Stomatal responses to environmental and endogenous signals in vascular plants are critical for regulating plant gas exchange with the atmosphere. In addition, stomatal closure is vital for minimizing water loss and preventing lethal embolism during drought. The vast majority of studies concerning stomatal physiology have focused on the angiosperms, where it is well-established that abscisic acid (ABA) is the major factor regulating stomatal responses to changes in leaf water status: the situation is much less clear in non-seed plants. For example, when fern species are drought-stressed and naturally synthesize ABA, this endogenous ABA is ineffectual in closing stomata. In contrast, a recent article observed stomatal closure by 15% when measuring gas exchange in leaves that were sprayed with high levels of exogenous ABA in the fern species Athyrium filix-femina. However, this response was only observed in plants acclimated to low VPD in a growth cabinet and not in plants of this species grown under high VPD or to any significant degree in two Dryopteris species. Cardoso et al. (10.1104/pp.18.01412) have measured the stomatal response to changes in vapor pressure difference (VPD) in two natural forms of the fern species Athyrium filix-femina, recently suggested to have stomata that are regulated by ABA. These two varieties have considerable variation in foliar anatomy and consequently leaf hydraulic properties, meaning similar changes in VPD should have very different effects on leaf water status The authors report that the two forms had considerable differences in key hydraulic traits, including leaf hydraulic conductance and capacitance, as well as the kinetics of stomatal response to changes in VPD. In both forms, however, the stomatal responses to VPD could be accurately predicted by a dynamic, mechanistic model that assumes guard cell turgor changes in concert with leaf turgor in the light, and not via metabolic processes including the level of ABA. During drought, endogenous ABA did not play a role in stomatal closure, and exogenous ABA applied to live, intact leaves did not induce stomatal closure. These results indicate that functional stomatal responses to changes in leaf water status in ferns are regulated by leaf hydraulics and not metabolism.
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