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

Young, A. J., Collins, J. C. and Russell, G. 1987. Ecotypicvariation in the osmotic responses of Enteromorpha intestinalis(L.) Link.—J. exp. Bot. 38: 1309–1324. The physiological basis for salt tolerance has been studiedin the euryhaline marine alga Enteromorpha intestinalis. Adaptationto dilute and concentrated seawaters has been investigated inthree separate populations of this alga: marine, rock pool andestuarine. Internal K⁺, Na⁺ and Cl^– levels have been determined usingtracer efflux analyses. K⁺ has been shown to be the major osmoticsolute within this alga. Cellular levels of Cl^– and, inparticular, Na⁺ are low although levels in the cell wall arehigh. Levels of these ions varied considerably between the separateplants; K⁺ levels within marine plants of E. intestinalis aretwo to four times those found in the other populations. Thetertiary sulphonium compound ß-dimethylsulphonio-propionateis maintained at relatively high levels, although it remainsfairly 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 wallsof these plants allow large changes in volume in the diluteconditions experienced in an estuary, while low turgor preventscell rupture. Thicker cell walls and small cells of the marineand rock pool plants assist in tolerating high and low externalosmotic potential—the estuarine plants respond poorlyto concentrated seawater. Key words: Enteromorpha, osmoregulation, ecotypes