SO[sub 2]: Apoplastic peroxidase is more sensitive than photosynthesis

Leaves of barley (Hordeum vulgare L.) were fumigated for 24h (-48h) measurements of maximum photosynthetic capacity were carried out. In parallel, intercellular washing fluids (IWF) from the leaves were collected and examined for their ability to peroxidatively oxidize sulfite. Although the leaves had taken up the SO[sub 2], as evidenced by an increase in total sulfur (as well as in the sulfate content) no reduction was found in the maximum ability to photoreduce CO[sub 2] under all conditions applied. On the other hand, there was marked increase in the activity of the IWF to oxidize sulfite to sulfate. Within 5 hours after the start of the fumigation, a distinct and significant rise in the sulfite-oxidizing activity was found. After one day, sulfite-oxidation rates of the cell wall extracts had more than doubled, in comparison to unfumigated controls. In extracts of leaves from fumigated plants, the increase of the sulfite oxidizing activity was negligible during the time of the experiment. Only extracts isolated from the leaf cell walls showed an enhanced activity. Unfumigated control plants never revealed a change in activity, neither in the leaf homogenate, nor in the IWF. These findings suggest that mechanisms in the cell walls of leavesmore » are much more sensitive to (and possibly triggered by) air pollutants like SO[sub 2], than reactions of the protoplast. The sulfite oxidation in the apoplast can be regarded as a forward defense mechanism to protect symplastic reactions.« less