Mangroves: Obligate or facultative halophytes? A review

Trees (Impact Factor: 1.65). 12/2011; 25(6). DOI: 10.1007/s00468-011-0570-x


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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    • "Several studies examine the physiological responses of plants to the spatial heterogeneity of soil salinity, as mentioned in the review by Bazihizina et al. (2012). However, as pointed out by Krauss et al. (2008), Wang et al. (2011), and then Krauss and Ball (2013), there is a lack of knowledge on the growth and physiological responses of halophytic species to fluctuating salinity. It appears that exposure to constant salinity could be less physiologically demanding than exposure to fluctuating salinity, due to the high energetic cost of acclimation investments (Krauss et al. 2008). "
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    ABSTRACT: Background Micro-tidal wetlands are subject to strong seasonal variations of soil salinity that are likely to increase in amplitude according to climate model predictions for the Caribbean. Whereas the effects of constant salinity levels on the physiology of mangrove species have been widely tested, little is known about acclimation to fluctuations in salinity. Aims and methods The aim of this experiment was to characterize the consequences of the rate of increase in salinity (slow versus fast) and salinity fluctuations over time versus constant salt level. Seedling mortality, growth, and leaf gas exchange of three mangrove species, Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle were investigated in semicontrolled conditions at different salt levels (0, 685, 1025, and 1370 mM NaCl). Results Slow salinity increase up to 685 mM induced acclimation, improving the salt tolerance of A. germinans and L. racemosa, but had no effect on R. mangle. During fluctuations between 0 and 685 mM, A. germinans and R. mangle were not affected by a salinity drop to zero, whereas L. racemosa took advantage of the brief freshwater episode as shown by the durable improvement of photosynthesis and biomass production. Conclusions This study provides new insights into physiological resistance and acclimation to salt stress. We show that seasonal variations of salinity may affect mangrove seedlings’ morphology and physiology as much as annual mean salinity. Moreover, more severe dry seasons due to climate change may impact tree stature and species composition in mangroves through higher mortality rates and physiological disturbance at the seedling stage.
    Full-text · Article · Apr 2014 · Plant and Soil
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    • "Acclimation, recovery, and opportunism of seedlings during a freshwater episode Only a few studies have investigated the effect of salinity fluctuations in swamp forests (Ball and Farquhar 1984; Lin and Sternberg 1993; Bompy et al. 2014). Several authors formulated the hypothesis that some plants may take advantage of low salinity periods to survive in hypersaline habitats (Orcutt and Nilsen 2000; Munns 2002; Hogarth 2007; Lambs et al. 2008; Wang et al. 2011). Our results obtained on P. officinalis seedlings confirm this hypothesis. "
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    ABSTRACT: Key message Pterocarpus officinalis is able to 1) improve its acclimation capacity if soil salinity increases slowly and 2) benefit from a freshwater episode. Abstract One likely effect of global change is an increase of the amplitude of salt variations in the soil of brackish coastal wetland forests. In the Antilles, such forests are dominated by the species Pterocarpus officinalis. The study aimed to determine the effect of 3 salinity levels (freshwater, moderate and hypersalinity – i.e. 0, 10 and 30 ‰, respectively) and 3 patterns of salinity variation (fast or slow salinity increase, fluctuating salinity) on the growth and ecophysiology of P. officinalis seedlings. P. officinalis proved tolerant to 10 ‰ salinity, even if at this salt concentration the water constraint altered the plant's water status and reduced stomatal conductance. No impact of the pattern of salinity variation was observed at 10 ‰. Seedlings were strongly affected by hypersalinity, but were able to acclimatize efficiently and to improve their performances (higher survival, total biomass and photosynthesis) when salinity increased slowly. Young P. officinalis were also able to take advantage of a freshwater episode on the longer term, certainly through leaf desalination associated with enhanced photosynthesis and water use efficiency. Higher soil salinity and more intense dry seasons in the context of climate change could affect the stand-level regeneration potential of P. officinalis seedlings.
    Full-text · Article · Feb 2014 · Trees
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    • "Trees of A. nitens and C. laurifolia grow vigorously at the savanna-end of the gradient, where they experience flooding of low height and short duration, and trees of Andira inermis, among others, grow to very large size in areas of the savanna far removed from the igapó. The apparent lack of need for flooding in these species contrasts with the requirement of salt in other flood-tolerant trees, mangroves, in many of which PN and growth increase in response to salinity up to an optimum (Wang et al., 2011). "
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    ABSTRACT: This review summarizes the research on physiological responses to flooding of trees in the seasonal black-water wetland of the Mapire River in Venezuela. Inter-annual variability was found during 8 years of sampling, in spite of which a general picture emerged of increased stomatal conductance (gs) and photosynthetic rate (PN) during the flooded period to values as high as or higher than in plants in drained wet soil. Models explaining the initial inhibitory responses and the acclimation to flooding are proposed. In the inhibitory phase of flooding, hypoxia generated by flooding causes a decrease in root water absorption and stomatal closure. An increase with flooding in xylem water potential (ψ) suggests that flooding does not cause water deficit. The PN decreases due to changes in relative stomatal and non-stomatal limitations to photosynthesis; an increase in the latter is due to reduced chlorophyll and total soluble protein content. Total non-structural carbohydrates (TNC) accumulate in leaves but their content begins to decrease during the acclimatized phase at full flooding, coinciding with the resumption of high gs and PN. The reversal of the diminution in gs is associated, in some but not all species, to the growth of adventitious roots. The occurrence of morpho-anatomical and biochemical adaptations which improve oxygen supply would cause the acclimation, including increased water absorption by the roots, increased rubisco and chlorophyll contents and ultimately increased PN. Therefore, trees would perform as if flooding did not signify a stress to their physiology.
    Full-text · Article · May 2013 · Frontiers in Plant Science
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