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

3. Hyphal transport of 32P and 15N

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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.

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