Ectomycorrhizal fungi and Pinus sylvestris: aluminium toxicity, base cation deficiencies and exudation of organic anions.

Keywords: aluminium (Al), base cations, BC:Alratio, magnesium (Mg), organic anions, oxalate, malonate, ectomycorrhizal fungi, Paxillus involutus, Pinus sylvestris (Scots pine)The finding of microscopic-small tunnels in mineral grains in podzol soils was the incentive for a research programme on rock-eating mycorrhizae. The research described in this thesis was part of that program. The tunnels were thought to be created by ectomycorrhizal (EcM) fungi through exudation of low molecular weight organic anions (LMWOA). LMWOA can detoxify Al by forming stabile non-toxic complexes and can enhance mineral weathering, thereby mobilising base cations for plant and fungus. In this study we demonstrated that a role for EcM-fungi in detoxification of Al and in mobilisation of base cations from mineral grains is likely since both toxic Al levels and low base cation supply induced LMWOA exudation by ectomycorrhizae.The risk for Al toxicity is often assessed by regarding the ratio of base cations (BC) to Al in the soil solution. Adverse effects on tree growth are expected at BC:Al<1. We demonstrated that Al toxicity is not determined by the BC:Alratio. Growth reductions in Pinus sylvestris and Picea abies seedlings were better predicted by the concentration of BC and Al in solution than by the BC:Alratio. At a constant BC:Al ratio of 1, shoot growth decreased with increasing concentrations of Al. Seedlings of P. sylvestris responded to Al by the exudation of LMWOA. Indeed, EcM fungi enhanced this exudation, suggesting a role for EcM-exuded LMWOA in detoxifying Al. The exudation was further enhanced when levels of toxic Al were combined with deficient Mg and P supply. EcM fungi did not physically hinder Al uptake and did not improve uptake of Ca or Mg under Al toxicity.Ectomycorrhizae affected the exudation of LMWOA and weathering of minerals when plants were deficient in Mg, K or P. The exudation of LMWOA (mainly oxalate and malonate) was nutrient-specific and EcM species-specific. Exudation of LMWOA by the EcM symbiosis could not be predicted from the exudation of LMWOA by seedlings or EcM fungi grown in pure culture. The oxalate exudation by ectomycorrhizae in response to Mg and K deficiencies correlated well with the ectomycorrhizal induced weathering of minerals containing K and Mg .Weathering of the K-containing muscovite was almost doubled if seedlings were colonised by P. involutus compared to seedlings colonised by other EcM fungi or without mycorrhizae. There was no effect of EcM fungi on the weathering of Mg-containing hornblende. Localised Mg addition stimulated EcM foraging under Mg deficiency. Hyphal foraging and transport of Mg to the roots was independent of P supply or P transport.