Effects of sulphur dioxide on leaf photosynthesis: the role of temperature and humidity

The effect of temperature and humidity on SO₂–induced photosynthetic depression was determined in gas exchange experiments with leaves of Vicia faba , L. Stomatal behaviour was sensitive to humidity resulting in higher uptake rates of SO₂ and stronger reductions of photosynthesis at low VPD (vapour pressure deficit). After a fumigation period of 2 h, when the photosynthetic rate had stabilized, photosynthesis of leaves exposed to SO₂ at 8°C was reduced much more than at 18°C at the same rate of SO₂ uptake. Data analysis with a mechanistic model revealed that this effect was due to the slower rate of S(IV) oxidation at lower temperatures, resulting in higher accumulation of S(IV) and thus stronger reduction of photosynthesis. These results were confirmed by experimental analyses of the S(IV) concentration in leaves following fumigation, which showed that more S(IV) accumulated in leaves exposed at a lower temperature. This may explain the high sensitivity of plants exposed to SO₂ under winter conditions, when both VPD and temperature are low.