Disturbances in stomatal behaviour caused by air pollutants

Many atmospheric pollutants, even when present at relatively low concentrations, may interfere with the control of stomatal aperture, and they thus have the potential to upset the water balance of the leaf or the whole plant. Although at high concentrations pollutants such as SO2 and O3 usually cause stomatal closure, at low concentrations stomatal conductance is often increased. As well as creating a risk of loss of control of water relations, this is likely to increase the dose of the pollutant entering the mesophyll. It is, however, difficult to generalize about the nature of the physiological disturbances caused by pollutants of variation in the responses between plants. In some cases the effects may be peculiar to one, or just a few, species. Two mechanisms underlying the interference with stomatal control have recently been identified, one involving O3 and the other CO2. In Aster tripolium (sea aster) stomata in detached epidermal strips close as the external Na concentration is increased, and it has been proposed that this phenomenon is involved in the regulation of salt loading of shoot tissues. Ozone has been shown to have the capacity to interfere with Na+-induced stomatal closure, and the possibility that it therefore disrupts an aspect of salinity tolerance in this species is worthy of further research. Elevated CO2, on the other hand, has been found to interfere with the control of water relations of beech (Fagus sylvatica): for a given degree of drought, stomatal conductance and rates of soil water depletion were significantly higher in elevated CO2 than in ambient air. It is normally assumed that atmospheric CO2-enrichment will lead to increased plant productivity and improved water economy, while also providing some protection against other atmospheric pollutants through partial stomatal closure. However, the response of beech indicates that in some species there may also be detrimental effects of CO2-enrichment of plant-water relations.