Effects of pH on mycorrhizal colonisation and nutrient uptake in cowpea under conditions that minimise confounding effects of elevated available aluminium

Effects of the arbuscular mycorrhizal (AM) fungi Gigaspora margarita and Glomus etunicatum on the growth of cowpea (Vigna unguiculata L. Walp.) were assessed at low pH by growing plants, with and without AM inoculation, individually in pots containing a mixture of sand and soil adjusted to pH 4.7, 4.9 or 5.2 at the start of the experiment, and with soluble aluminium (Al) concentrations at a sub-toxic level for the plant. Cowpea grew poorly in the absence of AM colonisation, particularly at pH 4.7. Growth was enhanced both by increasing the pH and by inoculating with the AM fungi, with plant responses greater with inoculation. The relative growth improvement by mycorrhizas (mycorrhizal growth response) was highest at pH 4.7, and decreased as the pH increased, although effects were not always significant. Gi. margarita was much more effective than G. etunicatum. There were differential effects of the two fungi on uptake of mineral elements. Plants inoculated with Gi. margarita took up a range of elements, including P and Zn as well as Al, to a much greater extent than those inoculated with G. etunicatum, regardless of medium pH. The effectiveness of Gi. margarita in increasing plant growth was closely correlated with colonised root length.