Effects of salinity on germination, seedling growth and physiology of three salt-secreting mangrove species

Propagules of three salt-secreting mangrove species, i.e. Acanthus ilicifolius L., Aegiceras corniculatum Blanco. and Avicennia marina (Forsk) Vierh., were germinated at salinities of 0, 5, 15, 25 and 35 parts per thousand (ppt). Their tolerance to salt stress was compared in terms of some parameters on germination, growth and physiology. Root initiations of Ae. corniculatum and Av. marina, two viviparous species, were about 3 and 6 d, respectively, not significantly affected by salinity, but those of Ac. ilicifolius, a non-viviparous species, were significantly delayed about 4 d at salinities over 25 ppt. Final seedling establishment percentages of Av. marina were 100% at all salinity treatments, while salinities over 25 ppt significantly reduced the values of Ae. corniculatum (28%) and Ac. ilicifolius (38%). As salinities increased from 0 to 35 ppt, decrease in relative growth rate (RGR) of Av. marina was only 5%, less than those of Ae. corniculatum (56%) and Ac. ilicifolius (70%). For each species, salt secretion from leaves increased with increases in salinity and the increases in salt secretion with every salinity increase of 10 ppt were about 0.9, 0.6 and 0.2 g m−2 d−1 for Av. marina, Ae. corniculatum and Ac. ilicifolius, respectively. When seedlings were cultured in fresh water, leaf tissue water of each species had salt concentrations around 2%, much higher than the environmental salinity (0 ppt). Under saline conditions from 5 to 35 ppt, salt concentrations in leaf tissue water of Av. marina maintained steady (4.3-5.0%), while the corresponding values increased from 2.4 to 4.5% in Ae. corniculatum and from 2.3 to 5.3% in Ac. ilicifolius. Evaporation and transpiration (evapo-transpiration) rates of these three species were similar under fresh water condition, but under saline conditions (5-35 ppt), Av. marina had significantly higher rates than Ac. ilicifolius and Ae. corniculatum. At salinities of 5-35 ppt, variations of evapo-transpiration were different between species with an order of Av. marina < Ae. corniculatum < Ac. ilicifolius. All of the parameters indicated that salt tolerance of the three salt-secreting mangrove species was in the descending order of Av. marina > Ae. corniculatum > Ac. ilicifolius.