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

Abstract

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 Km values for NH4+ uptake of 6, 35, and 55 μM, respectively, and Km values for K+ uptake of 24, 18, and 96 μM, respectively. Addition of 100 μM NH4+ raises the Km of K+ uptake by L. bicolor to 35 μM, while the Vmax remains unchanged. It is argued that the increase of Km 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 Km, for NH4+ uptake. The possibility that NH4+ and K+ are taken up by the same carrier can be excluded. The Km, 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+.