Halophytes

Halophytes

Halophytes are plants that are able to tolerate the high concentrations of salt found in coastal and marine habitats — conditions in which ordinary land plants, unable to cope with such salinity, cannot survive. Typical halophyte habitats include the muddy, salt-laden, tidally flooded margins of coasts and estuaries, where mangrove trees and their many associated species grow. These are, in effect, the land plants that can tolerate sea or marine salinity, occupying a narrow strip of habitat that most other flowering plants cannot colonise.

Surviving in such a salty environment poses a very specific physiological challenge: ordinary land plants absorb water by osmosis from soil water that has a lower salt concentration than their own root cells, but in a saline coastal soil the surrounding water can be as salty as, or saltier than, the plant's own tissues, which would normally make water uptake difficult and risk excessive salt entering the plant. Halophytes overcome this challenge through several distinct mechanisms.

Some halophytes practise salt exclusion, in which the root membranes are selectively permeable and actively prevent much of the dissolved salt from entering the plant in the first place, while still allowing water to be absorbed. Others practise salt excretion, using specialised salt glands present on their leaves that actively pump out excess salt that has already been absorbed, so that it is deposited on the leaf surface (sometimes visible as fine salt crystals) rather than accumulating inside the plant's tissues. A third mechanism is succulence — storing the water (and the salt dissolved in it) that is absorbed in large, fleshy, water-filled tissues, which dilutes the internal salt concentration to a level the plant's cells can tolerate, in much the same way that a xerophytic succulent stores water to survive drought. …