Physiological Responses of the Halophyte Salvadora persica to the Combined Effect of Salinity and Flooding

Miswak (Salvadora persica L.) is a subtropical shrub common in habitats with fluctuating soil salinity and sporadic floods. Little is known about its response to the combination of salinity and flooding (hypoxia) stresses. This study is an attempt to characterize the physiological response of Miswak to the combination of these two stresses in situ. We examined functional leaf traits, osmotic adjustment, gas exchange rates, chlorophyll a fluorescence and lipid peroxidation. Under stressful conditions, this species was able to shift from fast to slow growth and adjust its gas exchange rates in order to reduce water loss and limit the upward transport of toxic ions. In saline-dry(S) and saline-flooded (S-F) habitats stomatal conductance (g(s)) and stomatal density (SD) decreased as Na accumulated in the leaves. In contrast, in non-saline-flooded (NS-F) habitats SD increased. Na+ and K+ contributed greatly to leaf osmotic potential (Psi(pi)), especially under the combined effects of salinity and flooding. Proline concentrations increased too in root and leaf tissues possibility to protect macromolecules against the rising Na+ concentration. Furthermore, photorespiration and cyclic electron transport appear to have been upregulated in non-saline-flooded habitats and appear to play a protective role. However under prolonged saline stress, thermal dissipation was involved in PSII protection but not photo-inhibition. Still under saline and saline-flooded habitats, plants were able to reduce the flow of electrons through photosystems. Under saline-flooded habitats plants experience oxidative stress leading to photo-damage. A combination of morphological (smaller leaves) and physiological (accumulation of Na+ and K+ in the leaves) adjustments and increased osmo- (accumulation of proline) and photo- (reduction in chlorophyll content, increased non-photochemical quenching and reduced Photochemical quenching (q(p)) and linear electron transport rate) protection helped the plant overcome salinity, hypoxia and their combination in its native habitats. (c) 2018 Friends Science Publishers