Halophytes

A halophyte is a plant that grows in waters of high salinity, coming into contact with saline water through its roots or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs and seashores. The word derives from Ancient Greek ἅλας (halas) 'salt' and φυτόν (phyton) 'plant'. An example of a halophyte is the salt marsh grass Spartina alterniflora (smooth cordgrass). Relatively few plant species are halophytes—perhaps only 2% of all plant species.

The large majority of plant species are glycophytes, which are not salt-tolerant and are damaged fairly easily by high salinity.

One quantitative measure of salt tolerance (halotolerance) is the total dissolved solids in irrigation water that a plant can tolerate. Seawater typically contains 40 grams per litre (g/l) of dissolved salts (mostly sodium chloride). Beans and rice can tolerate about 1–3 g/l, and are considered glycophytes (as are most crop plants). At the other extreme, Salicornia bigelovii (dwarf glasswort) grows well at 70 g/l of dissolved solids, and is a promising halophyte for use as a crop. Plants such as barley (Hordeum vulgare) and the date palm (Phoenix dactylifera) can tolerate about 5 g/l, and can be considered as marginal halophytes.

Adaptation to saline environments by halophytes may take the form of salt tolerance or salt avoidance. Plants that avoid the effects of high salt even though they live in a saline environment may be referred to as facultative halophytes rather than 'true', or obligatory, halophytes.

For example, a short-lived plant species that completes its reproductive life cycle during periods (such as a rainy season) when the salt concentration is low would be avoiding salt rather than tolerating it. Or a plant species may maintain a 'normal' internal salt concentration by excreting excess salts through its leaves, by way of a hydathode, or by concentrating salts in leaves that later die and drop off.

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