Ozone, Sulphur Dioxide and Nitrogen Oxides: Some Effects on the Water Relations of Herbaceous Plants and Trees

Abstract

O₃, SO₂ and NO₂ have all been shown to exert effects on plants which are likely to influence water relations. In this paper a review of evidence (often confusing) from the literature is presented, followed by a description of two phenomena which we have recently studied in detail. It has been known for some time that plants exposed to SO₂, or to mixtures of SO₂ and NO₂, often display differences in stomatal behaviour from those in clean air. We have grown spring barley in SO₂ and NO₂ (24–35 ppb of each gas) and examined the ability of leaves to regulate water loss. The stomata were found to be much less responsive to ABA after exposure. In other respects the stomata of polluted plants appeared to function normally (e.g., they showed similar responses to CO₂). Accumulation of ABA in water-stressed leaves appeared to be unaffected by the pollution treatment. We have concluded that the impairment of stomatal sensitivity to ABA may adversely affect the water relations of polluted plants. In other experiments, young trees of Fagus sylvatica (beech) were exposed to realistic doses of ozone in summer. In well-watered trees ozone inhibited stomatal opening, but in trees growing in dry soil, and thus experiencing water stress, stomatal closure was significantly inhibited by ozone. We suggest that ozone may have two deleterious effects on beech: inhibition of CO₂ acquisition when water supplies are adequate, and interference with the protective role of stomatal closure during periods of water stress.