The effect of high-salinity seawater on gray mangrove, Avicennia marina (Forsk.) Vierh, vegetation growing along desert coasts of the United Arab Emirates was analyzed by ecological observation. Gray mangroves in swamps were severely affected by seawater with salinity between 41.5 ‰ and 80.2 ‰. They normally grew in seawater with salinity below 50 ‰, and dwarf stands resulted as the salinity increased to above 50 ‰. The number of seedlings surviving in highly saline swamps was markedly lower than that in swamps of low salinity. Salt tolerance experiments showed that seeds could not survive in seawater with a salinity higher than 55 ‰. However, seeds were able to survive up to 80 ‰ salinity when only the lower part of cotyledons including the radicle were placed in seawater. These seeds absorb high-salinity seawater through their roots, and cotyledons remove the harmful salts from the seawater through a desalination system. When the seedlings grown in coastal seawater (40 ‰) were transplanted into high-salinity seawater, the survival of seedlings decreased in 60 ‰ seawater, and seedlings were unable to survive in 80 ‰ seawater due to dehydration through their roots. These results show that different levels of salt tolerance exist between cotyledons and roots of gray mangrove, and that the level of salt tolerance in roots is high when seeds germinate in high-salinity seawater. This salt tolerance of seeds shows that they can grow in sand when the seawater salinity is below 50 ‰. When the salinity exceeds 50 ‰, seeds should be laid on the sand surface to attain a high germination ratio.
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