The development of vesicular-arbuscular mycorrhizal infection in plant root systems

Summary Many crop plants carry VA mycorrhizal infection. The fungi benefit their host principally by increasing the rate of phosphorus uptake from soils low in available phosphorus.Infection of seedling root systems occurs from soil-borne propagules and/or from neighbouring mycorrhizal roots. When, under the right conditions, the propagules germinate, a mycelium is produced which cannot growth extensively until mycorrhizal infection is established.The level of initial (primary) infection is related to propagule density. The relation can be described by a mathematical model which takes account of propagule germination rates, rates of hyphal growth through the soil and rates of root growth.Subsequent spread of secondary infection is by growth, along and between roots, of hyphae originating from established infection units and by the formation of new infection units. This spread can be modelled.If certain simplifying assumptions are made, the effects on the host of developing mycorrhizal infection can also be modelled and satisfactory predictions of infection spread and the mycorrhizal growth response obtained.In conclusion, some implications for agriculture and forestry are briefly discussed.