Nitrogen uptake by the floating macrophyte Lemna minor

Summary 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 NH 4 + and NO 3 − uptake kinetics of roots and fronds were measured on plants acclimated to three different NH 4 NO 3 concentrations. Lemna had the capacity to take up NH 4 + and NO 3 − 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.