Ecotypic Variation in the Osmotic Responses ofEnteromorpha intestinalis(L.) Link.

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Abstract
Young, A. J., Collins, J. C. and Russell, G. 1987. Ecotypic variation in the osmotic responses of Enteromorpha intestinalis (L.) Link.—J. exp. Bot. 38: 1309–1324. The physiological basis for salt tolerance has been studied in the euryhaline marine alga Enteromorpha intestinalis. Adaptation to dilute and concentrated seawaters has been investigated in three separate populations of this alga: marine, rock pool and estuarine. Internal K+, Na+ and Cl− levels have been determined using tracer efflux analyses. K+ has been shown to be the major osmotic solute within this alga. Cellular levels of Cl− and, in particular, Na+ are low although levels in the cell wall are high. Levels of these ions varied considerably between the separate plants; K+ levels within marine plants of E. intestinalis are two to four times those found in the other populations. The tertiary sulphonium compound β-dimethylsulphonio-propionate is maintained at relatively high levels, although it remains fairly insensitive to change in the external salinity. Changes in the tissue water content and cell volume are large, particularly within the estuarine plants. The thin cell walls of these plants allow large changes in volume in the dilute conditions experienced in an estuary, while low turgor prevents cell rupture. Thicker cell walls and small cells of the marine and rock pool plants assist in tolerating high and low external osmotic potential—the estuarine plants respond poorly to concentrated seawater.