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Little is known about long‐distance mesophyll‐driven signals that regulate stomatal conductance. Soluble and/or vapor‐phase molecules have been proposed. In this study, the involvement of the gaseous signal ethylene in the modulation of stomatal conductance inArabidopsis thalianaby CO₂/abscisic acid (ABA) was examined.

Jasmonic acid (JA) and salicylic acid (SA) regulate stomatal closure, preventing pathogen invasion into plants. However, to what extent abscisic acid (ABA), SA and JA interact, and what the roles of SA and JA are in stomatal responses to environmental cues, remains unclear. Here, by using intact plant gas‐exchange measurements in JA and SA single and double mutants, we show that stomatal responsiveness to CO₂, light intensity, ABA, high vapor pressure deficit and ozone either did not or, for some stimuli only, very slightly depended upon JA and SA biosynthesis and signaling mutants, includingdde2, sid2, coi1,jai1,myc2andnpr1alleles. Although the stomata in the mutants studied clearly responded to ABA, CO₂, light and ozone, ABA‐triggered stomatal closure innpr1‐1was slightly accelerated compared with the wild type. Stomatal reopening after ozone pulses was quicker in thecoi1‐16mutant than in the wild type. In intact Arabidopsis plants, spraying with methyl‐JA led to only a modest reduction in stomatal conductance 80 min after treatment, whereas ABA and CO₂induced pronounced stomatal closure within minutes. We could not document a reduction of stomatal conductance after spraying with SA. Coronatine‐induced stomatal opening was initiated slowly after 1.5–2.0 h, and reached a maximum by 3 h after spraying intact plants. Our results suggest that ABA, CO₂and light are major regulators of rapid guard cell signaling, whereas JA and SA could play only minor roles in the whole‐plant stomatal response to environmental cues in Arabidopsis andSolanum lycopersicum(tomato).

Low concentrations of CO₂cause stomatal opening, whereas [CO₂] elevation leads to stomatal closure. Classical studies have suggested a role for Ca²⁺and protein phosphorylation in CO₂‐induced stomatal closing. Calcium‐dependent protein kinases (CPKs) and calcineurin‐B‐like proteins (CBLs) can sense and translate cytosolic elevation of the second messenger Ca²⁺into specific phosphorylation events. However, Ca²⁺‐binding proteins that function in the stomatal CO₂response remain unknown.