Cellular compartmentation of zinc in leaves of the hyperaccumulator Thlaspi caerulescens

Cellular compartmentation of Zn in the leaves of the hyperaccumulator Thlas pi caerulescens was investigated using energy-dispersive x-ray microanalysi s and single-cell sap extraction. Energy-dispersive x-ray microanalysis of frozen, hydrated leaf tissues showed greatly enhanced Zn accumulation in th e epidermis compared with the mesophyll cells. The relative Zn concentratio n in the epidermal cells correlated linearly with cell length in both young and mature leaves, suggesting that vacuolation of epidermal cells may prom ote the preferential Zn accumulation. The results from single-cell sap samp ling showed that the Zn concentrations in the epidermal vacuolar sap were 5 to 6.5 times higher than those in the mesophyll sap and reached an average of 385 mM in plants with 20,000 mu g Zn g(-1) dry weight of shoots. Even w hen the growth medium contained no elevated Zn, preferential Zn accumulatio n in the epidermal vacuoles was still evident. The concentrations of K, Cl, P, and Ca in the epidermal sap generally decreased with increasing Zn. The re was no evidence of association of Zn with either P or S. The present stu dy demonstrates that Zn is sequestered in a soluble form predominantly in t he epidermal vacuoles in T. caerulescens leaves and that mesophyll cells ar e able to tolerate up to at least 60 mM Zn in their sap.