Effects of salinity on water potential and salt concentrations in xylem of mangrove trees

Abstract

The effects of salinity on water potential and salt concentrations were investigated in four species of mangroves. Seedlings of Avicennia alba, Bruguiera gymnorrhiza, Heritiera littoralis and Xylocarpus granatum were grown at salinities of 0, 10, 20, 30, 40 and 60 ppt in a greenhouse at the Department of Botany, Faculty of Science, Chulalongkorn University. All four species could grow at salinities of 0, 10, 20, 30 and 40 ppt. Plant biomass was closely linked to salinity. A salinity of 10 ppt was found to be optimal for growth. After 11 months growth at this salinity biomass accumulation in Avicennia alba, Bruguiera gymnorrhiza and Xylocarpus granatum were 17.7, 94.8 and 63.6 g/plant, respectively. Heritiera littoralis grew best at 0 ppt salinity, in which biomass accumulation after 11 months of growth was 44.4 g/plant. Seedlings of all species died at a salinity of 60 ppt. Na and Cl concentrations in Avicennia alba and Bruguiera gymnorrhiza xylem sap increased with increasing salinity. Na and Cl concentrations in Avicennia alba xylem sap at 40 ppt salinity were 114.23 and 113.64 mmol/l, respectively. Na and Cl concentrations in Bruguiera gymnorrhiza xylem sap at 40 ppt salinity were 6.96 and 4.10 mmol/l, respectively. Analysis of pressure-volume curves showed that shoot water potential in all four species decreased with increasing salinity due to a reduction in osmotic potential. The decrease in osmotic potential was attributed to increasing solute concentrations, mainly Na and Cl, in the leaves of all species except Heritiera littoralis, which had remarkably little Na and Cl in the leaves. The results indicated that Bruguiera gymnorrhiza, which does not have salt-secreting glands, was more efficient in excluding salt than Avicennia alba, which has salt-secreting glands