Osmotic and hydraulic adjustment of mangrove saplings to extreme salinity.

Salinity tolerance in plant species varies widely due to adaptation and acclimation processes at the cellular and whole-plant scales. In mangroves, extreme substrate salinity induces hydraulic failure and ion excess toxicity and reduces growth and survival, thus suggesting a potentially critical role for physiological acclimation to salinity. We tested the hypothesis that osmotic adjustment, a key type of plasticity that mitigates salinity shock, would take place in coordination with declines in whole-plant hydraulic conductance in a common garden experiment using saplings of three mangrove species with different salinity tolerances (Avicennia germinans L., Rhizophora mangle L. and Laguncularia racemosa (L.) C.F. Gaertn., ordered from higher to lower salinity tolerance). For each mangrove species, four salinity treatments (1, 10, 30 and 50 practical salinity units) were established and the time trajectories were determined for leaf osmotic potential (Ψs), stomatal conductance (gs), whole-plant hydraulic conductance (Kplant) and predawn disequilibrium between xylem and substrate water potentials (Ψpdd). We expected that, for all three species, salinity increments would result in coordinated declines in Ψs, gs and Kplant, and that the Ψpdd would increase with substrate salinity and time of exposure. In concordance with our predictions, reductions in substrate water potential promoted a coordinated decline in Ψs, gs and Kplant, whereas the Ψpdd increased substantially during the first 4 days but dissipated after 7 days, indicating a time lag for equilibration after a change in substratum salinity. Our results show that mangroves confront and partially ameliorate acute salinity stress via simultaneous reductions in Ψs, gs and Kplant, thus developing synergistic physiological responses at the cell and whole-plant scales.