Many plants develop toxicity symptoms and have reduced growth rates when supplied
with ammonium (NH4
+) as the only source of inorganic nitrogen. In the present study,
the growth, morphology, NH4
+ uptake kinetics and mineral concentrations in the tissues
of the free-floating aquatic plant Salvinia natans (water fern) supplied exclusively
with NH4
+–N at concentrations of 0.25–15mM were investigated. S. natans grew well,
with relative growth rates of c. 0.25 g g−1 d−1 at external NH4
+ concentrations up to 5mM,
but at higher levels growth was suppressed and the plants had small leaves and short
roots with stunted growth. The high-affinity transport system (HATS) that mediate NH4
+
uptake at dilute NH4
+ levels was downregulated at high NH4
+ concentrations with lower
velocities of maximum uptake (Vmax) and higher half-saturation constants (K1/2). High
NH4
+ levels also barely affected the concentrations of mineral cations and anions in the
plant tissue. It is concluded that S. natans can be characterized as NH4
+-tolerant in line
with a number of other species of wetland plants as growth was unaffected at NH4
+
concentrations as high as 5mM and as symptoms of toxicity at higher concentrations
were relatively mild. Depolarization of the plasma membrane to the equilibrium potential
for NH4
+ at high external concentrations may be a mechanism used by the plant to
avoid excessive futile transmembrane cycling. S. natans is tolerant to the high NH4
+ levels
that prevail in domestic and agricultural wastewaters, and the inherent high growth
rate and the ease of biomass harvesting make S. natans a primary candidate for use in
constructed wetland systems for the treatment of various types of nitrogen-rich wastewaters