Growth and Salt Balance of the Mangroves Avicennia marina (Forsk.) Vierh. And Rhizophora stylosa Griff. In Relation to Salinity

Abstract

Seedlings of A. marina and R. stylosa were grown for 12 months in nutrient solutions containing 0, 25, 50, 75 and 100% seawater. Plants of both species grew poorly without sodium chloride in the culture medium. Growth was greatly stimulated in 25% seawater which, within the range of treatments used, was the optimum salinity for growth by both species. Dry matter production by R. stylosa fell sharply between 25 and 50% seawater, whereas A. marina displayed an extended growth response in up to 75% seawater. Plants of A. marina grown on solutions of increasing salinity accumulated increasing amounts of sodium and chloride in all tissues. As a result, all tissues had osmotic potentials that were considerably more negative than that of the solution on which they were grown. This implied that the reduction in growth displayed by plants of A. marina grown at high salinity was due to inhibition of growth by high concentrations of sodium and/or chloride, rather than to an effect of water stress. By contrast, the leaves and stem of R. stylosa contained relatively low concentrations of sodium and chloride, with the result that the osmotic potentials of these tissues were close to, or more positive than, that of the solution on which the plants were grown. This suggests that the poor shoot growth of this species at high salinities was due to water stress. The results of this study support the view that A. marina, which has salt-secreting glands in its leaves, is more salt-tolerant than R. stylosa, which does not possess salt-secreting glands.