Solute accumulation and osmotic adjustment characteristics of the mangrove Avicennia marina under NaCl-induced salinity stress

Osmotic adjustment characteristics of mangrove leaves (Avicennia marina) were investigated over 45 days of growth under three NaCl concentrations (0, 200 and 600 mM NaCl). Maximum seedling growth occurred at 200 mM NaCl. Leaf water potential and osmotic potential were more negative under saltier conditions. The salinity treatments caused an increase in the leaf contents of Na(+), Cl(-), glycine betaine, total free amino acids and proline. However, inorganic ions (especially Na(+) and Cl(-)) played a key role in osmotic adjustment in A. marina leaves, as they contributed 70% and 76% to these adjustments in 200 and 600 mM NaCl-treated plants, respectively. NaCl treatment induced a decrease in the organic solute contribution. Thus, growth of A. marina was stimulated by the 200 mM NaCl treatment due to effective osmotic adjustment, mainly by accumulation of inorganic ions. Reduction in growth when A. marina was subjected to high salinity stress (600 mM NaCl) appears to have been caused for the most part by the accumulation of excess NaCl (ionic effect), exacerbated by additional organic solute synthesis, rather than an osmotic effect.