ZN-65 UPTAKE IN SUBTERRANEAN CLOVER (TRIFOLIUM-SUBTERRANEUM L) BY 3 VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI IN A ROOT-FREE SANDY SOIL

The uptake of Zn by the hyphae of three vesicular-arbuscular (VA) mycorrhizal fungi, Acaulospora laevis Gerdemann & Trappe, Glomus sp. resembling Glomus fasciculatum (Thaxter) Gerdemann & Trappe emend. Walker & Koske and Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders, in association with subterranean clover Trifolium subterraneum L. (cv. Seaton Park) was examined. The plants were grown in PVC tubes for 5 weeks before transfer to an experimental system, where a root compartment was separated from a root-free hyphal compartment by a 37 mum nylon mesh. Zn-65 was placed in the hyphal compartment at distances of 0, 10, 20 or 40 mm from the roots. For the non-mycorrhizal control, Zn-65 was placed at 0 or 10 mm from the roots. At 14 and 28 days after transfer (DAT) the youngest opened leaves (YOL) were harvested, and at 21 and 35 DAT whole plants were harvested. Dry weights, P and Zn concentrations, root length and mycorrhizal colonization, length of external hyphae and specific Zn-65 activity were recorded. At all harvests Zn-65 activity in shoots of plants colonized by A. laevis was greater than for non-mycorrhizal plants and plants colonized by either Glomus sp. or S. calospora. Plants inoculated with A. laevis absorbed Zn-65 placed 40 mm from the roots, whereas no Zn-65 uptake from this distance occurred with other treatments. The ability of plants colonized by A. laevis to access Zn-65 40 mm from the roots was associated with a much greater hyphal length at this distance from the roots than for the two other mycorrhizal fungi. However, immediately adjacent to the roots there was a greater length of external hyphae for Glomus sp. than for A. laevis. Hyphal length was much lower for S. calospora at all distances from the roots than for the other mycorrhizal fungi. The results show great differences in the abilities of the hyphae of VA mycorrhizal fungi to grow in root-free soil and to absorb and transport labelled Zn to the host plants in a soil with a luxury supply of Zn.