Antioxidant systems in the leaf apoplast compartment of Pinus pinaster Ait. and Pinus radiata D. Don. Plants exposed to SO₂

Summary. The activities of guaiacol peroxidase (GuPOD), ascorbate peroxidase (ASAp), superoxide dismutase (SOD) and ascorbate/glutathione cycle (AGC) enzymes, together with ascorbate (ASC) and glutathione contents, were determined in apoplastic‐fluid and cell‐wall fractions of needles of Pinus pinaster Ait. and Pinus radiata D. Don. exposed for up to 6 months to SO 2 (0.01 ppm or 0.30 ppm) in fumigation chambers. AGC enzyme activities (monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) were in all cases undetectable, as was glutathione content. In needles of P. pinaster plants exposed to SO 2 , ascorbate content and all enzyme activities considered (except AGC enzymes) increased. The increases were most marked in response to the higher SO 2 concentration. In needles of P. radiata, similar but less marked responses were observed. These findings suggest a) that enzyme activities and ascorbate contents increase in order to deal with the reactive oxygen intermediates produced during long‐term contamination with SO 2 , and b) that P. pinaster has more effective defences against contamination of this type than P. radiata.