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

SUMMARY Subterranean clover (Trifolium subterraneum L. cv. Nuba) was grown alone or in symbiosis with Glomus intraradices Schenck and Smith in containers divided by a fine nylon mesh into a root compartment (RC) and a root‐free hyphal compartment (HC). Extraradical hyphae spread into the HC where 32P and 15NH4+ were applied at 2 or 5 cm distance from the RC, when the plants were 42 d old. The time‐course of hyphal transport of the tracers was followed by measuring the content of 32P and 18N in leaflets sampled at various times during a 30 d labelling period. Plants colonized by G. intraradices had accumulated more of the applied tracers than the non‐mycorrhizal controls at the end of the experiment and hyphal transport of both P and N could be demonstrated. The levels of 32P in the leaflets of mycorrhizal plants already exceeded those of non‐mycorrhizal plants after 3 and 4‐5 d with the tracers applied at 2 and 5 cm distance from the RC, respectively. Leaflets of non‐mycorrhizal controls contained only traces of 32P, but considerable amounts of 15N, and mycorrhizas increased the concentration of 15N‐labelled N in leaflets only with tracers applied at 5 cm distance from the RC. The total recovery of applied 15N was 70 % higher in mycorrhizal than in non‐mycorrhizal plants when tracers were applied at 2 cm distance from the RC. When the distance from the RC and the tracers was increased to 5 cm, the total recovery of applied loN was 175% higher in mycorrhizal than in non‐mycorrhizal plants. The content of 32P and 15N in the external hyphae at the various distances from the RC confirmed that hyphal transport of P and N was directed towards the host plant.