We analysed plant growth, ion accumulation, leaf water relations, and gas exchange of Avicennia germinans (L.) L. subjected to a long-term, controlled salinity gradient from 0 to 55 ‰. Growth and leaf area were affected by salinity higher than 10 ‰. As salinity increased, the predawn leaf water potential (Ψw) and leaf osmotic potential (Ψs) decreased. Leaf Ψw was at least -0.32 MPa lower than the Ψw of solution. Na+ and K+ ions explained about 78 % of decrease in Ψs. K+ tissue water concentration decreased by more than 60 % in all salinity treatments as compared with those grown at 0 ‰. Inversely, Na+ concentration in tissue water increased with nutrient solution salinity. The maximum net photosynthetic rate
(PN) and stomatal conductance (gs) decreased by 68 and 82 %, respectively, as salinity increased from 0 to 55 ‰; the intercellular CO2 concentration (Ci) followed the same trend. The PN as a function of Ci showed that both the initial linear slope and upper plateau of the PN vs. Ci curve were markedly affected by high salinity (40 and 55 ‰). and N. Suárez, E. Medina.
This study assessed the effect of leaf age on construction cost (CC) in the mangrove species Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle growing in their natural habitat. Leaf osmolality values were species-specific, the highest in A. germinans (1 693 mmol kg-1) and the lowest in L. racemosa (1 270 mmol kg-1). In the three species, contents of chlorophyll (a+b) (Chla+b) and nitrogen (N) per unit of leaf area were maximal in adult leaves and tended to decline with age. Leaf mass to leaf area ratio (LMA) and ash content increased during leaf ageing. Similarly, as leaves aged, a significant increase in leaf construction cost per leaf area (CCa) was observed, while per leaf mass (CCm) it remained almost constant, suggesting a sustained production of leaf compounds as leaves became older. CC was positively correlated with LMA and heat of combustion (Hc) per leaf area, suggesting differences among species in the quantity and composition of expensive compounds. Leaf half lifetime (t0.5) showed contrasting values in the three mangrove species (60, 111, and 160 d in L. racemosa, R. mangle, and A. germinans, respectively). Overall, L. racemosa was the species with less expensive leaves to construct while leaves of A. germinans and R. mangle had the highest CCm and CCa, respectively. Leaf longevity was positively correlated with the ratio between CC and maximum photosynthetic rate (Pmax), clearly showing the existence of a balance between leaf costs and benefits.