Abstract: Xanthoparmellia mexicana was successfully isolating and culturing from its two symbionts and,preliminarily re-symbiosising its thalli in vitro performed from pure cultured photobiont and mycobiont.The solution exposure method was applied,confirming the toxic effect of sulfite solution with pH4.0.The biochemical responses of lichen thalli and cultured symbionts to the short time stress of SO₂ were studied.The chlorophyll PQa value of photobiont was correlated with its absorption of SO₂.Chlorophyll a was more sensitive than chlorophyll b;chlorophyll was damaged most severely by the exposure of 0.5mg稬^-1 of SO₂ fumigation,but was aggravated with the increase in the concentration of SO₂ by way of solution exposure.The activity of acid phosphatase in lichen was mainly dependent on the photobiont.Reduced glutathione(GSH) was activated by oxidative stress of SO₂.GSHwas confirmed to be highly correlated with SO₂ stress,and was proposed to be a significant and prospective biomarker of the status of lichen antioxidant system and the oxidative damage in lichen.The mycobiont was proposed to mainly resist the oxidative stress by SO₂,while the photobiont was damaged more easily with energy depletion.

Key words: Xanthoparmellia mexicana, SO₂, Antioxidant defense system