Organic anion exudation by ectomycorrhizal fungi and Pinus sylvestris in response to nutrient deficiencies

Abstract

Summary• Low molecular weight organic anions (LMWOA) can enhance weathering of mineral grains. We tested the hypothesis that ectomycorrhizal (EcM) fungi and tree seedlings increase their exudation of LMWOA when supply of magnesium, potassium and phosphorus is low to enhance the mobilization of Mg, K and P from mineral grains.• Ectomycorrhizal fungi and Pinus sylvestris seedlings were cultured in symbiosis and in isolation on glass beads with nutrient solution or with sand as a rooting medium, with a complete nutrient supply or with Mg, K, P or N in low supply. Concentrations of all dicarboxylic LMWOA in the rooting medium were measured.• Nonmycorrhizal seedlings released predominantly malonate. Colonization with Hebeloma longicaudum decreased the amount of organic anions exuded, whereas Paxillus involutus and Piloderma croceum increased the concentration of oxalate but not the total amount of LMWOA. Phosphorus deficiency increased the concentration of LMWOA by nonmycorrhizal and EcM seedlings. Magnesium deficiency increased the concentration of oxalate by nonmycorrhizal and EcM seedlings, but not the concentration of total LMWOA. Paxillus involutus grown in pure culture responded differently to low nutrient supply compared with symbiotic growth.• Ectomycorrhizal fungi did not increase the total concentration of LMWOA compared with nonmycorrhizal seedlings but, depending on the fungal species, they affected the type of LMWOA found.

Rock-eating mycorrhizas: their role in plant nutrition and biogeochemical cycles

“…The enhanced exudation in this experiment was apparently not an effect of Mg or P deficiency alone, because Mg or P deficiency without Al did not induce the exudation of oxalate. This outcome contrasts with results obtained before (Van Schöll et al 2006a), where increased exudation of oxalate in response to Mg and P deficiency was observed. Possibly, this discrepancy is due to differences in experimental circumstances, which might also explain the substantially lower concentration of LMWOA found here.…”

“…It remains however uncertain if this response is regulated by plant exudation or by fungal exudation when both organisms are grown in symbiosis. The exudation of oxalate and malonate by P. sylvestris seedlings and P. involutus in response to nutrient deficiencies differed strongly when grown either in pure culture or in symbiosis (Van Schöll et al 2006a). Increased weathering was not per se reflected by increased uptake into the shoot, and with some EcM fungi the weathering products may have been retained partly in the mycelium.…”

“…Apart from oxalate, EcM fungi exude other LMWOA (citrate) and protons that also enhance weathering but are not accounted for in the model. Under deficiencies, exudation (Van Schöll et al 2006a;Van Hees et al 2006b) and hence weathering can be up-regulated, further enhancing EcM fungal weathering. Van Hees et al (2006a) concluded that K weathering rates, induced by EcM conifers, were 3-10 times larger than historical weathering rates.…”

“…The weathering potential of oxalate via complexation with Al, as indicated by the stability constant (log K Al =6.5), is higher than that of malonate (log K Al =5.7; Fox et al 1990). In culture solution, Mg deficiency increased exudation of oxalate, but there were no significant differences between non-mycorrhizal or EcM seedlings (Van Schöll et al 2006a). Only seedlings colonised by P. involutus significantly increased oxalate exudation under K deficiency.…”

“…The weathering of the K and Mg containing minerals by non-mycorrhizal and EcM tree seedlings corresponded well to their exudation of oxalate in response to omission of K and Mg. Non-mycorrhizal pine seedlings exuded mainly malonate (50-80% of all LMWOA; Van Schöll et al 2006a). There were strong effects of EcM fungal colonisation on exudation of LMWOA, especially of oxalate (Table 1): Seedlings colonised by P. involutus significantly increased the exudation of oxalate, whereas colonisation by H. longicaudum decreased the exudation of total LMWOA and malonate compared to the nonmycorrhizal seedlings.…”