Kinetics of phosphate absorption by mycorrhizal and non‐mycorrhizal Scots pine seedlings

Abstract

Kinetics of net phosphate (Pi) uptake was measured on intact ectomycorrhizal and non‐mycorrhizal Pinus sylvestris seedlings using a semihydroponic cultivation method. The depletion of Pi in a nutrient solution was assessed over a 160–0.2 μM Pi gradient. Growth of the pine seedlings was P limited and measurements were performed 7 and 9 weeks after inoculation. Three ectomycorrhizal fungi were studied: Paxillus involutus, Suillus bovinus and Thelephoraterrestris. Pi uptake was extremely fast in plants colonised by P. involutus. The Pi concentration dropped below 0.2 μM within 4–5 h. In plants colonised with S. bovinus this occurred in 5–6 h and in plants associated with T. terrestris 8 h were needed to run through the whole concentration range. Non‐mycorrhizal plants of similar size and nutrient status decreased Pi to a concentration between 1 and 2 μM in 18 h. Data were curve fitted to a two‐phase Michaelis‐Menten equation. The apparent kinetic constants, Km and Vmax, for the high affinity Pi uptake system of the pine roots could be estimated accurately. Vmax of this system was up to 7 times higher in pines associated with P. involutus than in non‐mycorrhizal seedlings. The intact extraradical mycelium greatly increased the absorption surface area of the roots (Vmax). Non‐mycorrhizal plants had a Km between 7.8 and 16.4 μM Pi. Plants mycorrhizal with P. involutus had Km values between 2.4 and 7.2, plants colonised with S. bovinus had a Km between 5.1 and 12.3, and seedlings associated with T. terrestris had a Km from 4.6 to 10.1 μM Pi. All 3 ectomycorrhizal fungi had a strong impact on the Pi absorption capacity of the pine seedlings. The results also demonstrated that there is substantial heterogeneity in kinetic parameters among the different mycorrhizal root systems.