Nitrogen Uptake by the Floating Macrophyte Lemna minor

• Both roots and leaves of free-floating plants can potentially take up nutrients. In this study, the ability and relative contribution of roots and fronds for N uptake by the floating macrophyte Lemna minor was investigated. • The NH4+ and NO3- uptake kinetics of roots and fronds were measured on plants acclimated to three different NH4 NO3 concentrations. • Lemna had the capacity to take up NH4+ and NO3- through both roots and fronds; uptake kinetics for the two tissue types were comparable on an area basis. The overall contribution of root and frond to whole-plant uptake, estimated from measured kinetic characteristics, varied depending on plant N status (the root contribution increased from 32 to 73% for N-satiated and N-depleted plants, respectively). • The shift in the balance between root and frond contribution to whole-plant uptake resulted from a 1.5-38 times greater increase in the area-specific uptake capacity and affinity of roots relative to fronds, combined with a larger decrease in the minimum concentration for uptake (C min) for roots than fronds. At the morphological level, root-frond surface area increased with declining N supply, which might be beneficial to the plants since the area return per unit biomass invested was nine times greater for roots than for fronds.