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 NO3 −, NH4 +, or both at equimolar concentrations (500μM). Plants supplied with only NO3 − had lower growth rates (0.17±0.01gg−1 d−1), shorter roots, smaller leaves with less chlorophyll than plants supplied with NH4 + alone or in combination with NO3 − (RGR=0.28±0.01gg−1 d−1). Ammonium was the preferred form of N taken up. The maximal rate of NH4 + uptake (V max) was 6–14 times higher than the maximal uptake rate of NO3 − and the minimum concentration for uptake (C min) was lower for NH4 + than for NO3 −. Plants supplied with NO3 − had elevated nitrate reductase activity (NRA) particularly in the roots showing that NO3 − was primarily reduced in the roots, but NRA levels were generally low (<4μmolNO2 − g−1 DWh−1). Under natural growth conditions NH4 + is probably the main N source for S. natans, but plants probably also exploit NO3 − when NH4 + concentrations are low. This is suggested based on the observation that the plants maintain high NRA in the roots at relatively high NH4 + levels in the water, even though the uptake capacity for NO3 − is reduced under these conditions. [Copyright &y& Elsevier]