Exposure of plants for few days to high relative humidity (RH) increases the plant sensitivity to water scarcity afterwards. High vulnerability of high RH-exposed plants to water loss is because of decreased stomatal ability to close in response to stomatal closing stimuli such as desiccation and phytohormone abscisic acid (ABA) application (stomatal malfunctioning).
Occurrence of stomatal malfunctioning depends on the duration of exposure to high RH. Changes in stomata morphology or leaf anatomy are not the reasons for stomatal malfunctioning, but changes in the ABA signalling pathway in the cells controlling stomatal aperture are the reasons for lack of suitable stomatal response to closing stimuli after prolonged exposure to high RH. Remarkable natural variation was found for stomatal responses in Arabidopsis accessions after 4-day exposure to high RH. A threshold for foliar ABA level and expression of a gene involved in both ABA-dependent and -independent pathways was found to be crucial for keeping stomatal responsive to closing stimuli after long-term exposure to high RH.