DAILY FLUCTUATIONS OF ANTIOXIDANTS IN BEAN (PHASEOLUS-VULGARIS L) LEAVES AS AFFECTED BY THE PRESENCE OF AMBIENT AIR-POLLUTANTS

During the period 0800-1700 h (GMT) of a summer day, young leaves were collected every 20 min from Phaseolus vulgaris L. cv. Horticultural plants grown in open-top field chambers located at an urban site in northern Italy and exposed either to ambient levels of gaseous air pollutants or to filtered ambient air. Ascorbic and dehydroascorbic acids, GSH and GSSG, superoxide dismutase, ascorbate peroxidase, dehydroascorbate reductase, GSSG reductase, GSH peroxidase, catalase, guaiacol peroxidase, chlorophylls, carotenoids, soluble protein and dry weight were measured in these leaves. The main differences between treated and control leaves were observed during the period 1100-1530 h and concerned superoxide dismutase, catalase, ascorbate peroxidase, ascorbate/dehydroascorbate and GSH/GSSG ratios, chlorophylls, carotenoids and dry weight. On the basis of the pollution climate observed at the experimental site on the day of leaves sampling, ozone appeared to be the causative agent of the observed divergences among the time patterns of antioxidants in treated and control plants. It was deduced that ozone can induce oxidative stress via the production of superoxide radical anion and hydrogen peroxide.