New insights into fern stomatal behavior based on leaf responses to light, CO2, and VPD in a common garden

Background/Question/Methods

It is critical that we characterize stomatal responses to environment, and factors that underlie their variation within and across species to better predict plant performance. Ferns vary remarkably in leaf form and habitat preference but associations of stomatal behavior with their diversity and distribution have yet to be explored. We tested responses of stomatal conductance (g_s) to irradiance, vapor pressure deficit (VPD), and CO₂concentration for 13 phylogenetically and morphologically diverse fern species from open and shaded habitats within tropical rainforest, grown under high and low irradiance treatments. We hypothesized significant variation in magnitude stomatal responses associated with native and growth irradiance environments, and tested for coordination among stomatal responses to different stimuli, and for coordination between stomatal responses and other leaf flux and structural traits.

Results/Conclusions

We observed striking variation in the sensitivity, direction and magnitude of fern stomatal responses to environmental stimuli. Species showed stronger g_s responses to irradiance than to VPD and CO₂. Species native to high irradiance showed greater g_s responsiveness to irradiance and CO₂, whereas shade habitat species showed greater responsiveness to VPD. Within species, the high irradiance growth environment generated higher flux rates and greater magnitude responses to all three environmental factors. Across species, the apparent wrong way response (WWR) under increasing VPD was highly variable and of longer duration but comparable size to those seen for angiosperms. There was a correlation of g_s response to higher VPD with that in response to high irradiance. Responses of g_s to CO₂ were highly variable, including significant increases or decreases, and were mostly uncorrelated with responses to other factors. Responsiveness of g_s to irradiance and VPD were negatively related to leaf density whereas responsiveness to CO₂ was positively related to leaf density. Ferns have more sophistication and complexity in stomatal control than previously understood with multiple internal signals transduced by environmental factors. This is consistent with diverse optimization of hydraulic and metabolic needs to environmental dynamics at growth and evolutionary time scales.