Soil Salinity, Sun Exposure, and Growth of Acrostichum aureum, the Mangrove Fern

Abstract

Populations of Acrostichum aureum growing under contrasting conditions of soil salinity and sunlight exposure were studied in two sites in the north coast of Puerto Rico. Plant size and density of clumps under full sun exposure are larger in sites with lower soil salinity. In shady habitats with relatively high soil salinity (understory of Laguncularia forests), plants grow larger but occur at lower densities as compared with nonshaded sites. Apparently salt stress in shady habitats is reduced due to lower evaporative demands. Osmotic concentration of leaf cell sap increases markedly with soil salinity (except in shade plants), a process accompanied by corresponding increases in leaf water content per unit leaf area, and reductions in leaflet size and specific leaf area. The major solutes responsible for increases in cell sap osmotic pressure are Na+, Mg2+, and sucrose. However, the largest relative response to salinity is shown by the cyclitol D-1-0-methyl-muco-inositol, a cytoplasmic compatible solute. Thus, this cyclitol can be considered as an accurate indicator of salt stress in A. aureum. The Cl-/Na+ molar ratios in the cell sap are consistently higher in A. aureum compared to L. racemosa and R. mangle (2.5 against 1.2 and 1.5, respectively). The K/Na molar ratio based on total content per unit tissue dry weight is also higher in the fern (2.6) compared to typical mangroves (less than 0.5). These results indicate a difference in ion selectivity at the root level.