No abstract

We used pressure-volume analysis and dewpoint hygrometry to determine leaf water relation parameters of mangrove (Avicennia germinans L.) seedlings grown at salinities of 0, 8, 20 and 32 per thousand. Seedlings responded to an increase in salinity from 0 to 32 per thousand by an increase in leaf succulence as reflected in an increase in leaf water content per unit area from 300 to 360 g m(-2). Additionally, osmotic potential at full turgor decreased from -2.3 to -3.5 MPa and osmotic potential at zero turgor decreased from -2.7 to -4.3 MPa. Cell elasticity decreased as salinity increased from 0 to 32 per thousand, as indicated by a progressive increase in volumetric modulus of elasticity from 19 to 27 MPa. Increased leaf succulence enabled leaves to sequester large amounts of solutes without adversely increasing cell osmotic pressure. On the other hand, osmotic adjustment facilitated turgor maintenance as water potential diminished. Salinity-induced decreases in tissue elasticity generated greater water potential differences between leaves and soil under saline conditions than under non-saline conditions.

We assessed the effect of salinity on plant growth and leaf expansion rates, as well as the leaf life span and the dynamics of leaf production and mortality in seedlings of Avicennia germinans L. grown at 0, 170, 430, 680, and 940 mol m −3 NaCl. The relative growth rates (RGR) after 27 weeks reached a maximum (10.4 mg g −1 d −1 ) in 170 mol m −3 NaCl and decreased by 47 and 44% in plants grown at 680 and 940 mol m −3 NaCl. The relative leaf expansion rate (RLER) was maximal at 170 mol m −3 NaCl (120 cm m −2 d −1 ) and decreased by 57 and 52% in plants grown at 680 and 940 mol m −3 NaCl, respectively. In the same manner as RGR and RLER, the leaf production (P) and leaf death (D) decreased in 81 and 67% when salinity increased from 170 to 940 mol m −3 NaCl, respectively. Since the decrease in P with salinity was more pronounced than the decrease in D, the net accumulation of leaves per plant decreased with salinity. Additionally, an evident increase in annual mortality rates (λ) and death probability was observed with salinity. Leaf half-life (t 0.5 ) was 425 days in plants grown at 0 mol m −3 NaCl, and decreased to 75 days at 940 mol m −3 NaCl. Thus, increasing salinity caused an increase in mortality rate whereas production of new leaves and leaf longevity decreased and, finally, the leaf area was reduced.

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 speciesspecific, 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) (Chl a+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 (CC a ) was observed, while per leaf mass (CC m ) 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 (t 0.5 ) 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 CC m and CC a , respectively. Leaf longevity was positively correlated with the ratio between CC and maximum photosynthetic rate (P max ), clearly showing the existence of a balance between leaf costs and benefits.