GROWTH OF VESICULAR-ARBUSCULAR MYCORRHIZAL MYCELIUM THROUGH BULK SOIL

Soil mycelia of vesicular-arbuscular mycorrhizal (VAM) fungi not only extend the range of plant roots for nutrient uptake but also may connect roots, allowing the transfer of small amounts of nutrients between plants. A greenhouse experiment was conducted to determine the range and the rate of advance of VAM hyphae in root-free bulk soil. Plants were grown in three-part containers with a VAM donor plant (soybean, Glycine max [L.] Merr.) and an initially non-VAM receiver plant (corn, Zea mays L.), separated by a soil bridge delimited on both sides by screens (44-mu-m openings). The screens permitted passage of the hyphae of the VAM fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe between the donor and the receiver plants, but retained the roots. The length of the soil bridge (3, 6, or 9 cm) and the texture of the soil (1:2, 1:1, or 2:1 soil/sand) were varied. The advance of the hyphal front was estimated by timing colonization in the receiver plants. The rate of advance in the soil-sand mixes was 2.3 cm/wk with soil penetration of at least 90 mm. Spore production was inhibited in the heavier soils, and decreased in the soil of receiver plants with increasing distance between donor and receiver plant. Growth of the receiver plants was increased by early VAM development. Rates of hyphal growth may influence competitive relationships in plant groupings where mycorrhizal inoculum is sparse.