Inhibition of hyphal growth of a vesicular-arbuscular mycorrhizal fungus in soil containing sodium chloride limits the spread of infection from spores

The hyphal growth of some vesicular-arbuscular (VA) mycorrhizal fungi is inhibited by increasing concentrations of NaCl in soil, and this may influence the formation of mycorrhizas. Our objective was to examine the effect of NaCl on the distance that hyphae of a VA mycorrhizal fungus, Gigaspora decipiens Hall and Abbott can grow through soil and infect roots of a known host plant, Trifolium resupinatum. L. cv. Kyambro. An experimental procedure was developed to enable the effect of NaCl on the pre-symbiotic growth of hyphae of a VA mycorrhizal fungus to be studied separately from other phases of the life-cycle. Roots 1 cm from the spores were infected after 15 d in soils moistened with 0 or 75 mm NaCl, indicating that the initial hyphal growth was not inhibited in soil moistened with 75 mm NaCl. With time, hyphal growth became increasingly inhibited by NaCl, as roots 2.5 cm from the spores were not infected after 26 d in soil moistened with 75 mm NaCl, but were infected in soil without NaCl added. In soil moistened with 150 mm NaCl, roots 1 cm from the spores were infected after 26 d, but roots 2 cm from the spores were not infected after 36 d. Increasing concentrations of NaCl inhibited either the hyphal growth or the infectivity of G. decipiens hyphae after they had grown through soil. The latter may be unlikely, because the infectivity of G. decipiens hyphae placed directly onto the roots was not affected by NaCl. Increasing concentrations of NaCl also inhibited the spread of infection after the initial infection had occurred, and this is probably due to NaCl in the soil inhibiting hyphal growth as well as possibly affecting the supply of carbohydrates from the plant to the fungus.