Depression of net photosynthesis in Vicia faba L. exposed to sulphur dioxide and ozone

Abstract

In most laboratory investigations of plant responses to air pollution, plants are exposed to a single pollutant gas. However, in rural atmospheres, pollutants seldom occur singly; for example, in central England, where summertime mean concentrations of sulphur dioxide are typically 10–30 p.p.b. (parts in 10⁹ by volume) depending on the site, there are 10–30 days each summer when concentrations of ozone generated photo-chemically exceed 80 p.p.b. for at least 1 h (ref. 1). Maximum hourly ozone concentrations of 130–260 p.p.b. have been reported^1–3. Similarly, maximum hourly SO₂ concentrations in rural central England exceed 50 p.p.b. for a few hours each month. We have previously shown⁴ that net photosynthetic rates P_N of field beans, Vicia faba L. c.v. Dylan, were depressed, in comparison with rates in clean air, on exposure to SO₂ concentrations as low as 40 p.p.b. Responses of photosynthesis to mixtures of SO₂ and O₃ seem to have been studied only at higher concentrations atypical of the natural environment^5,6. We report here that the addition of ozone in the concentration range 60–150 p.p.b. to atmospheres containing 40 p.p.b. SO₂ led to decreases in P_N of V. faba L. larger than those observed in either gas alone.