Membrane-Mediated Decrease in Root Exudation Responsible for Phorphorus Inhibition of Vesicular-Arbuscular Mycorrhiza Formation

Abstract

The mechanism responsible for P inhibition of vesicular-arbuscular mycorrhiza formation in sudangrass (Sorthum vulgare Pers.) was investigated in a P-deficient sandy soil (0.5 .mu.g P/gram soil) amended with increasing levels of P as superophosphate (0, 28, 56, 228 .mu.g/g soil). The root P content of 4-wk-old plants was correlated with the amount of P added to the soil. Root exudation of amino acids and reducing sugars was greater for plants grown in P-deficient soil than for those grown in the P-treated soils. The increase in exudation corresponded with changes in membrane permeability of P-deficient roots, as measured by K+ (86Rb) efflux, rather than with changes in root content of reducing sugars and amino acids. The roots of P-deficient plants inoculated at 4 wk with Glomus fasciculatus were 88% infected after 9 wk as compared to less than 25% infection in P-sufficient roots; these differences were correlated with root exudation at the time of inoculation. For plants grown in P-deficient soil, infection by vesicular-arbuscular mycorrhizae increased root P which resulted in a decrease in root membrane permeability and exudation compared to nonmycorrhizal plants. Evidently, under low P nutrition, increased root membrane permeability leads to net loss of metabolites at sufficient levels to sustain the germination and growth of the mycorrhizal fungus during pre- and postinfection. Subsequently, mycorrhizal infection leads to improvement of root P nutrition and a reduction in membrane-mediated loss of root metabolites.