No abstract

Short-term phosphate uptake rates were measured on intact ectomycorrhizal and non-mycorrhizal Pinus sylvestris seedlings using a new, non-destructive method. Uptake was quantified in semihydroponics from the depletion of P i in a nutrient solution percolating through plant containers. Plants were grown for 1 or 2 months after inoculation at a low relative nutrient addition rate of 3 % d −" and under P limitation. Four ectomycorrhizal fungi were studied : Paxillus involutus, Suillus luteus, Suillus bovinus and Thelephora terrestris. The P i -uptake capacity of mycorrhizal plants increased sharply in the month after inoculation. The increase was dependent on the development of the mycobionts. A positive correlation was found between the P i -uptake rates of the seedlings and the active fungal biomass in the substrate as measured by the ergosterol assay. The highest P i -uptake rates were found in seedlings associated with fungi producing abundant external mycelia. At an external P i concentration of 10 µM, mycorrhizal seedlings reached uptake rates that were 2.5 (T. terrestris) to 8.7 (P. involutus) times higher than those of non-mycorrhizal plants. The increased uptake rates did not result in an increased transfer of nutrients to the plant tissues. Nutrient depletion was ultimately similar between mycorrhizal and non-mycorrhizal plants in the semihydroponic system. Net P i absorption followed Michaelis-Menten kinetics : uptake rates declined with decreasing P i concentrations in the nutrient solution. This reduction was most pronounced in nonmycorrhizal seedlings and plants colonized by T. terrestris. The results confirm that there is considerable heterogeneity in affinity for P i uptake among the different mycobionts. It is concluded that the external mycelia of ectomycorrhizal fungi strongly influence the P i -uptake capacity of the pine seedlings, and that some mycobionts are well equipped to compete with other soil microorganisms for P i present at low concentrations in soil solution.

S U M M A R YThe decomposition and the nitrogen and phosphorus mineralization of fresh beech (Fagus sylvatica L.) leaf litter are described. Leaves were buried for up to 6 months in plant containers m which Scots pine (Pxnus sylvestris L.) seedlings were cultivated at a low rate of nutrient addition. The saprotrophic abilities of three ectomycorrhizal fungi, Thelephora terrestris Ehrh.: Fr., Suillus bovinus (L.: Ft.) O. Kuntze and Paxillus involutus (Batsch: Fr) Ft., were compared with the degradation caused by the iittet-decomposing basidiomycete, Lepista nuda (Bull.: Ft.) Cooke. Uninoculated leaves were included as controls. The investigation was performed at two different pH values since substrate pH is supposed to have an effect on the activities of extracellular enzymes of ectomycorrhizal fungi. The enzyme expression might also be largely influenced by the substrate they colonised. The mycorrhizal fungi caused only a low decomposition rate of the litter compared with that of L. nuda, and nitrogen was released only by L nuda. Leaves colonized by mycorrhizal fungi showed no net release of nitrogen; on the contrary, a small accumulation of N in the litter was observed. It therefore seems likely that the ectomyccrrhizal fungi studied do not have the ability to decompose efficiently the ligDOcellulose matrix of the relatively recalcitrant beech Jeaf litter. The degradation of this matrix seems to be essential for the fungi to gain access to the leaf nitrogen pool of fresh beech litter A direct release of nitrogen from organic compounds by ectomycorrhizal fungi seems therefore to be confined to the older litter layers. The beech leaf litter contained an important fraction of easily mineralizable phosphorus. P was not a growth limiting factor m the cultivation system, and could therefore accumulate m the leaf litter colonized by the ectomycorrhizal mycelium.

Summary• Change in the copper (Cu) sensitivity of Pinus sylvestris is presented, in response to the ectomycorrhizal fungi, Suillus bovinus and Thelephora terrestris , common mycobionts on metal contaminated sites.• Seedlings grown under phosphorus (P) limitation were exposed to a range of Cu concentrations. Plant and fungal development, P nutrition, sorption of Cu on roots and external mycelia as well as transfer of Cu to shoots were assessed.• Root growth and P nutrition were severely inhibited in nonmycorrhizal pines at elevated Cu compared with mycorrhizal plants. Excess Cu had little effect on the development of mycorrhizal roots and mycelia. Thelephora terrestris was less sensitive to Cu stress than S. bovinus . The extraradical mycelium of S. bovinus retained large amounts of Cu. However, binding of Cu in fungal tissue was not a prerequisite for low Cu sensitivity since T. terrestris absorbed considerably less Cu that S. bovinus .• Both ectomycorrhizal fungi protect P. sylvestris against Cu toxicity; a benefit that was not due to a metal dilution effect. The mechanisms of mycorrhizal amelioration of Cu toxicity are probably diverse and species-dependent.