Root‐zone acidity and nitrogen source affects Typha latifolia L. growth and uptake kinetics of ammonium and nitrate

The NH4+ and NO3– 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 NH4+ or NO3– 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 NH4+. 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 NO3–‐fed than for NH4+‐fed plants. The maximum uptake rate, Vmax, was highest for NH4+ at pH 6.5 (30.9 µmol h–1 g–1 root dry weight) and for NO3– at pH 5.0 (31.7 µmol 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, K½, was lowest at low pH for NH4+ and at high pH for NO3–. The changes in Vmax and K½ were thus consistent with the theory of increasing competition between cations and H+ at low pH and between anions and OH– at high pH. Cmin was independent of pH, but slightly higher for NO3– than for NH4+ (Cmin(NH4+) ∼ 0.8 mmol m–3; Cmin(NO3–) ∼ 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 NH4+ 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.