Osmolality and delta C-13 of leaf tissues of mangrove species from environments of contrasting rainfall and salinity

Neotropical species of the genera Rhizophora, Laguncularia and Avicennia grow in environments of variable salinity and flooding stress. Species of Rhizophora predominate in riverine and low-energy coastal fringe environments with continuous water movement, while Laguncularia racemosa, and particularly Avicennia germinans, grow in areas with stagnating water. Avicennia germinans appears to have the largest range of salinity tolerance. The osmotic characteristics of Rhizophora spp., L. racemosa and A. germinans in riverine and coastal environments of north-eastern Venezuela are described and correlated with edaphic and climatic factors. Mature, fully-exposed leaves were collected in humid riverine sites (San Juan River Estuary, Monagas and Sucre States), and seasonally dry coastal fringe habitats (the Unare Coastal Lagoon, and the Chimana Islands off-shore Puerto de La Cruz, Anzoategui State). Cleaned leaf samples were frozen until measurement of leaf dimensions, chlorophyll, phosphorus and nitrogen contents, delta(13)C, osmolality of cell sap, and cell sap content of Na, K and Cl. Results indicate: (1) in all species, leaf sap osmolality is highly and positively correlated with interstitial water salinity, and negatively correlated with leaf area; (2) nitrogen and phosphorus contents of leaves are generally lower in dry areas, but average values are not significantly different. Therefore, it appears that nutrient deficiency is not a main factor determining variations in community structure. Nitrogen content per unit dry weight is, in general, twice as high in A. germinans compared to Rhizophora species and L. racemosa; (3) cell sap osmolality is mostly explained by the concentration of Na and Cl; (4) osmolality of riverine plants (957-1253 mmol kg(-1)) is lower than that of coastal plants (1558-1761 mmol kg(-1)); and (5) delta(13)C values are more negative in riverine (-27.4 to -28.1 parts per thousand) than in coastal plants (-25.4 to -27.2 parts per thousand), indicating a higher water-use efficiency in the latter. Coastal plants have a higher water-use efficiency but their growth is inhibited by salinity stress. (C) 1997 Academic Press Limited.