PHYSIOLOGICAL CHARACTERIZATION OF ROOT ZN2+ ABSORPTION AND TRANSLOCATION TO SHOOTS IN ZN HYPERACCUMULATOR AND NONACCUMULATOR SPECIES OF THLASPI

Radiotracer techniques were employed to characterize Zn-65(2+) influx into the root symplasm and translocation to the shoot in Thlaspi caeru lescens, a Zn hyperaccumulator, and Thlaspi arvense, a nonaccumulator. A protocol was developed that allowed us to quantify unidirectional Z n-65(2+) influx across the root-cell plasma membrane (20 min of radioa ctive uptake followed by 15 min of desorption in a 100 mu M ZnCl2 + 5 mM CaCl2 solution). Concentration-dependent Zn2+ influx in both Thlasp i species yielded nonsaturating kinetic curves that could be resolved into linear and saturable components. The linear kinetic component was shown to be cell-wall-bound Zn2+ remaining in the root after desorpti on, and the saturable component was due to Zn2+ influx across the root -cell plasma membrane. This saturable component followed Michaelis-Men ten kinetics, with similar apparent Michaelis constant values for T. c aerulescens and T. arvense (8 and 6 mu M, respectively). However, the maximum initial velocity for Zn2+ influx in T. caerulescens root cells was 4.5-fold higher than for T. arvense, indicating that enhanced abs orption into the root is one of the mechanisms involved in Zn hyperacc umulation. After 96 h 10-fold more Zn-65 was translocated to the shoot of T. caerulescens compared with T. arvense. This indicates that tran sport sites other than entry into the root symplasm are also stimulate d in T. caerulescens. We suggest that although increased root Zn2+ inf lux is a significant component, transport across the plasma membrane a nd tonoplast of leaf cells must also be critical sites for Zn hyperacc umulation in T. caerulescens.