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 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(-1) 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 mu M, 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.