Growth and Osmotic Relations of the Mangrove Avicennia marina, as Influenced by Salinity

Abstract

Seeds of the mangrove Avicennia marina were germinated and grown for up to 11 months on nutrient solution or nutrient solution containing 10, 25, 50, 75 or 100% natural seawater (500 mM sodium; 580 mM chloride). Early seedling development was most rapid in the absence of sodium chloride, but soon declined. As a result, biomass production in this treatment was poorest. The response was associated with the appearance of necrotic lesions. The growth optimum based on fresh or dry weight occurred on 10, 25 or 50% seawater. Plants receiving higher salinities, particularly full strength seawater, were slow to develop, and low in biomass, but healthy in appearance. Seeds taken from parent trees growing on tidal mudflats had osmotic potentials more negative than seawater, but contained little sodium or chloride, potassium being the most abundant inorganic ion. The osmotic potentials of the seedlings were more negative than those of the external watering solutions by at least 2 MPa in all of the treatments. Turgor pressures of approximately 0.8 MPa were evident for the salt-treated plants, but were much lower (0.2-0.3 MPa) for plants receiving only nutrient. Seedlings grown on nutrient alone accumulated mainly potassium (sodium and chloride being unavailable) and probably depended upon organic substances, in particular organic anions, to generate internal osmotic potential. Plants on 10-100% seawater treatments behaved as typical halophytes accumulating increasing levels of sodium and chloride as external salinity increased, even though the leaves can regulate steady-state ionic concentrations by means of salt glands. Sodium replaced potassium, but preferential potassium uptake was maintained over a wide range of external sodium concentrations. Chloride accumulation balanced approximately half of the cations present. The calculated osmotic contribution made by the cations (assuming complete charge balance by chloride) in the leaf extracts corresponded closely to measured osmotic potential.