Nitrogen nutrition of Salvinia natans: Effects of inorganic nitrogen form on growth, morphology, nitrate reductase activity and uptake kinetics of ammonium and nitrate

Abstract

In this study we assessed the growth, morphological responses, and N uptake kinetics of Salvinia natans when supplied with nitrogen as NO 3 −, NH 4 +, or both at equimolar concentrations (500 μM). Plants supplied with only NO 3 − had lower growth rates (0.17 ± 0.01 g g −1 d −1), shorter roots, smaller leaves with less chlorophyll than plants supplied with NH 4 + alone or in combination with NO 3 − (RGR = 0.28 ± 0.01 g g −1 d −1). Ammonium was the preferred form of N taken up. The maximal rate of NH 4 + uptake ( V max) was 6–14 times higher than the maximal uptake rate of NO 3 − and the minimum concentration for uptake ( C min) was lower for NH 4 + than for NO 3 −. Plants supplied with NO 3 − had elevated nitrate reductase activity (NRA) particularly in the roots showing that NO 3 − was primarily reduced in the roots, but NRA levels were generally low (<4 μmol NO 2 − g −1 DW h −1). Under natural growth conditions NH 4 + is probably the main N source for S. natans, but plants probably also exploit NO 3 − when NH 4 + concentrations are low. This is suggested based on the observation that the plants maintain high NRA in the roots at relatively high NH 4 + levels in the water, even though the uptake capacity for NO 3 − is reduced under these conditions.