THE INFLUENCE OF MONOVALENT CATIONS ON EFFLUX OF PHOSPHATE FROM THE ECTOMYCORRHIZAL BASIDIOMYCETE PISOLITHUS-TINCTORIUS

The influence of extracellular cation concentration on efflux of phosphate from mycelium of the ectomycorrhizal basidiomycete Pisolithus tinctorus has been studied in vitro. Inclusion of K+ (or to a lesser extent Na+) in the bathing solution at concentrations in excess of 10 mM resulted in a stimulation of phosphate efflux. A similar effect was observed in two other ectomycorrhizal basidiomycetes, but in only one of three ericoid ascomycetes. The presence of Ca2+ or Ba2+ (but not Mg2+) in the bathing solution prevented the monovalent cation-stimulated efflux. Pretreatment of P. tinctorius mycelium for 8 h with up to 50 mM K+ in the absence of Ca2+ had no effect on the subsequent rate of phosphate absorption. We discuss the mechanism involved in the efflux and consider the significance of the data in relation to nutrient exchange at the fungus-root interface.