Inhibition of hyphal growth of a vesicular-arbuscular mycorrhizal containing sodium chloride of infection from fungus in soil limits the spread 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 15d 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, bur were infected in soil without NaCl added. In soil moistened with 150mM NaCl, roots 1cm 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. (C) 1998 Elsevier Science Ltd. All rights reserved.