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Plants control water-use efficiency (WUE) by regulating water loss and CO 2 diffusion through stomata. Variation in stomatal control has been reported among lineages of vascular plants, thus giving rise to the possibility that different lineages may show distinct WUE dynamics in response to water stress. Here, we compared the response of gas exchange to decreasing leaf water potential among four ferns and nine seed plant species exposed to a gradually intensifying water deficit. The data collected were combined with those from 339 phylogenetically diverse species obtained from previous studies. In well-watered angiosperms, the maximum stomatal conductance was high and greater than that required for maximum WUE, but drought stress caused a rapid reduction in stomatal conductance and an increase in WUE in response to elevated concentrations of abscisic acid. However, in ferns, stomata did not open beyond the optimum point corresponding to maximum WUE and actually exhibited a steady WUE in response to dehydration. Thus, seed plants showed improved photosynthetic WUE under water stress. The ability of seed plants to increase WUE could provide them with an advantage over ferns under drought conditions, thereby presumably increasing their fitness under selection pressure by drought., (© 2021 The Authors New Phytologist © 2021 New Phytologist Foundation.)

• ABA is involved in stomatal closure in ferns, gymnosperms and angiosperms. • P type and R type ABA responses occur during dehydration and rehydration in gymnosperms as well as angiosperms. • Increasing and decreasing behaviour of foliar ABA content in angiosperms during dehydration. The variation in stomatal movement across vascular plants in response to abscisic acid (ABA) has received great attention in recent years. This review is an attempt to better understand the role of ABA in the stomatal movement of seed and seedless plants under dehydration and rehydration. Seed plants, i.e., gymnosperms and angiosperms, have two divergent ABA responses, the peaking type (P-type) and rising type (R-type), to induce stomatal closure under sustained drought stress. However, in the case of ferns and lycophytes, stomata of almost all species exhibit insensitive (I-type) behaviour to ABA. Consequently, seed plants have evolved an optimized water use efficiency to improve their succession in terrestrial ecosystems. During rehydration, the recovery of gas exchange is constrained by ABA accumulation under drought in R-type plants and constrained by hydraulics in P- and I-type plants. Thus, future studies should investigate the mechanisms underlying the divergence in stomata in response to ABA, focusing on P-type vs. R-type ABA responses in seed plants, the competition of seed plants, and the interaction between ABA and hydraulic pathways during rehydration. [ABSTRACT FROM AUTHOR]