Nitrate facilitates cadmium uptake, transport and accumulation in the hyperaccumulator Sedum plumbizincicola

The aims of this study are to investigate whether and how the nitrogen form (nitrate (NO 3 – ) versus ammonium (NH 4 + )) influences cadmium (Cd) uptake and translocation and subsequent Cd phytoextraction by the hyperaccumulator species Sedum plumbizincicola . Plants were grown hydroponically with N supplied as either NO 3 – or NH 4 + . Short-term (36 h) Cd uptake and translocation were determined innovatively and quantitatively using a positron-emitting 107 Cd tracer and positron-emitting tracer imaging system. The results show that the rates of Cd uptake by roots and transport to the shoots in the NO 3 – treatment were more rapid than in the NH 4 + treatment. After uptake for 36 h, 5.6 (0.056 μM) and 29.0 % (0.290 μM) of total Cd in the solution was non-absorbable in the NO 3 – and NH 4 + treatments, respectively. The local velocity of Cd transport was approximately 1.5-fold higher in roots (3.30 cm h −1 ) and 3.7-fold higher in shoots (10.10 cm h −1 ) of NO 3 – - than NH 4 + -fed plants. Autoradiographic analysis of 109 Cd reveals that NO 3 – nutrition enhanced Cd transportation from the main stem to branches and young leaves. Moreover, NO 3 – treatment increased Cd, Ca and K concentrations but inhibited Fe and P in the xylem sap. In a 21-day hydroponic culture, shoot biomass and Cd concentration were 1.51 and 2.63 times higher in NO 3 – - than in NH 4 + -fed plants. We conclude that compared with NH 4 + , NO 3 – promoted the major steps in the transport route followed by Cd from solution to shoots in S . plumbizincicola , namely its uptake by roots, xylem loading, root-to-shoot translocation in the xylem and uploading to the leaves. S . plumbizincicola prefers NO 3 – nutrition to NH 4 + for Cd phytoextraction.