Light regulation of root and leaf NO3 - uptake and reduction in the floating macrophyte Lemna minor.

•  The regulation of NO3 - uptake kinetics and reduction in relation to long- and short-term changes in irradiance was explored in roots and photosynthetic tissues of Lemna minor. •  The NO3 - uptake kinetics, nitrate reductase activity, plant morphology, chlorophyll and tissue NO3 - , organic-N, starch and sugars were measured on roots and fronds of L. minor grown at four combinations of irradiance- and NO3 - availability. •  Long-term acclimatizations paralleled those of terrestrial plants. Short-term changes in irradiance, however, changed frond NO3 - uptake proportionally with frond chlorophyll and N content, indicating a relationship between responsiveness and the metabolic potential of the plants. Root uptake changed to balance the change in frond uptake keeping whole plant uptake varying by < 40%. Nitrate reductase activity was primarily located in the roots and was correlated with frond uptake, indicating a transport of NO3 - from shoot to root before reduction. •  This study shows that irradiance can affect the contribution of root and leaf uptake by aquatic plants and that roots play a major role in NO3 - reduction despite large foliar uptake.