Salicylic acid induces stomatal closure by modulating endogenous hormone levels in cucumber cotyledons

Salicylic acid (SA) is one of the most important signaling molecules in plant growth and defense responses to biotic and abiotic stresses. Here, the effect of exogenous SA on the stomatal movements was investigated in cotyledons of cucumber (Cucumis sativus L.) seedlings. Application of different SA concentrations could induce the reduction in stomatal aperture and conductance, especially at a concentration of 0.5 mM. Using the isolated epidermal strips, stomata were found to close notably in response to exogenous SA, even at a concentration as low as 0.001 mM. Further study showed that a SA-induced decrease in the stomatal aperture was intensified by the higher SA concentrations, longer exposure, and lower pH of the medium. In addition, to understand the relationship between stomatal closure and endogenous hormone contents, the levels of ABA, IAA, and gibberellin (GA₃) were assayed under SA treatment. SA significantly increased endogenous ABA but not IAA and GA₃ content. A significant negative correlation (p ≤ 0.01) was observed between stomatal conductance and the ratio of ABA to (GA₃ + IAA) during SA application. It was suggested that exogenous SA could change the balance of endogenous hormones and thereby induce stomatal closure in cotyledons of cucumber seedlings.