Kinetics of NH4+ and K+ uptake by ectomycorrhizal fungi. Effect of NH4+ on K+ uptake

NH₄⁺ and K⁺ uptake experiments have been conducted with 3 ectomycorrhizal fungi, originating from Douglas fir (Pseudotsuga menziesii (Mirb.] Franco) stands. At concentrations up to 250 μM, uptake of both NH₄⁺ and K⁺ follow Michaelis-Menten kinetics. Laccaria bicolor (Maire) P. D. Orton, Lactarius rufus (Scop.) Fr. and Lactarius hepaticus Plowr. ap. Boud. exhibit K_m values for NH₄⁺ uptake of 6, 35, and 55 μM, respectively, and K_m values for K⁺ uptake of 24, 18, and 96 μM, respectively. Addition of 100 μM NH₄⁺ raises the K_m of K⁺ uptake by L. bicolor to 35 μM, while the V_max remains unchanged. It is argued that the increase of K_m is possibly caused by depolarization of the plasma membrane. It is not due to a competitive inhibition of K⁺ by NH₄⁺ since the apparent inhibitor constant is much higher than the K_m, for NH₄⁺ uptake. The possibility that NH₄⁺ and K⁺ are taken up by the same carrier can be excluded. The K_m, values for K⁺ uptake in the two other fungi are not significantly affected by 100 μM NH₄⁺. Except for a direct effect of NH₄⁺ on influx of K⁺ into the cells, there may also be an indirect effect after prolonged incubation of the cells in the presence of 100 μM NH₄⁺.