Abstract
The grey mangrove, Avicennia marina, is frequently threatened by extreme salinity resulting from the reduced tidal influence and high water evaporation in warm regions throughout the world. This study assessed the effects of high salinity (900 mM NaCl) on A. marina seedlings under controlled conditions after 30 days of treatment. Plant dry weight and the net photosynthetic rate slightly reduced by the high salinity. Leaf water status, the intrinsic water use efficiency and internal CO2 concentration were enhanced. Sodium and chloride were preferentially accumulated in roots and stems. Thicknesses of the hypodermis, palisade mesophyll, vessel diameter, vessel wall, and palisade mesophyll cell layers increased under high salinity. The entire lamina and the spongy mesophyll thicknesses maintained invariant. Chloroplasts from palisade mesophyll cells were adequately preserved; grana stacks were visible, intergranal lamellae and thylakoids were visible and linear, and starch grains were large. In contrast, some of the spongy mesophyll chloroplasts were swollen and damaged. Our results showed that the facultative halophyte A. marina has a significant ability to protect leaf anatomy, palisade mesophyll chloroplasts ultrastructure and photosynthesis process under high salinity, which profoundly contributes to its high salt tolerance.