Salt balance in mangroves

The tidal zone of tropical seas is frequently lined with great mangrove forests, the dominating species of which belong to such genera as Rhizophora, Bruguiera, Sonneratia, and Avicennia. The outstanding feature of these plants is their adaptation to growing in seawater, and even though all do well in river estuaries they seldom penetrate inland beyond the direct action of ocean tides. A question which immediately presents itself is How do these plants handle the salt in the seawater ? Do the roots exclude it from the transpiration stream or do the plants possess special organs for eliminating such salts as may penetrate into the sap? What balance, if any, exists between osmotic potential of seawater, roots, and leaves on one hand and hydrostatic pressure and osmotic potential of the xylem sap on the other? Our aim has been to inquire into these matters. It has long been realized that various mangroves behave differently with respect to some of these parameters. It is thus easily ascertained that certain species accumulate salt on their leaves. In Aegialitis and Aegiceras salt crystals can be seen covering the leaves and in Avicennia and Acanthus ilicifolia one may easily taste the salt. In other species like Sonneratia, Rhizophora, Bruguiera, Ceriops, and Lumnitzera salt can neither be seen nor tasted (5, 17). Various authors agree that press juices of mangrove leaves have a high osmotic potential, being more or less isotonic with seawater (5, 6, 17). Walter and Steiner, using the same species of Rhizophora, Sonneratia and Avicennia in East Africa as we worked on in Australia, found that the press juices of roots also showed similar values. They determined the transpiration rate of mangrove leaves to be about one-third of that of ordinary plants (20). Some of the mangroves possess salt glands on the leaves, visible by naked eye as minute dimples in the surface. The histology of the glands has been described in Aegialitis by Ruhland (9) and Avicennia by Walter and Steiner (20), but no experimental studies seem to have been performed on these. However Ruhland (9) determined the amount of salt given off by the isolated leaves of statice (Limonium latifolium), and using leaf disks of the same species, Arisz, Camphuis, Heikens, and Van Tooren (2) found that these would secrete salt when floated on a saline solution. The secreted fluid under certain conditions became more concentrated than the medium. This process was stopped by cyanide and other respiratory poisons. Large mangrove stands are typically rooted in deep muck which is completely anaerobic from decomposing materials. In such habitats the roots are conspicuously swollen by a spongy pneumatic tissue which communicates to the air through a multitude of lenticels located on stilt roots (Rhizophora, Bruguiera) or special pneumatophores (Avicennia, Sonneratia). The ventilatory function of these structures has been studied in detail in Avicennia and Rhizophora (4, 16, 19).