Short-term phosphorus uptake rates in mycorrhizal and non-mycorrhizal roots of intact Pinus sylvestris seedlings

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.