Effects of Salinity Fluctuation on Photosynthetic Gas Exchange and Plant Growth of The Red Mangrove (Rhizophora mangle L.)

The red mangrove (Rhizophora mangle L.) in southern Florida occurs frequently in two distinct growth forms, tall and scrub plants, with the scrub form usually found in coastal inland areas having a higher fluctuation of soil water salinity. In the present study, effects of constant and fluctuating salinities on leaf gas exchange and plant growth of red mangrove seedlings were investigated under greenhouse conditions. Both constant and fluctuating salinity treatments significantly affected leaf gas exchange and plant growth of red mangrove seedlings. Seedlings subjected to the fluctuating salinity with the mean of both 100 and 250 mol m~ NaCl showed significantly lower photosynthesis and plant growth than those subjected to the corresponding constant salinity with the same mean. The photosynthetic and growth rates of the seedlings under these fluctuating treatments were as low as, or even lower than those expected if they were growing under the high constant salinity of their respective fluctuation treatments. Seedlings subjected to the fluctuating salinity with the mean of 500 mol m~ NaCl, however, demonstrated slightly higher CO2 assimilation rate and stomatal conductance, but the same plant growth rates as those under the constant 500 mol m~ NaCl treatment. These results suggest that, in general, fluctuating salinity has significant negative effects on photosynthesis and plant growth relative to constant salinity with the same mean. If this finding can be applicable to field situations, the low photosynthesis and plant growth observed previously in several scrub mangrove forests probably can be attributed in part to the salinity fluctuation of soil water in these mangrove forests.