Phosphate-limitation physiology in ectomycorrhizal pitch pine (Pinus rigida) seedlings

Foliar and root P concentrations, net H(2)PO(4) (-) (P(i)) 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 micro M P(i) in sand culture. Following a 6-week period of acclimation to the P(i) regimes, seedlings grown at 100 micro M P(i) had greater foliar and root P concentrations than seedlings grown at 10 micro M P(i). Mycorrhizal colonization increased the concentration of P in roots and, under P(i)-limiting conditions, this retention was at the expense of P translocation to foliage. There were no differences in P(i) uptake rates between non-mycorrhizal and mycorrhizal roots grown at 100 micro M P(i). However, mycorrhizal colonization enhanced P(i) uptake in seedlings grown at 10 micro M P(i), 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 non-mycorrhizal 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 P(i) limitation occurred only in non-mycorrhizal roots. These results highlight the importance of seedling acclimation to prevailing P(i) 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 P(i) acquisition under P-limiting conditions.