Mangroves: obligate or facultative halophytes? A review

Trees (Impact Factor: 1.93). 01/2011; 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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    ABSTRACT: Rhizophora mangle and Laguncularia racemosa cooccur along many intertidal floodplains in the Neotropics. Their patterns of dominance shift along various gradients, coincident with salinity, soil fertility, and tidal flooding. We used leaf gas exchange metrics to investigate the strategies of these two species in mixed culture to simulate competition under different salinity concentrations and hydroperiods. Semidiurnal tidal and permanent flooding hydroperiods at two constant salinity regimes (10 g L−1 and 40 g L−1) were simulated over 10 months. Assimilation ( 𝐴 ), stomatal conductance ( 𝑔 𝑤 ), intercellular CO2 concentration ( 𝐶 𝑖 ), instantaneous photosynthetic water use efficiency (PWUE), and photosynthetic nitrogen use efficiency (PNUE) were determined at the leaf level for both species over two time periods. Rhizophora mangle had significantly higher PWUE than did L. racemosa seedlings at low salinities; however, L. racemosa had higher PNUE and 𝑔 𝑤 and, accordingly, had greater intercellular CO2 (calculated) during measurements. Both species maintained similar capacities for A at 10 and 40 g L−1 salinity and during both permanent and tidal hydroperiod treatments. Hydroperiod alone had no detectable effect on leaf gas exchange. However, PWUE increased and PNUE decreased for both species at 40 g L−1 salinity compared to 10 g L−1. At 40 g L−1 salinity, PNUE was higher for L. racemosa than R. mangle with tidal flooding. These treatments indicated that salinity influences gas exchange efficiency, might affect how gases are apportioned intercellularly, and accentuates different strategies for distributing leaf nitrogen to photosynthesis for these two species while growing competitively.
    International Journal of Forestry Research 08/2013; 2013.
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    ABSTRACT: Scientists have discussed the halophytic nature of intertidal plants for decades, and have generally suggested that inherent differentiation of an obligate halophyte from a facultative halophyte relates strongly to whether the plant can survive in fresh water, and not much else. In this mini-review, we provide additional insight to support the pervasive notion that mangroves as a group are truly facultative halophytes, and thus add discourse to the alternate view that mangroves have an obligate salinity requirement. Indeed, growth and physiological optima are realized at moderate salinity concentrations in mangroves, but we maintain the notion that current evidence suggests that survival is not dependent upon a physiological requirement for salt.
    Trees 02/2012; 27(1). · 1.93 Impact Factor


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