Effects of NH 4+ concentration on growth, morphology and NH 4+ uptake kinetics of Salvinia natans

Abstract

Many plants develop toxicity symptoms and have reduced growth rates when supplied with ammonium (NH 4 +) as the only source of inorganic nitrogen. In the present study, the growth, morphology, NH 4 + uptake kinetics and mineral concentrations in the tissues of the free-floating aquatic plant Salvinia natans (water fern) supplied exclusively with NH 4 +–N at concentrations of 0.25–15 mM were investigated. S. natans grew well, with relative growth rates of c. 0.25 g g −1 d −1 at external NH 4 + concentrations up to 5 mM, 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 NH 4 + uptake at dilute NH 4 + levels was downregulated at high NH 4 + concentrations with lower velocities of maximum uptake ( V max) and higher half-saturation constants ( K 1/2). High NH 4 + 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 NH 4 +-tolerant in line with a number of other species of wetland plants as growth was unaffected at NH 4 + concentrations as high as 5 mM and as symptoms of toxicity at higher concentrations were relatively mild. Depolarization of the plasma membrane to the equilibrium potential for NH 4 + 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 NH 4 + 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.