Stomatal dynamics are regulated by leaf hydraulic traits and guard cell anatomy in nine true mangrove species

Stomatal regulation is critical for mangroves to survive water deficits and highly fluctuating ambient water availability in the hyper-saline intertidal zone. Despite the importance of stomatal regulation in mangroves very little is known about stomatal sensitivity to vapour pressure deficit (VPD), and the co-ordination of this trait with stomatal morphology and leaf hydraulic traits in these species. We measured the stomatal response to a step increase in vapour pressure deficit (VPD) in situ, stomatal anatomy, leaf hydraulic vulnerability and pressure-volume traits in nine true mangrove species of five families. We aimed to answer two questions: (1) Does stomatal morphology determine stomatal dynamics in response to a high VPD in mangroves and (2) do leaf hydraulic traits influence stomatal sensitivity to VPD in mangroves? We found that the stomata of mangrove plants highly sensitive to VPD, and that species with higher maximum stomatal conductance had slower stomatal responses to an increase in VPD, and that stomatal density and size were correlated with the speed of stomatal closure at high VPD across the closely-related species. We also found that a higher leaf capacitance (Cleaf) and more resistance to leaf hydraulic vulnerability were associated with slower stomatal responses to an increase in VPD. Our results demonstrate that the dynamics of the stomatal response to an increase in VPD are regulated by leaf hydraulic traits and stomatal morphology. Our work provides a quantitative framework to better understand stomatal regulation in mangroves in an environment with highly dynamic water availability.