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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. 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, plants which are not salt-tolerant, and are damaged fairly easily by high salinity.
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 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 (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, by way of a hydathode, or by concentrating salts in leaves that later die and drop off.
Halophytes as biofuel
Some halophytes are being studied for use as "3rd generation" biofuel precursors. Halophytes such as Salicornia bigelovii can be grown in harsh environments and typically do not compete with food crops for resources, making them promising sources of biodiesel or bioalcohol.
- Atriplex, the saltbush or orache genus
- Yerba mansa
- Tetragonia tetragonoides (Warrigal greens)
- Glenn, E. P., et al. (1999). Salt tolerance and crop potential of halophytes. Critical Review in Plant Sciences 18(2), 227-55. doi:10.1080/07352689991309207
- Glenn, E. P.; Brown, J. J.; O'Leary, J. W. (1998). "Irrigating Crops with Seawater", Scientific American, Vol. 279, no. 8, Aug. 1998, pp. 56-61.
- "Pressroom - Fact Sheet: Alternative Fuels". IATA. August 2008. Retrieved 2008-11-18.[dead link]
- Glenn, Edward P.; Brown, J. Jed; O'Leary, James W. (August 1998). "Irrigating Crops with Seawater" (PDF). Scientific American (USA: Scientific American, Inc.) (August 1998): 76–81 . Retrieved 2008-11-17.