The effects of S02 on stomatal aperture of attached sunflower leaves were observed with a remote-control light microscope system that permitted continuous observation of stomatal responses over periods of several hours. The relationship between actual stomatal aperture and stomatal conductance, measured with a porometer, also was Stomata play an important role in regulating the uptake of air pollutants such as SO2 (5,10,19,20,22,30), and stomatal aperture therefore is a major factor controlling the amount of leaf injury. Stomatal behavior also is affected by SO2 and stomatal response to SO2 usually has been evaluated by measurement of stomatal conductance or transpiration rate. However, such measurements represent the average behavior of many stomata, and they may be modified by such SO2-induced leaf injury as disorganization ofcells and excessive water loss through the epidermis, independently of stomatal aperture. As a result, measurements based on stomatal conductance or transpiration rate provide no information concerning the response of individual stomata to SO2. They also may not even represent the average response of stomata after the appearance of leaf injury. Omasa and Onoe (27) using a remote-control light microscope, recently observed a transient opening of sunflower stomata exposed to 2 l1-I' of SO2, varying greatly in extent even in neighboring stomata, and occurring before water-soaking was visible in the subsidiary cells. This phenomenon could not have been found without direct, continuous observation of stomatal behavior.There are many reviews of the interrelationships between stomatal response to S02 and leaf injury (4,8,9,14,17,22,31), and several mechanisms have been proposed to explain stomatal responses. However, these explanations are inconsistent with one another and inadequate because they do not fully explain the effects, or differentiate between reversible and irreversible effects. We therefore examined the reversible and irreversible effects of SO2 on stomata of attached sunflower leaves by means of a remote-control light microscope system and attempted to organize the data so as to explain stomatal behavior. We also examined the relationship between stomatal aperture and stomatal conductance during exposure to SO2 to learn the effect of leaf injury on stomatal conductance.