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.

The latitudinal distribution of mangrove species is limited mainly by low temperature. Leaf scorch and massive leaf fall are the predominant symptoms of frost damage. Nutrient resorption during leaf senescence is an important adaptation mechanism of mangroves. Abnormal defoliation disturbs nutrient resorption. We evaluated the effects of frost on nutrient loss of mangroves and the protective effects of warmer seawater inundation on reducing nutrient loss. On January 14, 2009, the most cold-tolerant mangrove Kandelia obovata at its naturally latitudinal limit (Fuding, China, 27°17′N) was exposed to freezing temperature (−2.4°C) for 4 h (minimum −2.8°C). The freezing air temperature occurred during flood tide, resulting that the flooded shoots were protected by warmer seawater. Frost caused 31.3% and 13.0% defoliation on the exposed shoots and the flooded shoots, respectively. Frost restricted nutrient resorption during leaf senescence. K. obovata resorbed 61% N and 42% P during normal leaf senescence, respectively. However, frost-damaged leaves only resorbed 13% N and 10% P during the course, respectively. Foliar N:P molar ratios were <31, suggesting N limitation. Tidal inundation can partially protect mangroves from frost damage. Reduced nutrient resorption efficiency and massive leaf fall caused by frost add pressure to mangroves under nutrient limitation at their latitudinal limits.

Xylem-tapping mistletoes are known to have generally higher transpiration rate (Tr), lower CO2 assimilation rate (A) and therefore lower water-use efficiency (WUE) than their hosts. There are long-standing contradictions in water relations and nitrogen use in photosynthesis. Gas exchange, chlorophyll fluorescence and nutrition components were investigated in a special mistletoe–host pair, Viscum ovalifolium–Sonneratia caseolaris, as the host was a mangrove growing in a saline environment. Our results show that both plants had high foliar N content, therefore it was consistent with the N-parasitism hypothesis, although the mistletoe had a lower Tr than its mangrove host. It was suggested that the mistletoe reduces its Tr under salt stress with N sufficient conditions. The mistletoe had a fundamental limitation of photosynthesis, and was photoinhibited with regard to high salinity, but it developed more photoprotection to thermal radiation. Additionally, both stomatal conductance (gs) and mesophyll conductance (gm) limitations on photosynthesis dominated in the mistletoe under salt stress even though it had a high foliar N content similar to the host.

Salinity is a vital factor that regulates leaf photosynthesis and growth of mangroves, and it frequently undergoes large seasonal and daily fluctuations creating a range of environmentsoligohaline to hyperhaline. Here, we examined the hypotheses that mangroves benefit opportunistically from low salinity resulting from daily fluctuations and as such, mangroves under daily fluctuating salinity (FS) grow better than those under constant salinity (CS) conditions. We compared growth, salt accumulation, gas exchange, and chlorophyll fluorescence of leaves of mangrove Bruguiera gymnorhiza seedlings growing in freshwater (FW), CS (15 practical salinity units, PSU), and daily FS (0-30 PSU, average of 4.8 PSU) conditions. The traits of FS-treated leaves were measured in seedlings under 15 PSU. FS-treated seedlings had greater leaf biomass than those in other treatment groups. Moreover, leaf photosynthetic rate, capacity to regulate photoelectron uptake/transfer, and leaf succulence were significantly higher in FS than in CS treatment. However, leaf water-use efficiency showed the opposite trend. In addition to higher concentrations of Na + and Cl − , FS-treated leaves accumulated more Ca 2+ and K + . We concluded that daily FS can enhance water absorption, photosynthesis, and growth of leaves, as well as alter plant biomass allocation patterns, thereby positively affecting B. gymnorhiza. Mangroves that experience daily FS may increase their adaptability by reducing salt build-up and water deficits when their roots are temporally subjected to low salinity or FW and by absorbing sufficient amounts of Na + and Cl − for osmotic adjustment when their roots are subsequently exposed to saline water. K E Y W O R D S fluctuating salinity, fluorescence, mangrove, osmotic adjustment, photosynthesis, salt tolerance

The ranging error model is generally very complicated in actual ranging technologies. This paper gives an analysis of the biased distance substitution and proposes an unbiased multilateral positioning method to revise the biased substitution, making it an unbiased estimate of the squared distance. An unbiased estimate of the multilateral positioning formula is derived to solve the target node coordinates. Through simulation experiments, it is proved that the algorithm can improve the positioning accuracy, and the improvement is more obvious when the error variance is larger. Experiments using SX1280 also show that the ranging conforms to the biased error model, and the accuracy can be improved by using the unbiased estimator. When the actual experimental error standard deviation is 0.16 m, the accuracy can be improved by 0.15 m.