Physiological responses of the lichen Xanthoparmelia mexicana to oxidative stress of SO2

The objective of present article is to investigate the role and significance of free radicals in the mechanism of injury in the lichen Xanthoparmelia mexicana to SO2 pollution. At 0.5 ppm SO2 exposure phaeophytinization of chlorophyll (PQa value) dropped by 10%. Chlorophyll a, b and protein content decreased with the increase of SO2. The activity of SOD enhanced significantly at 0.5 ppm exposure, but that of POD changed significantly only at 1.5 ppm exposure. GSH dramatically increased with SO2 concentration. Its relative content elevated 30 and 120% at 0.5 and 1.5 ppm exposures. At all exposures, the content of MDA remained nearly unchanged. The activity of M1PDH was correlated with SO2 stress intensity. Activity of G6PDH enhanced 130% at 0.5 ppm exposure compared with the control. Similarly, activity of trehalase enhanced 190% at 0.5 ppm exposure. Glucose content declined significantly with SO2 concentration increased from 0.28 mg g(-1) DW of the control to 0.14 mg g(-1) DW at 0.5 ppm exposures. These results suggested that the chain injury reaction of oxidative stress could be mechanistically related to the efflux of energy and substrate between the two lichen symbionts, such as the metabolism of mannitol, trehalose and glucose. (C) 1999 Elsevier Science B.V. All rights reserved.