Ca2+ Transport in Membrane Vesicles from Pinto Bean Leaves and Its Alteration after Ozone Exposure

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Abstract
The influence of ozone on Ca2+ transport in plant membranes from pinto bean (Phaseolus vulgaris L. var Pinto) leaves was investigated in vitro by means of a filtration method using purified vesicles. Two transport mechanisms located at the plasma membrane are involved in a response to ozone: (a) passive Ca2+ influx into the cell and (b) active Ca2+ efflux driven by an ATP-dependent system, which has two components: a primary Ca2+ transport directly linked to ATP which is partially activated by calmodulin and a H+/Ca2+ antiport coupled to activity of a H+-ATPase. The passive Ca2+ permeability is increased by ozone. A triangular pulse of ozone stimulates a higher influx of Ca2+ than does a square wave, even though the total dose was the same (0.6 microliter per liter × hour). Leaves exposed to a square wave did not exhibit visible injury and were still able to recover from oxidant stress by activation of calmodulin-dependent Ca2+ extrusion mechanisms. On the other hand, leaves exposed to a triangular wave of ozone, exhibit visible injury and lost the ability of extruding Ca2+ out of the cell.