots

“…(b)] after day 16. Earlier both host and fungus grew at constant but different relative growth rates until fractional infection reached a maximum, when the relative growth rates of both the total and infected root length became approximately equal, as observed by Buwalda et al (1984). Increase in infected length on the primary root and its laterals together was almost linear with time [ Fig.…”

“…2 (a), (b)], but the rate of extension of infection on the main axis itself declined from day 20 onwards. Buwalda et al (1984) have shown that infection by vesicular-arbuscular mycorrhizal fungi normally occupies discrete lengths of root bounded by advancing 'fronts', which they term 'infection segments'. In this experiment, the primary root encountered two layers of inoculum, 3 cm apart, hence there were two infection segments with four fronts formed initially.…”

“…We have therefore investigated in detail how infection develops, the density of internal hyphae and the relationship of these to entry points. An attempt has also been made to test the assumptions made by Buwalda, Stribley & Tinker (1984) for rates of spread of infection on single roots, when infection segments met.…”