THE KINETICS OF NH(4)+ AND NO3(-) UPTAKE BY DOUGLAS-FIR FROM SINGLE N-SOLUTIONS AND FROM SOLUTIONS CONTAINING BOTH NH(4)+ AND NO3(-)

The kinetics of NH4+ and NO3- uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V(max) of NH4+ uptake was higher than that of NO3- uptake. The K(m) of NH4+ uptake and K(m) of NO3- uptake differed not significantly. When both NH4+ and NO3- were present, the V(max) for NH4+ uptake became slightly higher, and the K(m) for NH4+ uptake remained in the same order. Under these conditions the NO3- uptake was almost totally inhibited over the whole range of concentrations used (10-1000 muM total N). This inhibition by NH4+ occurred during the first two hours after addition.