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Salinity is toxic to nearly all plants. A halophyte is a plant that naturally grows where it is affected by salinity in the root area or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs, and seashores. 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," and are damaged fairly easily by salinity.

Definition and Diversity
One quantitative measure of salt tolerance is the "total dissolved solids" in irrigation water that a plant can tolerate. Sea water typically contains 40 grams per liter (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 bigalovii (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 halophyes.

Why Salt is Toxic to Plants
The toxicity of saline soils can, in principle, be attributed to either the chlorine ions (Cl-) or to the sodium ions (Na+) that enter the plant through its roots. For woody perennials, it is apparently the Cl- ions that do most of the damage, but for most plants the Na+ ions are more damaging. The main source of Na+ toxicity to cell metabolism is due to the displacement of potassium ions (K+) in vital cell processes. Sodium and potassium are both alkali metal elements, and are chemically quite similar. However, while sodium ions may bind chemically to the same sites on enzymes as do potassium ions, but the functioning of the enzyme is disrupted. More than 50 enzymes in plant cells are activated by K+, and Na+ cannot substitute. K+ is also essential to protein synthesis, which Na+ also disrupts. Beyond this metabolic distress, there are several other deleterious effects of high Na+ concentrations in the soil and irrigation water, but the plant cell's need to maintain a reasonably low ratio of Na+ to K+ appears to be primary.

Adaptation Mechanisms
Adaptation to saline environments by halophytes may take the form of salt tolerance (see halotolerance) 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 or by concentrating salts in leaves that later die and drop off.