Uptake kinetics of zinc in hyper-accumulating species of Sedum alfredii Hance at the leaf cellular level

Vacuolar compartmentalization in leaves could play a major role in hyper-accumulation of heavy metals. However, little is known about the physiology of intracellular zinc (Zn) sequestration in plants. Radiotracer techniques were employed to characterize 65Zn uptake kinetics in leaf sections and leaf protoplasts of hyper-accumulating ecotype (HE) and non-hyper-accumulating ecotype (NHE) species of Sedum alfredii Hance. The time-dependent kinetics of 65Zn uptake in leaf of S. alfredii Hance consisted of two stages with an initial rapid stage followed by a longer but slower stage. Zn accumulation in leaf sections and leaf protoplasts of the HE was much higher than that of the NHE. Concentration-dependent kinetics of 65Zn uptake in leaf sections and leaf protoplasts of S. alfredii Hance yielded non-saturating kinetic curves that could be divided into linear and saturable components. This saturable component could be characterized by Michaelis-Menten equation, and there were no significant differences in the calculated Km between the two ecotypes, whereas Vmax values for HE were much higher than for NHE. Carbonyl cyanidem-chlorophenylhydr-azone (CCCP) treatment strongly inhibited 65Zn transport into protoplasts, especially in HE, indicating that we were measuring primarily true transmembrane Zn that was transported into S. alfredii Hance. These findings indicate that there was a carrier-mediated system in plasma membrane which could effectively move cytoplasmic Zn into the vacuole, and this carrier-mediated system play a role in the dramatic Zn hyper-accumulation expressed in Sedum alfredii Hance.