Utilization of AlPO₄ as a phosphorus source by ectomycorrhizal Pinus rigida Mill. seedlings

SUMMARY Mycorrhizal infection of roots of Pinus rigida Mill, seedlings by Pisolithus tinctorius Coker & Couch maintained normal foliar P relations when seedlings were exposed to Al in solution. To investigate how this symbiosis aids in P acquisition in the presence of Al, seedlings were grown in sand culture with solutions containing between 1 and 50 μM NaH 2 PO 4 (P i ) with or without insoluble AlPO 4 in the substrate. Mycorrhizal seedlings had a higher affinity for Pi than non‐mycorrhizal seedlings as judged by superior growth and foliar P contents at all P i levels tested. In addition, mycorrhizal seedlings, but not nonmycorrhizal seedlings, effectively extracted P from AlPO 4 through the dissolution of this insoluble salt. Measurement of matrix solution pH indicated that increased proton production, in association with high proliferation of mycorrhizal roots under P‐limiting conditions, led to increased AlPO 4 solubilization.