External hyphae of vesicular‐arbuscular mycorrhizal fungi associated with Trifolium subterraneum L.

summary Variation among VA mycorrhizal fungi in their ability to enhance phosphorus uptake and plant growth could be due to differences in the length, distribution and phosphorus uptake of external hyphae. Trifolium subterraneum L. was grown in a sandy soil in association with the VA mycorrhizal fungi Acaulospora laevis Gerdemann & Trappe, Glomus sp., Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders or left uninoculated. When mycorrhizas had become well established, plants were transferred to a two‐compartment system, where root growth into a hyphal compartment was restricted by a fine nylon mesh. Spread of hyphae into the hyphal compartment was monitored by sequential sampling of soil cores at different distances from the root compartment. Plants were harvested at three times. The spread of hyphae differed between fungi. The length density of hyphae of S. calospora declined approximately exponentially with increasing distance from the roots, whereas A. laevis maintained a plateau of constant hyphal density up to 7 and 11 cm from the roots after 28 and 47 d, respectively. Glomus sp. had an intermediate pattern of spread with a plateau closest to the roots followed by an exponential decline. The average rate of hyphal spread (mm d −1 ) was 3.1 for A. laevis but only 0.7–0.8 for Glomus sp. and S. calospora. In addition to its more extensive hyphal spread, A. laevis produced the largest increases in phosphorus uptake and plant growth. All mycorrhizal plants had a larger phosphorus inflow than the non‐mycorrhizal controls, but the phosphorus inflow per unit mycorrhizal root length was 25.3 times as high with A. laevis as with the two other fungi. These differences between the fungi became even more pronounced when the phosphorus inflow was expressed on the basis of hyphal length. This work indicates that the efficiency of phosphorus uptake by a VA mycorrhizal fungus is strongly affected by its spatial distribution of hyphae in the soil, and possibly also by differences in capacity for uptake by unit length of hypha.