Article

Mangroves: obligate or facultative halophytes? A review

Trees (Impact Factor: 1.87). 12/2011; 25(6). DOI: 10.1007/s00468-011-0570-x

ABSTRACT Salinity plays significant roles in regulating the growth and distribution of mangroves, and the salt tolerance mechanisms
of mangroves have been the focus of research for several decades. There are contradictory views regarding the relationship
between mangroves and salt: (1) Mangroves are facultative halophytes, i.e. freshwater is a physiological requirement and salt
water is an ecological requirement for mangroves because they are capable of growing in freshwater. The former prevents excess
respiratory losses while the latter prevents invasion and competition from non-halophytes. (2) Mangroves are obligate halophytes,
i.e. salt is necessary for their growth. Mangroves cannot survive in freshwater permanently and salt water is a physiological
requirement. Up to now, mangroves are usually considered as facultative halophytes. In this review, we provided five lines
of evidence to evaluate these two contradictory views: (1) the results of laboratory culture experiments and field investigations;
(2) the viviparous nature of mangroves; (3) the salt accumulation of mangroves under freshwater or low salinity; (4) the effect
of salinity on the photosynthetic rate and in vitro enzyme activities, and (5) the effects of salinity fluctuation on mangrove
growth and physiology. Contrary to widely accepted view, our evaluations of the aforementioned evidence suggest that mangroves
are obligate halophytes. Mangroves can grow in freshwater for a limited time by drawing upon the nutrients and salt reserves
in their hypocotyls while prolonged culture in freshwater is fatal to them. Mangroves have the ability to absorb Na+ and Cl− rapidly and preferentially under low-salinity conditions. Not all of the enzymes in mangroves are sensitive to salt. In fact,
the activities of some enzymes are even stimulated by low or moderate salinity. Plants grown under constant salinity in a
laboratory setting are unlikely to behave in the same way as those in their natural habitat with fluctuating salinity. Thus,
studies on the effects of freshwater or low salinity and salinity fluctuation on mangroves, as well as the physiological mechanisms
that allow maintenance of function under fluctuating salinity conditions should be strengthened in future research.

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