EXTERNAL HYPHAE OF VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI ASSOCIATED WITH TRIFOLIUM-SUBTERRANEUM L .1. SPREAD OF HYPHAE AND PHOSPHORUS INFLOW INTO ROOTS

Variation among VA mycorrhizal fungi in their ability to enhance phosp horus uptake and plant growth could be due to differences in the lengt h, distribution and phosphorus uptake of external hyphae. Trifolium su bterraneum L. was grown in a sandy soil in association with the VA myc orrhizal fungi Acaulospora laevis Gerdemann & Trappe, Glomus sp., Scut ellospora calospora (Nicol. & Gerd.) Walker & Sanders or left uninocul ated. When mycorrhizas had become well established, plants were transf erred to a two-compartment system, where root growth into a hyphal com partment was restricted by a fine nylon mesh. Spread of hyphae into th e hyphal compartment was monitored by sequential sampling of soil core s at different distances from the root compartment. Plants were harves ted at three times. The spread of hyphae differed between fungi. The l ength density of hyphae of S. calospora declined approximately exponen tially with increasing distance from the roots, whereas A. laevis main tained 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 a ddition to its more extensive hyphal spread, A. laevis produced the la rgest increases in phosphorus uptake and plant growth. All mycorrhizal plants had a larger phosphorus inflow than the non-mycorrhizal contro ls, but the phosphorus inflow per unit mycorrhizal root length was 2.5 -3 times as high with A. laevis as with the two other fungi. These dif ferences between the fungi became even more pronounced when the phosph orus inflow was expressed on the basis of hyphal length. This work ind icates that the efficiency of phosphorus uptake by a VA mycorrhizal fu ngus 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.