Ozone inhibits guard cell K⁺ channels implicated in stomatal opening

Ozone (O₃) deleteriously affects organisms ranging from humans to crop plants, yet little is understood regarding the underlying mechanisms. In plants, O₃ decreases CO₂ assimilation, but whether this could result from direct O₃ action on guard cells remained unknown. Potassium flux causes osmotically driven changes in guard cell volume that regulate apertures of associated microscopic pores through which CO₂ is supplied to the photosynthetic mesophyll tissue. We show in Vicia faba that O₃ inhibits (i) guard cell K⁺ channels that mediate K⁺ uptake that drives stomatal opening; (ii) stomatal opening in isolated epidermes; and (iii) stomatal opening in leaves, such that CO₂ assimilation is reduced without direct effects of O₃ on photosynthetic capacity. Direct O₃ effects on guard cells may have ecological and agronomic implications for plant productivity and for response to other environmental stressors including drought.