Physiological Response of Avicennia marina to Salinity and Recovery

In this study, the response of Avicennia marina (Forssk.) Vierh. to salinities of 0, 150, 300, 600, 900, and 1200 mM NaCl was investigated under controlled conditions after 2 and 4 months of exposure, and for the recovery from salinity. A. marina can survive under 1200 mM NaCl. Optimal fresh weight was obtained under 300 mM NaCl after 4 months of salt treatment. The fresh weight increment was attributed to adequate water feeding and a highly effective photosynthetic area. However, the dry weight declined progressively with the increase of salinity allowing us to classify A. marina as a facultative halophyte. Dry weight reduction was mainly due to a disruption in the net photosynthetic assimilation rate due to the accumulation of Na+ and Cl– in the photosynthetic organs. A. marina enhanced its stomatal conductance and water use efficiency under high salinity. In addition, A. marina excreted salt excess outside as it deposited cubic crystals on leaf surfaces. The excretion rates of sodium, chloride and potassium were optimal under 900 mM NaCl. The recovery from salinity had no beneficial effects on growth, water status and leaf electrolyte leakage. It decreased the accumulation of Na+ and Cl– and consequently the excretion rates. However, it enhanced the photosynthetic activity under 300–900 mM NaCl compared to non-recovered plants.