Figure 1.

Hypothetical mechanism(s) regulating stomatal closure or opening. ABA and ethylene can individually regulate stomatal closure via H₂O₂ and ETR1. While high levels of ABA can inhibit ethylene and vice versa. However, the presence of both ABA and ethylene (ABA–ET) induces antioxidant activity, which in turn reduces H₂O₂ contents and thus diminishes stomatal closure (half-open stomata). Under ozone stress, ethylene binds to ERT1, which inhibits the ABA response pathway leading to open stomata (indicated by yellow arrows).