THE MECHANISM OF ZINC UPTAKE IN PLANTS - CHARACTERIZATION OF THE LOW-AFFINITY SYSTEM

The mechanism of zinc influx was investigated using giant algal cells (Chara corallina Klein ex Will.esk. R.D. Wood), in which it was possib le to discriminate clearly between tracer zinc bound in the cell wall and actual uptake into the cell. It was shown that despite lengthy des orption, retention of zinc in slowly exchanging zinc pools in the cell wall can invalidate tracer influx measurements. A comparative study o f zinc desorption from isolated cell walls of wheat (Triticum aestivum L.) roots indicated exchange characteristics similar to that of Chara . Fractionation of Chara internodal cells taken directly from cultures showed that most of the cell-associated zinc was in the cell walls. T he cytoplasmic and vacuolar zinc concentrations were 56 mmol . m(-3) a nd 32 mmol . m(-3), respectively, for cells grown in a zinc concentrat ion of 0.1 mmol . m(-3). Influx of Zn-65 in Chara was linear over seve ral hours, with rapid transfer to the vacuole, but only slow efflux. I nflux occurred in a biphasic manner, which was tentatively attributed to the operation of two separate transport systems, a high-affinity sy stem which is saturated at 0.1 mmol . m(-3) and a low-affinity system which showed a linear dependence on concentration up to at least 50 mm ol . m(-3). Only the low-affinity system was examined in detail. Influ x through this system showed a strong dependence on external pH with a n optimum around 7 and was also stimulated by cytoplasmic acidificatio n. Influx was sensitive to metabolic inhibition, but not to blockers o f Ca2+ and K+ channels. Other characteristics included a slight sensit ivity to Mn2+ and Fe2+ but little sensitivity to high concentrations o f K+ or Na+. Influx was independent of membrane potential difference i n cells voltage-clamped at - 65 to - 300 mV.