Salt/water relationships in mangroves

Mangroves form extensive, unique communities in tropical coastal areas and tidal lowlands, dominating 60-75% of tropical shorelines. The aim of the present review is to summarize the current knowledge concerning the mechanisms underlying the most striking feature of these plants - their unique ability to obtain water from the surrounding sea. Mangroves are thought to accomplish this by rejecting potentially harmful salts. Some species actively excrete those salts leaking into the plant by means of specialized salt glands in their leaves. Mangroves are rooted in anaerobic soils, a condition giving rise to the spectacular aerial roots, such as pneumatophores and stilt roots, characteristic of mangroves, that provide oxygen to submerged tissues. We shall also discuss recent studies that have focused on physiological issues in mangroves, such as oscillatory behavior of their stomata, the structure and function of salt glands, and the compatible solutes in their leaves, which balance the osmotic pressure of the seawater. Salinity effects on their germination, distribution, CO₂ assimilation, respiration, and the functioning of some of their enzymes have also been examined. Finally, we shall draw attention to open questions related to the salt and water regime of mangroves and the underlying mechanisms responsible for their remarkable success in a hostile environment.