Organic acids produced by mycorrhizal Pinus sylvestris exposed to elevatedaluminium and heavy metal concentrations

A cultivation method was developed to enable exposure of ectomycorrhizal pl ants with intact extramatrical mycelium to solutions containing different c oncentrations of aluminium or heavy metals. Pinus sylvestris seedlings colo nized by Suillus variegatus (two isolates), Rhizopogon roseolus or Paxillus involutus (two isolates) were used. Seedlings were transferred to Petri di shes containing glass beads and exposed to elevated concentrations of Al, C d, Cu, or Ni in two ways: immediately following transfer; and after allowin g mycorrhizal seedlings to develop an extraradical mycelium that colonized the interface between the upper surface of the beads and the metal-containi ng solution. Production of organic acids in mycorrhizal and non-mycorrhizal systems was measured by withdrawing samples from the solution and analyzin g by HPLC. In most experiments, levels of oxalic acid were significantly hi gher in mycorrhizal treatments than in non-mycorrhizal controls. The measur ed levels of organic acids were variable, but the results obtained suggest that production of oxalic acid is stimulated by exposure to elevated Al in mycorrhizal seedlings colonized by S. variegatus and R. roseolus. Elevated Al concentrations also increased oxalic acid production by non-mycorrhizal seedlings significantly in two of four Al experiments performed, but the me asured concentrations were significantly lower than in corresponding mycorr hizal treatments in both cases. Malonic acid was found in the culture solut ion of non-mycorrhizal and P. involutus-colonized seedlings, but only trace amounts were found in S. variegatus or R. roseolus-infected seedlings. Cit ric, shikimic, lactic, acetic, propionic, fumaric, formic, iso-butyric and butyric acid were found in variable concentrations. Production of oxalic ac id by seedlings colonized by S. variegatus BL or P. involutus was not stimu lated by exposure to 0.44 mu M Cd or 17 mu M Ni. Exposure to 0.157 mM Cu in two separate experiments using P. involutus 87.017 and two strains of S. v ariegatus (BL and 159) appeared to stimulate production of oxalic acid irre spective of mycorrhizal status or species.