Stomata regulate leaf water potential in response to both gradual changes in soil water availability and rapid changes in leaf-to-atmospheric vapor pressure difference (D). Despite numerous studies that characterize each of these responses separately, it is unclear if patterns of short term regulation fit into existing characterizations of plants along a continuum of long term maintenance of minimum leaf water potential (isohydric to anisohydric classifications). Furthermore, these short term responses have been shown to be associated with anatomical characteristics of stomata. We used ten potted woody angiosperm species with previously quantified degrees of iso/anisohydry. In well-watered plants, we measured stomate size and density and stomatal response to step increases in D. Other functional traits assessed include integrated and instantaneous water-use-efficiency, quasi-steady state stomatal conductance in darkened and illuminated leaves, rate change in conductance during illumination, and minimum stomatal conductance when excised. We hypothesized that across diverse species, stomatal anatomy and responses to increased D will be strongly coordinated with each other and with degree of iso/anisohydry.
Stomatal responses to increased D were generally well predicted by degree of iso/anisohydry but were less well associated with stomatal anatomy. In more isohydric species, rate of stomatal closure (dgs/dt) was up to 5x faster than in more anisohydric species. In these more isohydric species, most closure occurred at low D (~1.5 kPa) with little further response at higher D (up to ~2.5 kPa). Conversely, more anisohydric species were most responsive to high D and largely unaffected by low D. Faster rates of closure and reduced sensitivity to low D were also associated with a greater stomatal density and leaf area occupied by stomata (size times density) but did not correlate directly with stomate size alone. The range of sensitivities to D were correlated with pressure-volume curve properties, where species exhibiting little response to high D had less negative turgor loss points and saturated osmotic potentials. Greater sensitivity to low D was additionally associated with greater integrated WUE and instantaneous WUE at low D. Our results indicate that short term stomatal responses are consistent with longer term strategies for water potential maintenance. The link between short-term and long-term stomatal responses may be facilitated by anatomical features associated with the area devoted to gas exchange within leaves rather than dimensions of individual stomata.