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 (NO₃ ^-) versus ammonium (NH₄ ⁺)) 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 NO₃ ^- or NH₄ ⁺. Short-term (36 h) Cd uptake and translocation were determined innovatively and quantitatively using a positron-emitting ¹⁰⁷Cd 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 NO₃ ^- treatment were more rapid than in the NH₄ ⁺ 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 NO₃ ^- and NH₄ ⁺ 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 NO₃ ^-- than NH₄ ⁺-fed plants. Autoradiographic analysis of ¹⁰⁹Cd reveals that NO₃ ^- nutrition enhanced Cd transportation from the main stem to branches and young leaves. Moreover, NO₃ ^- 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 NO₃ ^-- than in NH₄ ⁺-fed plants. We conclude that compared with NH₄ ⁺, NO₃ ^- 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 NO₃ ^- nutrition to NH₄ ⁺ for Cd phytoextraction.