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 Clrapidly 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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The growth of the Cd-hyperaccumulator Solanum nigrum L. and its physiological responses to a short-term (7 d) Cd stress and to exogenous methyl jasmonate (MeJA) were investigated. Compared with the leaves of S. nigrum, the roots were more liable to Cd and showed a significantly decreased dry mass and increased malondialdehyde content. Cd accumulation in the shoots and roots of S. nigrum were proportional to the Cd concentration in the hydroponic solution. The application of a low concentration of MeJA (0.01 μM) significantly reduced the translocation/accumulation of Cd in both the shoots and roots compared with a 40 mg dm -3 Cd treatment only. Moreover, 40 mg dm -3 Cd significantly decreased the activity of leaf superoxide dismutase, but 0.01 μM MeJA restored it. MeJA also enhanced the activity of catalase in the leaves but showed no significant effect on peroxidase activity. The content of both endogenous jasmonic acid (JA) and MeJA in the leaves of S. nigrum increased with the increase of exogenous MeJA concentration.

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