Salt tolerance in a Juncus roemerianus brackish marsh: Spatial variations in plant water relations

Halophytes residing in tidal marshes are often subjected to intense and varying environmental demands. These variations may occur in a matter of hours, with tidal-induced changes in hydrology and soil chemistry (including soil oxygen status, redox potentials, and salinity). Juncus roemerianus has been shown to occupy three dissimilar marsh zones, including lower marsh with minimally diluted saline waters, mid-marsh which undergoes periodic freshwater dilutions of tidal waters, and upper marsh which is principally maintained by upland freshwaters. The purpose of this study was to discern spatial patterns in plant water status of J. roemerianus. Leaf water potentials (ψleaf) were 1-2 MPa less than soil-osmotic potential (ψπsoil), with the greatest ψleaf (i.e., most negative; ca. − 4 MPa) generated within the lower marsh as plants experienced the highest salinities (18 ppt; during low tide). There was a 3-12% increase in relative water content (θ) in mid-marsh plants, in comparison to both lower- and upper-marsh individuals. Pressure-volume isotherms showed that plants along the shoreline (i.e., lower-marsh) had lower osmotic potential at full saturation (ψπsat), lower osmotic potential at turgor loss point (ψπtlp), and elevated bulk modulus of elasticity (ϵ). Whereas, no spatial differences in water content at turgor loss point (θtlp) and symplastic water fraction (θsym) were observed among the three marsh zones. Additionally, morphological and plant mass characteristics, including stomatal size, stomatal density, leaf abundance, and above-ground biomass, were greater in lower marsh plants compared to other two marsh zones. Overall, this study demonstrates the importance of soil-water salinity (as ψπsoil) in shaping spatial variations in plant water status within a J. roemerianus brackish marsh.