Effects ofpH and aluminum and manganese toxicity on mycorrhizal associations with sorghum and maize

Effects of H-ions (pH) and high Al and Mn on vesicular-arbuscular mycorrhiza (VAM) associations are not completely understood. Effects of pH and high Al and Mn on VAM associations were studied in plants grown in hydroponic sand culture experiments under greenhouse conditions. Initial experiments consisted of adding 2.0 mM 2-(N-morpholino)-ethanesulfonic acid (MES) buffer to stabilize pH at 4.0, 5.0, 6.0 and 7.0 for testing effectiveness of Glomus intraradix colonization in maize (Zea mays L.). The MES buffer had no effect on VAM colonization at pH 4.0, but enhanced colonization at higher pH values. Addition of MES buffer reduced shoot and root dry matter yields, increased Ca, Mg, Mn, and Zn shoot concentrations, but had no effect on shoot P concentrations. Colonization with VAM increased shoot Fe and Cu concentrations, but the VAM effect on P varied with pH. In other experiments, the VAM species Glomus etunicatum, Glomus intraradix, and two VAM isolates (A and B) were evaluated to determine their effectiveness in improving sorghum (Sorghum bicolor (L.) Moench) growth and nutrition at pH 4.0, 5.0, 6.0 and 7.0. The VAM species showed differential responses to pH, but VAM colonization generally increased as pH increased. Glomus etunicatum was the most effective VAM species regardless of pH, but VAM effectiveness was not related to root colonization. Amelioration of Al toxicity effects on sorghum by VAM and effects of Al on VAM colonization were determined in other experiments. Plants were grown with and without G. etunicatum and G. intraradix using nutrient solution at pH 4.8 with 0, 35, 70, and 105 $\mu$M Al. The VAM colonization increased with increasing Al. Glomus etunicatum was more effective in reducing Al toxicity symptoms than G. intraradix. Amelioration of Al toxicity symptoms showed an association with P and K nutrition. The effects of high Mn on VAM colonization and effectiveness on sorghum were determined. Sorghum was grown with and without G. etunicatum and G. intraradix in sand irrigated with nutrient solution at pH 4.8 with 18, 270, 540, and 1080 $\mu$M Mn. Manganese toxicity symptoms were more pronounced in VAM than in non-VAM plants, and toxicity symptoms were more evident in plants colonized with G. intraradix than with G. etunicatum. Increasing amounts of Mn in solution reduced VAM colonization