The NH(4)(+) and NO(3)(-) uptake kinetics by Typha latifolia L. were studied after prolonged hydroponics growth at constant pH 3.5, 5.0, 6.5 or 7.0 and with NH(4)(+) or NO(3)(-) as the sole N-source. In addition, the effects of pH and N source on H(+) extrusion and adenine nucleotide content were examined. Typha latifolia was able to grow with both N sources at near neutral pH levels, but the plants had higher relative growth rates, higher tissue concentrations of the major nutrients, higher contents of adenine nucleotides, and higher affinity for uptake of inorganic nitrogen when grown on NH(4)(+). Growth almost completely stopped at pH 3.5, irrespective of N source, probably as a consequence of pH effects on plasma membrane integrity and H(+) influx into the root cells. Tissue concentrations of the major nutrients and adenine nucleotides were severely reduced at low pH, and the uptake capacity for inorganic nitrogen was low, and more so for NO(3)(-)-fed than for NH(4)(+)-fed plants. The maximum uptake rate, V(max), was highest for NH(4)(+) at pH 6.5 (30.9 micro mol h(-1) g(-1) root dry weight) and for NO(3)(-) at pH 5.0 (31.7 micro mol h(-1) g(-1) root dry weight), and less than 10% of these values at pH 3.5. The affinity for uptake as estimated by the half saturation constant, K((1/2)), was lowest at low pH for NH(4)(+) and at high pH for NO(3)(-). The changes in V(max) and K((1/2)) were thus consistent with the theory of increasing competition between cations and H(+) at low pH and between anions and OH(-) at high pH. C(min) was independent of pH, but slightly higher for NO(3)(-) than for NH(4)(+) (C(min)(NH(4)(+)) approximately 0.8 mmol m(-3); C(min)(NO(3)(-)) approximately 2.8 mmol m(-3)). The growth inhibition at low pH was probably due to a reduced nutrient uptake and a consequential limitation of growth by nutrient stress. Typha latifolia seems to be well adapted to growth in wetland soils where NH(4)(+) is the prevailing nitrogen compound, but very low pH levels around the roots are very stressful for the plant. The common occurrence of T. latifolia in very acidic areas is probably only possible because of the plant's ability to modify pH-conditions in the rhizosphere.

The effects of solution pH on NH4+ uptake kinetics and net H+ extrusion by Typha latifolia L. were studied during short‐term (days) and long‐term (weeks) exposure to pH in the range of pH 3.5–8.0. The NH4+ uptake kinetics were estimated from depletion curves using a modified Michaelis‐Menten model. T. latifolia was able to grow in solution culture with NH4+ as the sole N source and to withstand a low medium pH for short periods (days). With prolonged exposure (weeks) to pH 3.5, however, the plants showed severe symptoms of stress and stopped growing. The solution pH affected NH4+ uptake kinetics. The affinity for NH4+, as quantified by the half saturation constant (K1/2) and Cmin (the NH4+ concentration at which uptake ceases), decreased with pH. K1/2 was increased from 7.1 to 19.2 mmol m−3 and Cmin from 2.0 to 5.7 mmol m−3 by lowering the pH in steps from 8.0 to 3.5. Vmax was, however, largely unaffected by pH (∼22 μmol h−1 g−1 root dry weight). Under prolonged exposure to constant pH, growth rates were highest at PH 5.0 and 6.5. At pH 8.0 growth was slightly depressed and at pH 3.5 growth completely stopped. NH4+ uptake kinetics were similar at pH 5.0, 6.5 and 8.0 whereas at pH 3.5 NH4+ uptake almost completely stopped. The ratio between net H+ extrusion and NH4+ uptake decreased significantly at low pH. The adverse effects of low pH on NH4+ uptake kinetics are probably a consequence of a reduced H+‐ATPase activity and/or an increased re‐entry of H+ at low pH, and the associated decrease in the electrochemical gradient across the plasma membranes of the root cells.