PHOSPHORUS AMENDMENT INHIBITS HYPHAL BRANCHING OF THE VAM FUNGUS GIGASPORA-MARGARITA DIRECTLY AND INDIRECTLY THROUGH ITS EFFECT ON ROOT EXUDATION

The effect of solution phosphorus (P) concentration upon growth of pre germinated spores of the vesicular-arbuscular mycorrhizal fungus Gigas pora margarita was examined in vitro. P at 1 mM significantly inhibite d branching of the primary germ tube. The number of branches and the t otal hyphal length were both significantly inhibited at 10 mM P. In ad dition, germinated spores exposed to exudates produced by Ri T-DNA-tra nsformed roots of Daucus carota L. grown in the presence of P showed s ignificantly less hyphal branching than those exposed to exudates prod uced by P-stressed roots. These phenomena could contribute to the obse rved inhibition of mycorrhiza formation by high P.