Salinity effects on the leaf water relations components and ion accumulation patterns in Avicennia germinans (L.) L. seedlings

Physiological traits involved in leaf water relations were evaluated in Avicennia germinans (L.) L. seedlings growing at different salinities in the field. Analysis of pressure-volume (P-V) curves and sap osmometry were combined to evaluate osmotic adjustment and cell elasticity, and the contribution of accumulated inorganic ions to osmotic potential was estimated. Seedlings growing in soils with interstitial water salinity above that of normal sea water showed a modification of the relationship between water potential and relative water content. Thus, their leaf osmotic potential at maximum turgor (Psi(pi(max))) and at zero turgor (gamma(pi(0))) was 1.41 and 1.82 MPa lower respectively, than that of the seedlings from the low salinity site. Volumetric moduli of elasticity (epsilon) were between 17 and 23 MPa. Thus, epsilon was about 6 MPa lower in high-salinity plants indicating that their cells were slightly more elastic. Tonic concentration analysis showed that Sigma [anions] and epsilon [cations] were higher in the high-salinity site (22-35%) while the water content per unit dry mass was only 12-17% lower. Reduction in water content was insufficient to explain the increase in ion concentration. Ion concentration explained 73 and 66% of the osmotic potential estimated by P-V curves for leaves from low- and high-salinity sites, respectively. In conclusion, this study provided evidence that leaves of A. germinans seedlings adapt to hypersaline soils by increasing solute concentration by 52% and cell elasticity by 26%. Both processes allow leaf water uptake and turgor maintenance over a large range of soil water potential.