APATITE AS A P SOURCE IN MYCORRHIZAL AND NONMYCORRHIZAL PINUS-SYLVESTRIS SEEDLINGS

The objectives of the study are firstly to test the ability of ectomyc orrhizal pine seedlings to use apatite as a P source in comparison wit h non-mycorrhizal pine seedlings and secondly, to determine if there i s a relation between exudation of organic acids and the ability to use apatite as a P source. Non-mycorrhizal Pinus sylvestris (L.) seedling s and seedlings ectomycorrhizal with 4 different isolates of ectomycor rhizal fungi were grown for 220 days in sand/peat filled pots with apa tite (Ca-5(F,OH)(PO4)(3)) as the sole P source. In an additional exper iment, non-mycorrhizal Pinus sylvestris (L.) seedlings and seedlings e ctomycorrhizal with 2 different isolates of ectomycorrhizal fungi were grown without any P source for 250 days. All other nutrients were sup plied in a balanced nutrient solution. Ectomycorrhizal seedlings grew less than non-mycorrhizal seedlings but ectomycorrhizal seedlings prod uced a large external mycelium not included in the biomass estimates. All seedlings in the present study had low shoot:root ratios compared to seedlings growing under optimal conditions. All seedlings grown wit h apatite as P source had higher foliar P concentrations (0.71-2.11 mg /g) than seedlings growing without any P source (0.57-0.75 mg/g) indic ating a significant ability to use apatite as a P source. Seedlings co lonized by Suillus variegatus and Paxillus involutus had higher concen trations and total contents of P in shoots compared with non-mycorrhiz al seedlings, indicating significant improvement of P uptake by these fungi in comparison with non-mycorrhizal seedlings or seedlings coloni zed Piloderma croceum. No clear relationship between exudation of orga nic acids and uptake of P was found. Seedlings colonized by S. variega tus reduced the pH of the soil more than seedlings colonized by P. inv olutus or non-mycorrhizal seedlings. It is suggested that S. variegatu s colonization improves the P uptake by reducing the pH of the soil wh ile I? involutus improves P uptake by having a greater ability to abso rb dissolved phosphate than non-mycorrhizal roots or roots colonized b y the other fungi used in the study.