Salt tolerance of mangroves and submerged aouatic plants

Mangroves and submerged aquatie plants, the seagrasses, share a niehe attribute of toleranee to broad and rapid ehanges in salinity. Among five seagrasses studied, Halodule, whieh often oeeurs as the sole oceupant of shallower and more hypersaline bays, has the greatest toleranee of salinity. Cymodocea and Halophila have the narrowest toleranee ranges, and Ruppia is the only one that can survivefor extended periods in nonsaline eonditions. In order of their deereasing toleranee to low salinity, the five seagrasses are Ruppia, Halodule, Thalassia,Cymodocea and Halophila. The mangroves also survived rapid changes in salinity but among three speeies studied, narrower toleranees were shown by Rhizophora and Laguncularia than by Avicennia. AlI survive for indefinite periods in nonsaline eonditions, but Avicennia withstands rapid ehanges to salinities over 118 OjOO.Among Avicennia plants, those of various age or stage of development showed different salt tolerance. Seedlings and younger p1antshad greater to1eraneeto hypersaline conditions. Although salt toleranee has been explored in these estuarine plants, many questions remain eoneerning the meehanisms underlying the diverse toleranee ranges of the seagrassesand of different ages and stages of development in mangroves.