The inter- and intracellular distribution of the elements calcium, potassium and sodium in non-mycorrhizal and mycorrhizal roots of Pinus sylvestris dependent on different external nutrient supply conditions was detected by energy-dispersive X-ray microanalysis after cryofixation, freeze-drying and pressure infiltration of the material. In non-mycorrhizal and mycorrhizal roots, calcium was mainly detectable in the apoplastic regions. The levels in vacuoles and cytoplasm were near the limits of detection by X-ray microanalysis. Incubation with high concentrations of potassium and sodium, or mycorrhizal infection with Suillus bovinus and Pisolithus tinctorius reduced the amounts of calcium detectable in the roots, especially in the apoplast of cortical cells. The studies revealed that: potassium is mainly localized in cytoplasm and cell walls; the cytoplasmic content is regulated over a wide range of external potassium concentrations; potassium levels in the inner parts of roots are higher than in the outer parts. Mycorrhizal infection with Suillus bovinus had no effect on the inter- and intracellular distribution of potassium in roots but, if the external supply was low, the potassium content in shoots was reduced. In non-mycorrhizal pine roots and those infected with Paxillus involutus an increase in the sodium content of all cell compartments was observed after treatment with high external concentrations of NaH2PO4. However, an increase in sodium content in mycorrhizas of S. bovinus was not detected. The X-ray microanalytical results are discussed in relation to the apoplastic movement of nutrients in non-mycorrhizal and mycorrhizal fine roots of pine and to the demand for these nutrients in different intracellular compartments.

The inter- and intracellular distribution of phosphate in non-mycorrhizal and ectomycorrhizal roots of Pinus sylvestris was analysed by energy-dispersive X-ray spectroscopy after cryofixation, freeze-drying, pressure infiltration with resin and dry cutting of the material. The results show (1) that in non-mycorrhizal roots the distribution of phosphate across the root cross section is relatively homogenous, (2) that the intracellular distribution of phosphate depends on the external supply, and (3) that the phosphate absorption can be increased by a supply of NH4. Under phosphate starvation, the phosphate is mainly localized in the cell cytoplasm, whereas under high external phosphate this element accumulates in the vacuoles of cortical cells. It can be inferred that at low external conditions phosphate is mainly located in the cytoplasm of cells to protect metabolism from deficiency, that phosphate is translocated to the shoot via symplastic and apoplastic pathways, and that especially the vacuolar pool size is regulated by the external phosphate supply. The nutrition of the host plant under phosphate starvation was improved by a mycorrhizal infection with Suillus bovinus. However, this effect might not be the same for all ectomycorrhizal infections and supply conditions. With high external phosphate an ectomycorrhizal infection with S. bovinus had no effect on intracellular phosphate within the roots, the shoot contents of mycorrhizal plants were less than those of non-mycorrhizal seedlings and translocation rate across the mycorrhizal interface appeared to be independent of the availability of phosphate. Intracellular phosphate in fungal cells of mycorrhizal roots was independent of the external supply and seemed to be regulated by the formation of polyphosphates. A supply of (NH4)2HPO4, which led to an increase in cytoplasmic phosphate levels in the hyphae of the Hartig net of the Paxillus involutus mycorrhiza also led to higher phosphate in the plant cell compartments. Therefore, it can be inferred that the translocation of phosphate from fungus to host plant across the mycorrhizal interface is regulated by the phosphate concentration in the cytoplasm of the Hartig net and by the efflux rate into the interfacial apoplast.

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 Pi 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−1 and under P limitation. Four ectomycorrhizal fungi were studied: Paxillus involutus, Suillus luteus, Suillus bovinus and Thelephora terrestris. The Pi-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 Pi-uptake rates of the seedlings and the active fungal biomass in the substrate as measured by the ergosterol assay. The highest Pi-uptake rates were found in seedlings associated with fungi producing abundant external mycelia. At an external Pi 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 Pi absorption followed Michaelis–Menten kinetics: uptake rates declined with decreasing Pi concentrations in the nutrient solution. This reduction was most pronounced in non- mycorrhizal seedlings and plants colonized by T. terrestris. The results confirm that there is considerable heterogeneity in affinity for Pi uptake among the different mycobionts. It is concluded that the external mycelia of ectomycorrhizal fungi strongly influence the Pi-uptake capacity of the pine seedlings, and that some mycobionts are well equipped to compete with other soil microorganisms for Pi present at low concentrations in soil solution.