Diurnal Pattern of Salt Secretion in Leaves of the Black Mangrove , Avicennia marina

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. MATERIALS AND METHODS of the salts in seawater. Second, accumulating salts (the 5-20% that enter the roots) are actively and efficiently secreted through salt glands (Berjak 1978, Fahn 1979, Ish-ShalomGordon and Dubinsky 1990). The aim of the study reported here is to describe the diurnal secretion activity of the salt glands of A. marina and the way this pattern is affected by environmental factors and leaf age. I Manuscript accepted 9 March 1992. 2 Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel. Avicennia marina (Forsk.) Vierh (Avicenniceae) is a coastal-swamp halophyte, common in subtropical and tropical regions such as North America, Vietnam, the Pacific region of India, eastern and southern Africa, eastern and southern Australia, and southern New Zealand (Chapman 1960, 1976, Teas 1984, Odum and Mclvor 1990). The natural distribution of the species reflects its wide salt tolerance, and previous studies have shown that under laboratory conditions A. marina can survive in a wide range of salinities, from 175% seawater (Clarke and Hannon 1970) to distilled water (Drennan and Pammenter 1982). The species is sensitive to low temperature (West 1956), but is the only mangrove Site Description that can thrive under desert conditions like The study was conducted on A. marina those found along the Red Sea shores. It plants from Shurat el Gharqana, in the Nabq forms monospecific stands along the southern area, on the Sinai coast of the Red Sea. shores of the Sinai Peninsula, from Ras The mangrove stands of the Nabq area have Muhamad to Shurat el Arwashie in the Nabq been studied by several authors (Leshem and area, the northernmost boundary of the Levison 1972, Por and Dor 1975, 1984, Por et mangrove formation. Elaborate mechanisms al. 1977, Evenari et al. 1985, Gara et al 1986) aH-ow-the----surviva-I-of-krmlrina in rni<is'---~T~h~e'-:-s'....'e'--'-s-'-t.'=a='-!nd~s£!o~f~A~.'!om'--.!..,ia~ri~n~a~a-!..!rHec<'U.gd-.roJJw~inL< 1 metahaline environment. First, simple nonPliocene fossil reefs and associated deposits, metabolic ultrafiltration through selective as well as on recent subfossile coral reefs. The permeable membranes rejects from 80% salinity in this habitat can be as high as 4.7% (Waisel et al. 1986) to 95% (Scholander 1968) (Por et al. 1977), above that of the adjacent open sea, which reaches a maximum of 4.1 % (Reiss and Hottinger 1984). The A. marina plants are rooted in the tidal zone, exposed to fluctuations in water levels (50-70 cm