The mechanism of zinc uptake in plants : characterisation 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 possible to discriminate clearly between tracer zinc bound in the cell wall and actual uptake into the cell. It was shown that despite lengthy desorption, retention of zinc in slowly exchanging zinc pools in the cell wall can invalidate tracer influx measurements. A comparative study of 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. The cytoplasmic and vacuolar zinc concentrations were 56 mmol. m -3 and 32 mmol. m -3 , respectively, for cells grown in a zinc concentration of 0.1 mmol. m -3 . Influx of 65 Zn in Chara was linear over several hours, with rapid transfer to the vacuole, but only slow efflux. Influx occurred in a biphasic manner, which was tentatively attributed to the operation of two separate transport systems, a high-affinity system 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 mmol. m -3 . Only the low-affinity system was examined in detail. Influx through this system showed a strong dependence on external pH with an optimum around 7 and was also stimulated by cytoplasmic acidification. Influx was sensitive to metabolic inhibition, but not to blockers of Ca 2+ and K + channels. Other characteristics included a slight sensitivity to Mn 2+ and Fe 2+ but little sensitivity to high concentrations of K + or Na + . Influx was independent of membrane potential difference in cells voltage-clamped at -65 to -300 mV.