Tolerance and avoidance: Two contrasting physiological responses to salt stress in mature marsh halophytes Juncus roemerianus Scheele and Spartina alterniflora Loisel

For most plants, elevated salinities can promote both hyperionic and hyperosmotic stress, often resulting in decreased growth and increased mortality. In previous studies involving plant–water relations, two contrasting physiological mechanisms to water stress have emerged: ( i) stress-tolerance, which can be achieved through osmotic adjustment and changes in tissue elasticity, and ( ii) stress-avoidance, which restricts further water loss through decreased stomatal conductance and changes in leaf morphology and/or orientation. While these processes have been well characterized in angiosperms during drought, few studies have considered these responses in halophytes during salt-stress. In this study, experimental microcosms were used to evaluate salt-tolerance and salt-avoidance in two contrasting coastal-marsh halophytes, Juncus roemerianus and Spartina alterniflora. In mature S. alterniflora, preacclimated to freshwater, only salt-tolerance mechanisms (osmotic adjustment and increased tissue rigidity) were observed during high salinity conditions. In contrast, physiological modifications observed in mature J. roemerianus involved salt-avoidance through decreased stomatal conductance. These physiological responses are consistent with zonation patterns in these plants, wherein S. alterniflora resides in the lower marsh and must contend with long-term salt exposure and J. roemerianus inhabits the upper reaches of salt-marshes where salinities tend to be lower and where salt-stress often involves transient exposure to high salinities.