Vegetation patterns in James Bay coastal marshes. II. Physiological adaptation to salt-induced water stress in three halophytic graminoids

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Abstract
The responses of three halophytes to an incremental increase in soil-water salinity have been studied under greenhouse conditions. Biomass accumulation of both the roots and shoots of each species decreased markedly as osmotic stress increased. Differences in salt sensitivity accounted for the species natural separation along a salinity gradient identified from the low marsh to the high marsh. The three species utilized different strategies to metabolically lower their internal osmotic potential during salt stress. Puccinellia phryganodes reduced cell water content and accumulated high concentrations of proline. Carex paleacea exhibited no adjustment in water retention, but accumulated high concentrations of glycine betaine and a small amount of proline. The most salt-sensitive species, Scirpus americanus, reduced cell water content while it accumulated high concentrations of glycine betaine. The reliance on proline as an osmoregulant in P. phryganodes is suggested to be an adaptation to its highly fluctuating salinity regime. There is no definitive evidence that suggests that any of the species accumulate carbohydrates in response to saline stress. The loss of reducing sugars in C. paleacea and S. americanus grown under moderately saline conditions (i.e., 5 and 10 ppt) may be attributed to a significant reduction in the photosynthetic capacity of the species. The subsequent increase in carbohydrates at higher salinities may have been due to their accumulation during severe osmotic stress.
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