Mature propagules of two mangrove species, Avicennia marina and Rhizophora stylosa, were collected at Cape Ferguson, north Queensland in February 1981 and transferred to a shadehouse at the Australian Institute of Marine Science. The shadehouse was naturally lit and screening reduced the photon irradiance incident on the plants to about one-third of that outside the shadehouse. Air temperature inside the shadehouse was not controlled and ranged between a minimum of 17°C in winter and a maximum of 37°C in summer.Avicennia marina propagules were placed in a tray of 25% seawater for 2 days to allow the pericarp to be shed. They were then planted out into trays containing washed beach sand saturated with 25% seawater to promote root formation. Root formation by Rhizophora stylosa propagules was induced by standing the basal end of the propagule in 25% seawater.After 4 weeks, seedlings were transferred to individual 2.4 litre plastic pots containing one of 5 treatments. Salinity treatments of 0, 25, 50, 75 and 100% seawater were prepared by mixing seawater with tapwater, and a solution containing major and minor nutrients was added to each treatment. Four plants of each species were placed in each treatment. The culture solutions were initially changed monthly and later at 2 weekly intervals, as the seedlings became larger. The water level in the pots was maintained by adding tapwater containing < 1 mM sodium and chloride.In February 1982, the number of leaves remaining on each plant was counted and the number lost through senescence was obtained by counting leaf scars. Plants were then washed thoroughly with tapwater to remove surface salt, blotted with paper towelling, and subdivided into component parts, which were weighed and then dried to constant weight at 80°C. The total area of leaves remaining on the plant was measured with a Licor leaf-area meter. The dry weight and area of leaves lost through senescence was calculated from the number of leaves lost and, respectively, the mean dry weight and mean area per living leaf.The dried plant samples were ground for 30 seconds in a tungsten carbide mill. For analyses of sodium, potassium, calcium and magnesium, 1 g dry weight was pressure digested with 10 ml of concentrated HNO3 in a polycarbonate bottle for 16 h at 60°C. Extracts were analysed by the Australian Mineral Development Laboratory, Adelaide, S.A. using an inductively coupled plasma emission spectrometer calibrated using NBS leaf samples. For chloride, 100 mg of ground sample was digested at 80°C in 20% (v/v) HNO3, filtered, and the extracts analysed for chloride with a Radiometer CMT10 coulometric chloride titrator.
This study was undertaken to examine salt tolerance in Avicennia marina (which has salt secreting glands in its leaves) and Rhizophora stylosa (which does not have these glands), by monitoring growth under different salinity regimes over a period of one year. The contribution of the more common cations and anions in seawater to osmotic adjustment in both species was also examined.
