THE PROBLEM OF VARIABILITY IN STOMATAL RESPONSES, PARTICULARLY APERTURE VARIANCE, TO ENVIRONMENTAL AND EXPERIMENTAL CONDITIONS

S ummary The extreme variability observed in stomatal responses to identical treatments has hindered the drawing of specific conclusions about how stomata respond to environmental factors and the mechanisms that drive these responses. This review considers stomatal aperture as a characteristic response and the ramifications of the observed within‐sample variance in aperture. In the wake of past research efforts, the efficacy of experimental techniques developed to observe stomatal aperture is reviewed, environmental, morphological and physiological factors that are thought to contribute to aperture variance are discussed, and the desirability of implementing procedures to reduce aperture variance is assessed. By taking into account these considerations, researchers may be better able to plan their future efforts.