Stomata and plant water relations: does air pollution create problems?

Small changes in the gaseous composition of the atmosphere have many different impacts on terrestrial plants. Some of the most important involve changes in stomatal control of leaf conductance. Evolution has provided highly complex mechanisms by which stomata respond to a wide range of environmental factors to balance the conflicting priorities of carbon gain for photosynthesis and water conservation. These mechanisms involve direct responses of the guard cells to aspects of the aerial environment, and hormonal communication within the plant enabling conductance to be adjusted according to soil moisture status. Various aspects of these delicately balanced mechanisms can be disturbed by air pollutants. Impairment of the regulation of plant water use by SO2 and O3 has been known for some years, but there are still many obstacles to our understanding of the variations in response between species, or even between genotypes of the same species. A surprising outcome of some recent studies is the suggestion that CO2 pollution may disrupt the control of water relations in some species because their stomata do not close sufficiently in CO2-enriched air. It has often been taken for granted that the elevation of atmospheric CO2 would lead to economies in water use by plant canopies, but the underlying assumptions are now being seriously questioned.