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

Kinetics of net phosphate (P-i) uptake was measured on intact ectomycorrhizal and non-mycorrhizal Pinus sylvestris seedlings using a semihydroponic cultivation method. The depletion of P-i in a nutrient solution was assessed over a 160-0.2 mu M P-i 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 Thelephora terrestris. P-i uptake was extremely fast in plants colonised by P. involutus. The P-i concentration dropped below 0.2 mu M 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 P-i to a concentration between 1 and 2 mu M in 18 h. Data were curve fitted to a two-phase Michaelis-Menten equation. The apparent kinetic constants, K-m and V-max, for the high affinity P-i uptake system of the Dine roots could be estimated accurately. V-max 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 (V-max). Non-mycorrhizal plants had a K-m between 7.8 and 16.4 mu M P-i. Plants mycorrhizal with P. involutus had K-m values between 2.4 and 7.2, plants colonised with S. bovinus had a K-m between 5.1 and 12.3, and seedlings associated with T. terrestris had a K-m from 4.6 to 10.1 mu M P-i. All 3 ectomycorrhizal fungi had a strong impact on the P-i absorption capacity of the pine seedlings. The results also demonstrated that there is substantial heterogeneity in kinetic parameters among the different mycorrhizal root systems.