EFFECTS OF OXYGEN AND NITRATE ON AMMONIUM UPTAKE KINETICS AND ADENYLATE POOLS IN PHALARIS-ARUNDINACEA L AND GLYCERIA-MAXIMA (HARTM) HOLMB

We studied the effects of oxygen (aerated versus O-2 depleted similar to 0.5 mg l(-1) O-2) and nitrate (none versus 10 mu mol l(-1)) on the ammonium uptake kinetics and adenylate pools in two wetland plants differing in their degree of flood tolerance (Phalaris arundinacea L. and Glyceria maxima (Hartm.) Holmb.). The study was performed as a random block design in a growth chamber. The NH4+-uptake kinetics were estimated by using a computerised nonlinear parameter estimation procedure to fit the differential farm of a modified Michaelis-Menten model to solution depletion curves. The uptake kinetics for NH4+ differed between the two species: V-max was significantly higher for P. arundinacea (24.7 to 29.6 mu mol h(-1) g(-1) root dry weight) than for G. maxima (4.6-10.3 mu mol h(-1) g(-1) root dry weight). The NH4+ concentration at which uptake ceases (C-min) was 0.2 to 0.5 mu mol l(-1) for P. arundinacea and significant higher (1.1-2.7 mu mol l(-1)) for G. maxima. K-m varied between 3.1 and 6.2 mu mol l(-1) for P. arundinacea, and 1.6 and 3.0 mu mol l(-1) for G. maxima. The different uptake kinetics of the two species reflect the different structure of their root systems: P. arundinacea has an extensive root system consisting of many thin roots whereas G. maxima has fewer but thicker roots. The uptake kinetics also suggest that P. arundinacea is adapted to growing at lower ambient NH4+ concentrations than G. maxima. Oxygen had no consistent effect on NH4+ uptake kinetics. However, the plants that had NO3- in the nutrient solution as well as NH4+ had slightly higher V-max values and lower C-min and K-m values than those without NO3-. Thus, both species were able to sustain their uptake characteristics at low external O-2 concentrations, probably because of internal aeration through the air-space tissue of the plants. Nitrate deprivation also lowered the energy charge ratio and adenine nucleotide content in roots. The roots recovered quickly from NO3- deprivation once NO3- was resupplied. The stresses imposed by partially O-2-depleted conditions and lack of nitrate were therefore relatively mild and reversible. It seems that the inherent aerenchyma development under aerated conditions in these species is sufficient to maintain adequate root oxygenation under partially O-2-depleted conditions.