Ammonium and nitrate uptake by the floating plant Landoltia punctata

Abstract

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.

Fig. 1.

Net ion fluxes along the root length of Landoltia punctata as noted in the bars at the top: root cap (hatched), meristem zone (black), distal elongation zone (white), proximal elongation zone (grey) and mature zone (speckled). (A) H⁺, (B) NH₄⁺, (C) NO₃⁻. Numbers indicate the initial flux values. Flux measurements were taken while roots were exposed to the basal solution containing (in mm): NH₄NO₃, 0·5, 1·0 or 2·5; CaCl₂, 0·05; MgSO₄, 0·025; NaCl, 0·56; K₂SO₄, 0·025; NaH₂PO₄, 0·032; and NaHCO₃, 0·2.

Fig. 1.

Net ion fluxes along the root length of Landoltia punctata as noted in the bars at the top: root cap (hatched), meristem zone (black), distal elongation zone (white), proximal elongation zone (grey) and mature zone (speckled). (A) H⁺, (B) NH₄⁺, (C) NO₃⁻. Numbers indicate the initial flux values. Flux measurements were taken while roots were exposed to the basal solution containing (in mm): NH₄NO₃, 0·5, 1·0 or 2·5; CaCl₂, 0·05; MgSO₄, 0·025; NaCl, 0·56; K₂SO₄, 0·025; NaH₂PO₄, 0·032; and NaHCO₃, 0·2.

Fig. 2.

Net fluxes (nmol m⁻² s⁻¹) of NH₄⁺ and NO₃⁻ at roots (the mature zone, 1·2 mm from the root tip) and fronds of Landoltia punctata exposed to different media (0·5, 1·0 or 2·5 mm NH₄NO₃). Values are means±s.e. (n=5). Different lower-case letters indicate significant differences (P≤0·05).