Salt balance in mangroves

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Abstract
Some species, like Aegialitis and Avicennia, eliminate large quantities of salts through special glands on the leaves; others, like Rhizophora and Sonneratia, do not. The excreted fluid is often more concentrated than seawater, and shows a marked diurnal cycle with maximum in the daytime. The xylem sap in the salt-secreting species carries about 0.2%-0.5% NaCl; the non-secreting species 10 times less. The osmotic potential of the mangrove sap is therefore at most a few atm. Three different approaches indicate that the sap pressure is usually below ambient, but that it seldom becomes negative and then only by a few atm. It would therefore seem premature to postulate that the salt separation by the roots is a simple ultrafiltration, for this would demand a permanent sap pressure of -20 atm. The root system of mangroves is ventilated by air, and it seems more likely that the separation involves a case of active transport.

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