Ammonium and Nitrate Uptake by the Floating Plant Landoltia punctata

BACKGROUND AND AIMS: Plants from the family Lemnaceae are widely used in ecological engineering projects to purify wastewater and eutrophic water bodies. However, the biology of nutrient uptake mechanisms in plants of this family is still poorly understood. There is controversy over whether Lemnaceae roots are involved in nutrient uptake. No information is available on nitrogen (N) preferences and capacity of Landoltia punctata (dotted duckweed), one of the best prospective species in Lemnaceae for phytomelioration and biomass production. The aim of this study was to assess L. punctata plants for their ability to take up NH₄⁺ and NO₃⁻ by both roots and fronds. METHODS: NO₃⁻ and NH₄⁺ fluxes were estimated by a non-invasive ion-selective microelectrode technique. This technique allows direct measurements of ion fluxes across the root or frond surface of an intact plant. KEY RESULTS: Landoltia punctata plants took up NH₄⁺ and NO₃⁻ by both fronds and roots. Spatial distribution of NH₄⁺ and NO₃⁻ fluxes demonstrated that, although ion fluxes at the most distal parts of the root were uneven, the mature part of the root was involved in N uptake. Despite the absolute flux values for NH₄⁺ and NO₃⁻ being lower in roots than at the frond surface, the overall capacity of roots to take up ions was similar to that of fronds because the surface area of roots was larger. L. punctata plants preferred to take up NH₄⁺ over NO₃⁻ when both N sources were available. CONCLUSIONS: Landoltia punctata plants take up nitrogen by both roots and fronds. When both sources of N are available, plants prefer to take up NH₄⁺, but will take up NO₃⁻ when it is the only N source.