Failure to Observe Stomatal ABA Responses in a Fern and a Lycophyte

Stomatal closure induced by abscisic acid (ABA) is an important mechanism by which plants reduce transpiration under drought stress and is well studied in seed plants. However, ABA-induced stomatal closure in ferns and lycophytes has been reported in some studies but not in others, even within the same species, such as the fern Nephrolepis exaltata and the lycophyte Selaginella uncinata. Here, ABA-induced stomatal behaviour and ABA signalling components, including guard cell potassium ion (K⁺) and calcium ion (Ca²⁺) fluxes and vacuoles, were analysed in the seed plant Vicia faba, the fern N. exaltata and the lycophyte S. uncinata. We found that ABA dramatically induced stomatal closure (by approximately 80%) in V. faba but did not induce stomatal closure in N. exaltata or S. uncinata. Similarly, downstream effects of ABA signalling pathways, including the accumulation of reactive oxygen species (ROS) and nitric oxide (NO), the activation of Ca²⁺ influx, the efflux of K⁺ and the disaggregation of large vacuole compartments, were observed in V. faba but were absent in N. exaltata and S. uncinata, except for ROS accumulation. Our findings show that, under the growth conditions used, the stomata of N. exaltata and S. uncinata do not respond to ABA.