The need for speed: fast stomatal closure requires a high ABA sensitivity
The fast stomatal responses of angiosperms confers a better capacity to respond to variations in environmental cues and thus provided a great competitive advantage to this group of plants during land plant colonization (Raven 2014). Despite the fact that the mechanisms by which stomatal speediness is regulated are unclear (Lawson & Vialet-Chabrand 2018), especially among different phylogenetic groups, our results strengthen the hypothesis that the slower fern stomatal response is associated to a reduced capacity to respond to ABA (Brodribb & McAdam 2011; McAdam & Brodribb 2012; Cardoso et al. 2019). This idea is supported by the results in which exogenous application of ABA rapidly reduced g s in the angiosperm, whilst fern stomata did not respond to this phytohormone in the time period analysed. It is important to emphasize, however, that ferns stomata requires longer time to respond to stomatal closure stimulus such as dark and high CO2 concentration (Franks & Britton-Harper 2016; Limaet al. 2019). Thus, we cannot currently conclude that the stomata of the ferns investigated here lack ABA responsiveness.
Interestingly, the stomata of the ferns used here responded to exogenous application of sucrose and mannitol, and to changes in the CO2 concentration and during the light-to-dark transition. Furthermore, our results revealed that the diel course ofg s is similar between ferns and angiosperms in relative terms, with a maximum g s observed in the initial period of the day, as typically observed under tropical conditions (Antunes et al. 2012, 2017). This result indicates that fern stomata is able to respond to the natural circadian rhythm, which is controlled by a complex regulatory network associated to changes in environmental cues such as temperature, air humidity, VPD, CO2 concentration and light quality and quantity (Gardner et al. 2006; Graf et al. 2010; Shalit-Kanehet al. 2018). Taken together, these results strengthen the idea that fern stomata are responsive to environmental cues, although the velocity of these responses are much lower than those observed in angiosperms (Franks & Britton-Harper 2016). Given the discrepancy between ferns stomatal responses to ABA and sucrose, this suggests that ferns first acquired ABA-independent mechanisms for the regulation of stomatal speediness. In the next section, we discuss how mesophyll-derived metabolites may contribute to explaining the slower stomatal responses found in ferns and how fern stomata can respond to environmental cues such as darkness and high CO2concentration with no or at least reduced ABA sensitivity.