Abstract. Water translocation in mangrove seedlings is often affected by water stress conditions such as drought, hyper-salinities and their frequent variations. This study was therefore aimed at studying the wood anatomical responses of xylem tissue and hydraulic conductivity of Rhizophora mucronata Lam., a common species in mangrove planting, under different levels of drought [25%, ~50% and ~100% of water holding capacity (WHC)] and soil salinity [high salinity (35 psu), moderate salinity (15 psu) and freshwater (0 psu)]. As wood anatomical responses, significantly higher vessel density, vessel grouping (P<0.001) along with narrow vessel elements (P<0.001) were observed in plants grown in the 25% and 50% WHCs and high salinity treatments. All these anatomical responses are more directed towards avoidance of vessel cavitation which is commonly found under water deficit conditions. The results showed that R. mucronata plants failed to maintain efficient transportation of water when the field capacity was 50% of WHC or lower and the level of salinity was 35 psu or greater, as evident by the reduction of water conductive areas, vessel areas and hydraulic conductivity (P<0.05). Overall, water use efficiency of R. mucronata seedlings under the imposed water stress conditions has remarkably reduced and it further indicated that such imposed stress conditions directly affect the survival of planted seedlings as depicted by the significantly low survival in 25% and 50% of WHCs and high salinity. Therefore, in-depth study on lagoon hydrology including inundation levels, water depth, salinity and the selection of correct tidal positioning is highly recommended as prerequisites in mangrove planting.

A method to measure the salt secretion by mangroves which are open to the sea spray was developed and used to measure the salt secretion by mangrove species of Rekawa lagoon, an ecosystem with the highest diversity of true mangroves in southern Sri Lanka. Out of the twelve species of mangroves, only four species i.e. Acanthus ilicifolius, Aegiceras corniculatum, Avicennia marina, and Avicennia officinalis proved to be able to secrete salts. Under the salinity regime (26.2 ± 4.41 ppt) existed during the experimental period, the salt secretion by leaves of these four species were 47.2 ± 18.3, 35.1 ± 16.0, 149.3 ± 45.9, and 81.6 ± 30.5 mg salt cm −2 day −1 respectively. This result corroborates the published records and, hence validates the technique used in this study to measure the salt secretion. These four species exhibited increases in salt secretion with increases in soil salinity, consistent with previous reports. Results of this study also shows that the capacity to secrete salts at any given salinity was different between four species, following an order of Av. marina > Av. officinalis > Ac. ilicifolius, and Ae. corniculatum. The salt secretion by these species immediately after reducing the soil salinity was increased significantly implying an opportunistic removal of salt, which was accumulated in the plant body under high saline condition. Capacity for salt secretion by the four species as well as the magnitude of the increase of salt secretion as a response to increasing soil salinity, vary in parallel to the variations in salt tolerance given in published reports for the same species.

Due to the predicted threats of global warming and sea level rise, the salt tolerance and salt accumulative abilities of plants have become popular contentious topics. Mangroves are one of the major groups of salt tolerant plants and several mechanisms are known as instrumental in their salt tolerance. Salt excretion through leaf drop is given as one, but its validity is questioned by some recent works compelling the necessity for further studies. Knowledge of the salt contents in different mangrove plants is a pre requisite for such studies. Hence, this study aimed to quantify and compare the salt content in mature leaves of nine mangrove species in Sri Lanka., i.e. Aegiceras corniculatum, Avicennia marina, Avicennia officinalis, Bruguiera gymnorrhiza, Bruguiera sexangula, Ceriops tagal, Excoecaria agallocha, Lumnitzera racemosa, Rhizophora apiculata and Rhizophora mucronata which are growing in the same mangrove system; the Rekawa lagoon in Sri Lanka. Two species of non mangrove plants, Gliricidia sepium and Artocarpus heterophyllus, which were growing in inland areas were also selected for comparison. The concentration of Na + in leaves was considered as a measure of the salt concentration. The Na + in leaves was extracted by acid digestion and quantified by flame photometry. The salt content of mangroves was measured under two contrasting hydrological situations: at the highest and lowest water levels of the lagoon. Rekawa lagoon can be considered as a 'barrier built estuary', the highest water level occurs when the lagoon mouth is blocked due to the formation of the sand bar and the water level is increased by fresh water inflow, inundating the total mangrove area and decreasing the soil/water salinity. The water level of the lagoon becomes lowest when the lagoon mouth is opened (naturally or by dredging) and lagoon water is flushed out to the sea. Then the salinity of lagoon water becomes high due to sea water influx. The results showed that the concentration of Na + in mangrove leaves was 3 to 12 times higher compared to that in leaves of selected non mangroves. Statistical analysis revealed that the variations in Na + content in leaves of different mangrove species were same under both hydrological regimes. E. agallocha and R. mucronata showed the highest salt content whilst A. corniculatum B. sexangula showed the lowest salt content. The three species, A. marina, A. officinalis and L. racemosa, showed the second highest salt content and the remaining two species C. tagal and B. gymnorhiza, showed the second lowest salt content. Apparently the interspecific variation in the concentration of Na + in mangrove leaves follow the interspecific variations in the salinity tolerance reported for the same species.

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Alligator weed (Alternanthera philoxeroides (Mart.) Griseb) is an invasive weed in different countries of South America, North America, Australia and Asia. It can grow in range of habitats from aquatic to terrestrial environments. It is one of the non-native plants which was invading natural and agricultural eco systems in Sri Lanka and has recorded as a threat to the native species and thereby to the plant bio diversity. It was an accidental introduction to Sri Lanka due to misidentification as a commonly cultivated leafy vegetable A. sessilis (Mukunuwenna). So it is highly possible to include A. philoxeroides in the market samples of leafy vegetables due to this similarity. A. philoxeroides can propagate through vegetative fragmentation and these vegetative propagules can be dispersed by water in aquatic habitats. An over view of all aspects of A. philoxeroides including its geographical distribution and spread in the world, morphology and ecological aspects, environmental impacts, success of the management options used and present status of this weed in Sri Lanka is presented by this article. Management of alligator weed through chemical, biological, and mechanical means are effective up to some extent in Sri Lanka but it is still a problem in many areas Therefore well-planned, long-term mechanism to evaluate the role of different factors which is responsible for rapid infestation is essential to control the weed. Future research should be focused more on integration weed management approaches in both natural and agricultural ecosystems and to identify the weed from its congener, A. sessilis.