Assessment and Comparison of salt Content in Mangrove Plants in Sri Lanka

Abstract

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.