Beyond the rhizosphere: growth and function of arbuscular mycorrhizal external hyphae in sands of varying pore sizes

Research on nutrient acquisition by symbiotic arbuscular mycorrhizal (AM) fungi has mainly focused on the root fungus interface and less attention has been given to the growth and functioning of external hyphae in the bulk soil. The growth and function of external hyphae may be affected by unfavourable soil environments, such as compacted soils in which pores may be narrow. The effects of pore size on the growth of two AM fungi (Glomus intraradices and G. mosseae) and their ability to transport P-33 from the bulk soil to the host were investigated. Trifolium subterraneum L. plants were grown individually in 'single arm cross-pots' with and without AM fungi. The side arm was separated from the main compartment by nylon mesh to prevent root penetration. It contained three zones: 5 mm of soil: sand mix (HC1); 25 mm of media treatment (HC2); and 20 mm of P-33-labelled soil (HC3). There were four media treatments; soil and three types of quartz sand with most common continuous pore diameters of 100, 38 and 26 mum. AM plants had similar growth and total P uptake in all treatments. However, plants grown with G. intraradices contained almost three times more P-33 than those grown with G. mosseae, indicating G. intraradices obtained a greater proportion of P at a distance from the host roots. Differences in P acquisition were not correlated with production of external hyphae in the four media zones and changes in sand pore size did not affect the ability of the fungi studied to acquire P at a distance from the host roots. Production of external hyphae in HC2 was influenced by fungal species and media treatment. Both fungi produced maximum amounts of external hyphae in the soil medium. Sand pore size affected growth of G. intraradices (but not G. mosseae) and hyphal diameter distributions of both fungi. The results suggest that not only are G. mosseae and G. intraradices functionally complementary in terms of spatial phosphorus acquisition, they are also capable of altering their morphology in response to the soil environment.