The salt glands of some mangrove plants remove excess salt using ion transporters that help create a concentrated sodium solution.

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Mangroves are shrubs or small trees that are found in coastal areas where ordinary plants cannot survive. One difficulty they face in their environment is the different salinity of the tides that come in and out from the coast. 

When the mangrove’s root tissues are exposed to salt water, the concentration of salt in the vessels of the root is lower than the concentration of salt in the water surrounding the plant. This concentration gradient would tend to drive salt ions across the plant tissue’s membranes into its cells. However, mangroves have various salt tolerance mechanisms that vary with species: they can exclude salt, accumulate salt, and/or excrete salt. Plants that exclude salt prevent it from entering the membranes of their roots. In other plants that do end up containing excess salt, some  accumulate it into older leaves so it can be shed with the leaves. Others excrete salt, in much higher concentration than seawater, through glands on their leaves. 

Research on the mechanism of salt excretion has led to the hypothesis that a network of channels and pumps moves salt (specifically, sodium ions) between plant cells to the glands that eventually excrete the excess salt. The cytoplasm (inner material) of each plant cell is connected by channels in the cell membranes, enabling cells to communicate, exchange resources, and transfer excess sodium ions. The membranes of the cells closest to the salt glands contain specialized proteins that pump sodium from the cell into the gland. First, proton pumps (H+-ATPases) use chemical energy from the energy-transporting molecule ATP to drive protons into a compartment and establish a proton concentration gradient. Then an ion exchanger, the sodium-hydrogen antiporter, uses the energy of the proton gradient to move sodium ions and protons in opposite directions, at the same time. The process of protons flowing down their concentration gradient releases energy needed by the sodium-hydrogen antiporter to move sodium ions to a compartment already high in sodium. Parts of the gland that aren’t  in contact with the cell are surrounded by a cuticle that prevents ions from flowing back into the cells. The sodium solution becomes concentrated and builds up pressure in the salt gland, which then secretes the salt as a concentrated solution .

This strategy was contributed by Natalie Chen.

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“[Regarding salt glands in general] previous studies on the salt gland ultrastructure in Spartina foliosa (Levering and Thomson, 1971) and T[amarix] aphylla (Thomson et al., 1969) demonstrated that cuticles were present around the salt glands, and they formed a thick barrier from the mesophyll and the external environment. New findings of Distichlis spicata showed that these ions were transported into the salt gland through the bottom penetration area that was not covered by the cuticles of the salt gland, and the cuticles can prevent the ions from backflowing into the mesophyll (Semenova et al., 2010). Ions accumulated in the salt gland via the bottom penetration area and plasmodesmata generated fluid pressure due to the presence of the cuticle, and then secreted through salt gland pores.” (Yuan et al. 2016: 6)

Journal article
Progress in Studying Salt Secretion from Salt Glands in Recretohalophytes: How do Plants Secrete Salt?Frontiers in Plant ScienceJune 30, 2016
Fang Yuan, Bingying Leng, and Baoshan Wang

“As with all multicellular salt glands (Thomson, 1975; Thomson et al., 1988), the cuticle encloses the glands, extending outward from the basal cell along the sides of the glands. ” (Dschida et al. 1992: 504)

Journal article
Epidermal Peels of Avicennia germinans (L.) Stearn: A Useful System to Study the Function of Salt GlandsAnnals of BotanyMay 18, 1992
W. J. Dschida, K. A. Platt-Aloia, and W. W. Thomson

“…we suggest that ions are taken up [and] transported symplastically [through cell cytoplasm and channels] through the glands, and released from the symplast [area beneath the plasma membrane] to the exterior of the glands with the subsequent appearance and accumulation of salt secretions on the surface of the leaves. Also, the initial uptake into the symplast from the leaf apoplast [area within cell walls] is energy dependent, involving the H+/ ATPase [proton pump] in the plasma membrane of the cells with the establishment of an electrochemical proton gradient. This electrochemical proton gradient is utilized by cation carriers and/or channels for uptake. Ion movement through the symplast to the secretory cells of the glands is probably diffusive and cell to cell via plasmodesmata [connecting channels] (Fitzgerald and Allaway 1991). Outward release of the ions from the secretory cells also probably involves the similar establishment of an electrochemical proton gradient that drives the action of cation carriers and/or channels. We note that this model has many similarities to hypothesis of ion transport across roots (Hanson 1978; Clarkson 1991), and there are strong similarities in the evidential bases for these, both structurally and physiologically.” (Balsamo et al. 1995: 667)

Journal article
Electrophysiology of the salt glands of Avicennia germinansInternational Journal of Plant SciencesNovember 3, 2016
Ronald A. Balsamo, Michael E. Adams and William W. Thomson

Journal article
Nitric oxide enhances salt secretion and Na+ sequestration in a mangrove plant, Avicennia marina, through increasing the expression of H+-ATPase and Na+/H+ antiporter under high salinity.Tree PhysiologyOctober 28, 2010
Juan Chen, Qiang Xiao, Feihua Wu, Xuejun Dong, Junxian He, Zhenming Pei, Hailei Zheng, and Torgny Näsholm

Journal article
Salt tolerance mechanisms in mangroves: a reviewTreesFebruary 11, 2010
Asish Kumar Parida, and Bhavanath Jha

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