External hyphae of vesicular—arbuscular mycorrhizal fungi associated with <i>Trifolium subterraneum</i> L.

summary Phosphorus transport by hyphae of the three VA mycorrhizal fungi, Acaulospora laevis Gerdemann &amp; Trappe, Glomus sp. and Scutellospora calospora (Nicol. &amp; Gerd.) Walker &amp; Sanders, associated with Trifolium subterraneum L. was investigated by means of radiotracer techniques. Plants with roots heavily colonized by each mycorrhizal fungus were transplanted to two‐compartment systems, where a hyphal compartment was separated from the main compartment by a fine mesh preventing root penetration. The hyphal compartment contained layers of 32 P‐labelled soil, which were placed at 0, 1, 2.5, 4.5 or 7 cm from the root compartment. A time‐course study over 37 d showed that Glomus sp. transported most 32 P to shoots over soil‐root distances shorter than 1 cm. In contrast, A. laevis transported most 32 P to shoots over soil–root distances longer than 1 cm. This ability of A. laevis to transport phosphorus over longer distances than Glomus sp. parallels previous observations that hyphae of A. laevis spread faster and further in soil than hyphae of the Glomus sp. Scutellospora calospora transported much less 32 P to plants, but accumulated more 32 P in its hyphae, than the two other fungi. The higher specific radioactivity in the hyphae of S. calospora than of A. laevis and Glomus sp. indicated a retarded translocation of 32 P in its hyphae or retarded transfer of 32 P across its interface with the host. However, the poor phosphorus transport by S. calospora might also have resulted from its reaction to root trimming at transplanting; percentage root colonization by S. calospora decreased markedly after transplanting to the labelling system.