Spartina alterniflora (cordgrass), a halophyte.

A halophyte is a salt-tolerant plant that grows in soil or 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'. Halophytes have different anatomy, physiology and biochemistry than glycophytes.[1] 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. Information about many of the earth's halophytes can be found in the ehaloph database.

The large majority of plant species are glycophytes, which are not salt-tolerant and are damaged fairly easily by high salinity.[2]

Classification

Halophytes can be classified in many ways. According to Stocker (1933), it is mainly of 3 kinds by habitat, viz.

  1. Aqua-halines (aquatic plants)
    • Emerged Halophytes (most of the stem remains above the water level)
    • Hydro-halophytes (whole or almost whole plant remains under water)
  2. Terrestro-halines (terrestrial plants)
    • Hygro-halophytes (grow on swamp lands)
    • Mesohalophytes (grow on non-swamp, non-dry lands)
    • Xero-halophytes (grow on dry or mostly dry lands)
  3. Aero-halines (epiphytes and aerophytes)

Again, according to Iversen (1936), these plants are classified with respect to the salinity of the soil on which they grow.[3]

  1. Oligo-halophytes (amount of NaCl in the soil is 0.01 to 0.1%)
  2. Meso-halophytes (amount of NaCl in the soil is 0.1 to 1%)
  3. Euhalophytes (amount of NaCl in the soil is >1%)

For comparison, seawater has a salinity of about 3.5%. See water salinity for other reference levels.

Habitats of halophytes

Major habitats where halophytes flourish include mangrove swamps, sand and cliff shorelines in the tropics, salt deserts and semi-deserts, the Sargasso Sea, mudflats and salt marshes, kelp forests and beds, salt lakes and salt steppes of the Pannonian region, wash fringes, isolated inland saline grasslands, and in places where people have brought about salination.[4]

Salt tolerance

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.[5] 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.[2]

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.

Pneumatophores of Grey mangrove

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 salt glands, or by concentrating salts salt bladders in leaves that later die and drop off.[1]

In an effort to improve agricultural production in regions where crops are exposed to salinity, research is focused on improving understanding of the various mechanisms whereby plants respond to salinity stress, so that more robust crop halophytes may be developed. Adaptive responses to salinity stress have been identified at molecular, cellular, metabolic, and physiological levels.[6]

Examples

Some halophytes are:

Taxon Common name(s) Habitat type Tolerance type
Anemopsis californicayerba mansa, lizard tailHygro
Atriplexsaltbush, orache, orachXero
Attalea speciosababassuMeso
Panicum virgatumswitchgrassMeso, Xero
Salicornia bigeloviidwarf glasswort, pickleweedHygroEu (seawater)
Spartina alterniflorasmooth cordgrassEmerged, HygroEu (seawater)
Tetragonia tetragonoideswarrigal greens, kōkihi, sea spinachHygroEu (seawater)
Dunaliella(a green alga)HydroEu (seawater)
Sesuvium portulacastrumsea purslane, shoreline purslaneHygroEu (seawater)
SuaedaSeep-weedsHygroEu (seawater)
Halimione portulacoidessea purslaneHygroEu (seawater)
Sarcocornia fruticosasaltworts ?

Uses

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.[5][7][8]

Phytoremediation

Halophytes like Suaeda salsa can store salt ions and rare-earth elements absorbed from soils in their tissues.[9] Halophytes can therefore be used in Phytoremediation measures to adjust salinity levels of surrounding soils.[10] These measures aim to allow glycophytes to survive in previously uninhabitable areas through an environmentally safe, and cost effective process.[11] A higher concentration of halophyte plants in one area leads to higher salt uptake and lower soil salinity levels.[9]

Different species of halophytes have different absorption capabilities.[10] Three different halophyte species (Atriplex patula, Atriplex hortensis, and Atriplex canescans) have been found to rehabilitate soils contaminated with road salt over varying lengths of time.[11]

See also

References

  1. 1 2 Physiology of halophytes, T. J. FLOWERS, Plant and Soil 89, 41-56 (1985)
  2. 1 2 Glenn, E. P.; et al. (1999). "Salt tolerance and crop potential of halophytes". Critical Reviews in Plant Sciences. 18 (2): 227–55. doi:10.1080/07352689991309207.
  3. ↑ "Halophytes: Classification and Characters of Halophytes". 29 January 2015.
  4. ↑ Kapler, Adam. 2019. Habitats of Halophytes. In: Halophytes & Climate Change: Adaptive Mechanisms and Potential Uses. Edited by Mirza Hassanuzzaman, Sergey Shabala, & Masayuki Fujita. CAB International. Pp. 19-37.
  5. 1 2 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.
  6. ↑ Gupta, Bhaskar; Huang, Bingru (3 April 2014). "Mechanism of Salinity Tolerance in Plants: Physiological, Biochemical, and Molecular Characterization". International Journal of Genomics. 2014: 701596. doi:10.1155/2014/701596. PMC 3996477. PMID 24804192.
  7. ↑ "Fact Sheet: Alternative Fuels". IATA. December 2013. Archived from the original on 2014-02-01. Retrieved 2014-01-28.
  8. ↑ Bresdin, Cylphine; Glenn, Edward P.; Brown, J. Jed (2016), "Comparison of Seed Production and Agronomic Traits of 20 Wild Accessions of Salicornia bigelovii Torr. Grown Under Greenhouse Conditions", Halophytes for Food Security in Dry Lands, Elsevier, pp. 67–82, doi:10.1016/b978-0-12-801854-5.00005-4, ISBN 978-0-12-801854-5, retrieved 2022-03-05
  9. 1 2 Liang, Jiaping; Shi, Wenjuan (2021). "Cotton/halophytes intercropping decreases salt accumulation and improves soil physicochemical properties and crop productivity in saline-alkali soils under mulched drip irrigation: A three-year field experiment". Field Crops Research. 262: 108027. doi:10.1016/j.fcr.2020.108027. S2CID 230576810.
  10. 1 2 Brito, Pedro; Caetano, Miguel; Martins, Marcelo D.; Caçador, Isabel (December 2020). "Effects of salt marsh plants on mobility and bioavailability of REE in estuarine sediments". Science of the Total Environment. 759: 144314. doi:10.1016/j.scitotenv.2020.144314. PMID 33338692. S2CID 229325441.
  11. 1 2 Mann, Ellen; Rutter, Allison; Zeeb, Barbara (October 2020). "Evaluating the efficacy of Atriplex spp. in the phytoextraction of road salt (NaCl) from contaminated soil". Environmental Pollution. 265 (Pt B): 114963. doi:10.1016/j.envpol.2020.114963. PMID 32806446. S2CID 221162683.
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