Osmolality and δ13C of Leaf Tissues of Mangrove Species from Environments of Contrasting Rainfall and Salinity

Abstract

Neotropical species of the generaRhizophora, LagunculariaandAvicenniagrow in environments of variable salinity and flooding stress. Species ofRhizophorapredominate in riverine and low-energy coastal fringe environments with continuous water movement, whileLaguncularia racemosa, and particularlyAvicennia germinans, grow in areas with stagnating water.Avicennia germinansappears to have the largest range of salinity tolerance.The osmotic characteristics ofRhizophoraspp.,L. racemosaandA. germinansin 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, Anzoátegui State). Cleaned leaf samples were frozen until measurement of leaf dimensions, chlorophyll, phosphorus and nitrogen contents, δ13C, 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 inA. germinanscompared toRhizophoraspecies andL. racemosa; (3) cell sap osmolality is mostly explained by the concentration of Na and Cl; (4) osmolality of riverine plants (957–1253mmol kg−1) is lower than that of coastal plants (1558–1761mmol kg−1); and (5) δ13C values are more negative in riverine (−27·4 to −28·1‰) than in coastal plants (−25·4 to −27·2‰), 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.