Diurnal Pattern of Salt Secretion in Leaves of the Black Mangrove, Avicennia marina, on the Sinai Coast of the Red Sea

Avicennia marina (Forsk.) Vierh has a typical diurnal pattern of salt gland activity, with high secretion rate during the day, a peak at noon, declining after sunset, and remaining low throughout the night. The main factor affecting the daily secretion rhythm was radiation, with a 2-hr time lag between the radiation level and the corresponding secretion. This lag might be a result of a salt accumulation phase, or of the time needed to build up photosynthetic product pools, needed to generate ATP through the respiration process, for active salt secretion. Differences between daily secretion patterns of young and mature leaves were not significant, and temperature had little effect on controlling the secretion rhythm

Isotopic fractionation during cellulose synthesis in two mangrove species: Salinity effects

Carbon, non-exchangeable hydrogen, and oxygen isotope ratios of cellulose ofAvicennia germinans andRhizophora mangle plants hydroponically grown under different salinities (0, 18, 45% sea water, but with irrigation waters having the same isotopic ratios) were measured to determine the possibility of using isotopic ratios of plant tissues as biological recorders of sea level rise. There was a large variability in the δD values of leaf nitrated cellulose between different treatments and even within a single treatment for bothA. germinans andR. mangle. Thus, δD values of non-exchangeable hydrogens of cellulose cannot be used as a historical tracer for utilization of ocean water or freshwater by mangroves. In contrast, δ18O values of cellulose were not significantly different between different salinity treatments for both mangroves, indicating that δ18O of cellulose can be used as a sea water tracer. δ13C values of cellulose did not vary directly with salinity as has been observed with other plants. δ13C values of cellulose fromA. germinans were the lowest for plants growing at 18% sea water, with cellulose from plants growing in 0 and 45% sea water having significantly higher δ13C values. δ13C values of cellulose fromR. mangle were the highest for plants grown in 45% sea water, with plants grown in 0 and 18% sea water having equally lower δ13C values

Water relations of coastal plant communities near the ocean/freshwater boundary

Salinity and isotope ratios were determined in water from several wells in the Florida Keys, and tidal inlets. Both D/H and O/ O ratios of water from wells and tidal inlets were highly correlated to their salinity. Water from standing pools was enriched in deuterium and oxygen-18 relative to their salinity because of evaporation processes. O/ O and D/H ratios of stem water from plants of several different communities at Sugar Loaf Key, ranging from hardwood hammocks to mangroves, were highly correlated to their predawn water potential. The correlation was consistent with the presence of high salinity in waters with high O and D content. Most individuals from each community were either utilizing water with isotopic characteristics typical of freshwater or of ocean water, while only a few individuals had stem water with isotopic ratios intermediate to these two water sources