Saline habitats occur along bodies of saltwater, e.g., coastal salt marsh, and inland within high-evaporation basins, saline lakes, and lowlands of dryland and desert topography. The electrolytes sodium (a cation) and chloride (an anion) are extremely toxic to most plants at relatively low soil water concentrations, due to deleterious effects on cellular metabolism and ultrastructure.
A saline habitat often has a low diversity of plants, sometimes even just one dominant species, because so few species are able to resist salt damage, and those that do have special microhabitats that are optimal.
Salt resistance is the reaction of an organism to salt stress. Resistance can involve either salt tolerance or salt avoidance.
Halophytes are often classified as excretives and succulents. Another classification recognizes excluders versus includers.
The genus Atriplex (Family Chenopodiaceae), saltbush, is worldwide in saline shoreline and inland habitats. On the surfaces of the leaf are vesiculated trichomes (hairs). Each trichome has a stalk and a balloon-like tip, the bladder cell. Species of Atriplex are excluders. The leaves sequester excess electrolytes in the bladder cells, which release the salt back into the environment when they are ruptured. The plant accumulates sodium, potassium, and chloride ions in the bladder using active transport to remove these toxins from the mesophyll cells and place them instead within the vacuole of the bladder cell. Leaves of Atriplex have a silvery reflectance, due to the presence of this layer of trichomes, which has also been shown to prevent some ultraviolet light from reaching the leaf tissues.
Excreting salt glands occur in numerous unrelated plant groups certain grasses that live in alkali sink and salt pans and in warm temperate to tropical salt marshes. The salt excreting species occur in three tribes of grasses, Sporoboleae, Chlorideae, and Aeluroideae (Family Poaceae). These include such well-known tidal saltmarsh plants as the cordgrasses (Spartina alterniflora, S. patens), alkali grass (Puccinellia phryganodes), saltgrass (Distichlis spicata), and shoregrass (Monanthochloe littoralis). Short, two-celled trichomes have evolved as collecting chambers for salts, principally between the cuticle and the wall of the cap cell. From there it is forced through tiny pores in the stretched cuticle.
Frankenia (Family Frankeniaceae) is a genus found both in coastal salt marsh and inland saline habitats. Saltcedar (Tamarix, Family Tamaricaceae) originated in inland saline habitats of Asia, but readily grows around the world in arid and semiarid regions as an uncontrolled weed along rivers and standing water. Other genera of Tamaricaceae grow in saline soils. All of these taxa have salt glands, i.e., trichomes, in which salts are accumulated and excreted, likely through cuticular pores.
Several common mangroves secrete salts from leaves. So much crystalline salt can occur on the leaf of black mangrove, Avicennia germinans, that it appears whitish. Aegialitis annulata is a salt-excreting mangrove of Family Plumbaginaceae, a group in which other species are salt excreters in salt marsh and oceanside (Limonium and Armeria) or have chalk glands and live in dry, alkaline habitats.
Halophytic succulence is demonstrated in many genera of plants that inhabit saline environments, such as the mud flats of coastal salt marsh, but also is commonly observed on the leading edge of vegetation on coastal beaches, i.e., in strand vegetation. Common examples of succulence can be found in Family Chenopodiaceae:
Succulence (actually fleshiness) also occurs in other famous and widespread species, such as Batis maritima, Sesuvium portulacastrum, Trianthema portulacastrum, and many Old World species of Zygophyllum. True succulents in the ice plant family, Aizoaceae, are also commonly found in saline habitats: Aizoon and Mesembryanthemum. The large central vacuole in thin-walled celled of these watery leaves and stems accumulate substantial quantities of salts. By depositing ions of salts in vacuoles, the toxicity is partitioned from the cytoplasm and organelles of the cells. As in Atriplex, salts are removed from the plant when the leaf of stem segment is shed.
Among the halophytes, the seagrasses, which are submerged plants of shallow marine meadow, have salt tolerance, because the osmolity of cytoplasm is adjusted to match the osmolity of the seawater, so that desiccation of the plant does not occur. This contrasts the strategies of excluders and succulents, which are primarily salt avoiding mechanisms, attempts to reduce salt concentrations within the cytoplasm of photosynthetic cells.
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