Phosphate-limitation physiology in ectomycorrhizal pitch pine

H2PO4 (Pi) uptake rates, and root surface acid phosphatase (APase) rates were assessed in pitch pine (Pinus rigida Mill.) seedlings inoculated with the ectomycorrhizal fungi Laccaria bicolor (Maire) Pat., Paxillus involutus (Batsch.) Fr., or Pisolithus tinctorius (Pers.) Coker and Couch, and grown at 10 or 100 μM Pi in sand culture. Following a 6-week period of acclimation to the Pi regimes, seedlings grown at 100 μM Pi had greater foliar and root P concentrations than seedlings grown at 10 μM Pi. Mycorrhizal colonization increased the concentration of P in roots and, under Pi-limiting conditions, this retention was at the expense of P translocation to foliage. There were no differences in Pi uptake rates between non-mycorrhizal and mycorrhizal roots grown at 100 μM Pi. However, mycorrhizal colonization enhanced Pi uptake in seedlings grown at 10 μM Pi, with rates 1.3-, 2.6-, and 3.3-fold greater in roots colonized with L. bicolor, P. involutus, and P. tinctorius, respectively, than in nonmycorrhizal roots. Root acid phosphatase (APase) activity was greater in non-mycorrhizal roots than in roots colonized with any of the three mycorrhizal fungi, and increases in activity in response to Pi limitation occurred only in non-mycorrhizal roots. These results highlight the importance of seedling acclimation to prevailing Pi availability and the role of mycorrhizal fungi in altering the allocation of P between roots and shoots. The activities of the APase systems of the mycorrhizal species tested do not support the hypothesis that this enzyme system plays an important role in Pi acquisition under P-limiting conditions.