What Is a Halophyte?
A halophyte is a type of plant that is able to survive in and around land or water high in salt content. These plants may spend their entire life cycle in high saline environments. Most commonly, halophytes are found growing along ocean shorelines, but can also grow in swamps, marshlands, and desert conditions. Some halophytes are known as facultative, which means they are able to survive with or without saline. Other halophytes, known as obligate halophytes, are only able to survive in habitats with high salt concentrations. Researchers estimate that only around 2% of plant species are considered halophytes.
Adaptation to Salt
Halophyte plants thrive in high salt conditions by employing a number of adaptation methods, including tolerance, resistance, and avoidance.
Salt tolerance occurs when the cellular function of a plant operates in the same manner with high salt concentrations as it would operate in freshwater conditions.
Salt resistance occurs when plants allow salt levels to build up within their cells and when plants are able to filter and eliminate salts. When halophytes allow salt to build up at the cellular level, they are known as salt accumulators.
Salt avoidance takes place in one of two ways. Plants can avoid salt by growing their roots into deeper levels of soil, where the concentration of salt is much lower, or by preventing reproduction and growth during certain seasons of the year. For example, germination in salt avoidant halophytes may take place during the rainy season only, when salt content is diluted. These plants are sometimes referred to as salt excluders.
Types of Halophytes
Halophytes occupy one of three categories: aquatic-haline, terrestro-haline, and aero-haline.
Aquatic-haline plants are those that are almost entirely submerged in saltwater. In some cases, only the root system and a small portion of the plant are under water. These aquatic-haline plants are known as emerged halophytes.
Terrestro-haline plants are those that grow in soil with high saline content. In this case, contact between the plant and salt occurs only with roots and soil. These plants tend to be located in arid environments, with the exception of those found in swamps. The terrestro-haline plants found on dry land are known as xero-halophytes.
Aero-haline plants come into contact with salt via droplets of saltwater that move through the air. These saltwater droplets are typically found in coastal regions, where ocean waves crashing against the shore create saltwater spray and fog conditions carry saltwater over the coastal areas. However, in some cases dust blowing across desert-like areas carries salt that is deposited on nearby plants.
Some halophytes are able to survive in both saline and non-saline environments. Some examples of facultative halophytes include plants that belong to the following families: Cyperaceae, Juncaceae, and Gramineae.
The Cyperaceae family is comprised of more than 5,000 species, the majority of which are found in tropical areas of South America and Asia. One of the most well-known examples of a Cyperaceae plant is the papyrus sedge. The papyrus sedge grows in swamp-like habitats and around lake shores throughout Africa, particularly in the Mediterranean regions and in Madagascar. This plant was used by Ancient Egyptians to create one of the first types of paper ever used.
The Juncaceae family is comprised of around 464 species, most of which belong to the Juncus genus. These plants are characterized as slow-growing and are typically considered either a type of sedge or grass. Over history, many Juncaceae plants have been used to make mats or as home insulation. The Juncus effusus, for example, is found on all continents except Antarctica. This plant, also known as the common rush, grows in wetlands, marshes, moors, and riparian areas. It can also be found along streams and ditches.
The Gramineae family is made up of approximately 12,000 species, most of which are considered flowering grasses. These facultative halophytes are one of the most important agricultural plants and include crops such as rice, wheat, barley, bamboo, millet, and maize.
Obligate halophytes are those plant species that require a high saline concentration for survival. One example of this is the Salicornia genus of plants, which belong to the Amaranthaceae family. These halophyte plants are considered succulents and commonly grow along coastal beaches, salt marshes, and mangroves throughout the Americas, Europe, and southern regions of Asia.
Glasswort is a specific example of a Salicornia plant. This species is found in the high saline environments of ocean shorelines in England. Its name refers to the fact that once burned, glasswort ash is an essential component in manufacturing glass and even soap. Given its high salt content, its ash creates sodium carbonate, sometimes known as alkali.
Salt Resistance or Salt Accumulators
Salt accumulating halophytes have a cellular function distinct from other types of plants. Salt resistant plants have developed a specialized cell known as salt glands. These salt glands are able to store extra salt found within the plant system and are located near the surface of plant leaves. When the salt gland is filled to capacity, the gland breaks open and releases the extra salt to the exterior of the plant leaf. This action leaves a fine layer of salt over the outside of the plant. However, in some cases the salt gland does not break, which ultimately causes the plant to die.
Salt Avoidance or Salt Excluders
Some halophytes regulate their exposure to high saline conditions by adapting their reproductive cycle. Other salt excluders practice defoliation, also known as leaf dropping. This reaction occurs because the salt content is forced to accumulate in the small stem that connects the leaf to the principal stem. Once the salt maximum has been reached, the leaf and its stem are dropped from the plant.
Other salt excluders have developed specialized root systems that prevent salt from being absorbed by the plant. Additionally, some roots have a specialized cellular structure in which the inner part of the cell is covered in a wax-like substance. This substance prevents the cells from absorbing saline. Another adaptation in some salt avoidant halophytes is that the roots actually extract salt from the plant and deposit it into the surrounding soil.