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

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Abstract 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 ( ψ leaf ) were 1–2 MPa less than soil-osmotic potential ( ψ π soil ), with the greatest ψ leaf (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 ( ψ π sat ), lower osmotic potential at turgor loss point ( ψ π tlp ), and elevated bulk modulus of elasticity ( ϵ ). Whereas, no spatial differences in water content at turgor loss point ( θ tlp ) and symplastic water fraction ( θ sym ) 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 ψ π soil ) in shaping spatial variations in plant water status within a J. roemerianus brackish marsh.