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Excessive soluble aluminum (Al3+) in acidic soils is toxic to the external hyphae of arbuscular mycorrhizal fungi but it can be alleviated by other soil factors. A glasshouse experiment was conducted to study the effect of increased Al3+ concentration on the growth of the external hyphae of Gigaspora margarita in the presence of other plants near the host plants. The experiment used compartmentalized pots to facilitate the growth of mycorrhizal-inoculated host plants, external hyphae of the fungus and not mycorhizal-inoculated neighboring plants in different compartments; and measuring the effects of Al3+ and of the neighboring plants on the growth of the fungal hyphae independently. Increased concentration of Al3+ in soil affected the growth of external hyphae of G. margarita negatively. However, the hyphal length density of the fungus was much higher in the pots with neighboring plants than that in the other ones, despite the Al toxicity. This indicates that the hyphae could be taken away from the toxic effect of Al3+ by the stimulating growth from roots of the neighboring plants.

The effect of soil phosphorus (P) administered to the host plant on the growth of external hyphae of Gigaspora margarita was investigated in a glasshouse experiment using pots divided into three compartments i.e. one for donor plants (DPC), one for external fungal hyphae (EHC) and one for receiver plants (RPC) respectively. The DPC was filled with a sterilized soil/sand mix previously set up to have 5, 16 or 26 mg Bray-1 P kg-1 soil (P1, P2,or P3; categorized as low, intermediate or high level of P-availability) at pH 5.3 and inoculated with and without the fungal inoculums, while the RPC was filled with P3 without inoculation. Two pre-germinated seeds of cowpea were then grown there for 2 weeks before filling the EHC with the original sterilized soil/sand mix having pH 4.6 and 12 mg Al3+ kg-1 soil. These plants were harvested after further grown for 4-8 weeks. P fertilizer induced different growth conditions of host plants, which could control the production of external hyphae by the fungal partner. In supporting G. margarita to develop an optimum extent of external hyphae in acidic soils with a toxic level of Al3+, cowpea plants required soil P availabilities at about the intermediate level.

The effectiveness of arbuscular mycorrhizal (AM) fungi has often been attributed to growth of their external hyphae, whilst the hyphae themselves may be subjected to the effects of severe soil conditions. The growth of external hyphae of Gigaspora margarita and Glomus etunicatum and their functions in cowpea growth have been studied at low soil pH using a pot system making is possible for the hyphae to grow separately from their host’s roots. Pots had two compartments, one for roots (RC) and one for hyphae (HC). The RC was a cylindrical bag made of 30 ìm nylon mesh that retains the roots but allows the hyphae to pass through, placed centrally and surrounded by the HC. Initially, the RC was filled with 120 g of a soil/sand mixture (pH 5.3), inoculated with G. margarita, G. etunicatum or free fungal inoculants. A pre-germinated cowpea seed was grown in the compartment for two weeks before the HC was filled with 580 g of the mix in which the pH had been adjusted to 4.6, 4.9 or 5.2. Growth of the plants and of the fungal hyphae in the HC was assessed 6 weeks later. The two fungi differed in their responses to soil pH levels in their growth of external hyphae although they colonized plant roots in the same way. At pH 4.6, the hyphae of G. etunicatum grew more weakly than those of G. margarita. Increasing the pH enhanced the growth of G. etunicatum’s hyphae but reduced G. margarita’s. In relation to their external hyphal functions, G. margarita was able to improve its shoot dry weight and P uptake of cowpea plants higher than G. etunicatum. These findings highlight the ability of developing an extensive external hyphal network under adverse conditions of excessive H+ ions as an important characteristic for the effectiveness of AM fungi in acidic soils.