Phosphorus Distribution in Red Pine Roots and the Ectomycorrhizal Fungus Hebeloma arenosa

Red pines (Pinus resinosa Ait.) were grown in a pasteurized sandy loam either unamended with phosphate or fertilized with one of two levels of phosphate (34 or 136 mg/kg) as superphosphate, and with and without addition of Hebeloma arenosa inoculum. Shoot and total dry weights of mycorrhizal seedlings grown in soil unamended with P were greater than those for nonmycorrhizal seedlings grown in the same soil, but less than the dry weights of seedlings grown in soil amended with middle to high levels of P. Mycorrhizal infection was inhibited at the highest level of P amendment. (31)P nuclear magnetic resonance spectra of intact mycorrhizal roots showed the presence of two dominant peaks, orthophosphate (Pi) and polyphosphate (polyP). The polyP peak was absent in spectra of nonmycorrhizal roots. The ratio for areas under the two peaks, Pi/polyP, was 1.8 for mycorrhizal roots grown in both unamended soil and soil that had received middle levels of superphosphate. Apparently, the fungus strongly mediates the supply of phosphate to the tree through the production of polyP, even at growth-limiting levels of soil P, and regulates compartmentalization of P in the mycorrhizal roots.