Differential Salt Tolerance Strategies in Three Halophytes from the Same Ecological Habitat: Augmentation of Antioxidant Enzymes and Compounds

Understanding the salt tolerance mechanism in obligate halophytes provides valuable information for conservation and re-habitation of saline areas. Here, we investigated the responses of three obligate halophytes namely <i>Arthrocnemum macrostachyum</i>, <i>Sarcocornia fruticosa</i> and <i>Salicornia europaea</i> to salt stress (0, 100, 200, 400 and 600 mM NaCl) during their vegetative growth with regard to biomass, ions contents (Na⁺, K⁺ and Ca²⁺), chlorophyll contents, carotenoids, phenolic compounds, flavonoids, and superoxide dismutase, peroxidase and esterase activities. <i>S. europaea</i> showed the lowest biomass, root K⁺ content, Chl a/b ratio, and carotenoids under salinity. This reduction of biomass is concomitant with the increase in proline contents and peroxidase activity. On the other hand, the promotion of growth under low salinity and maintenance under high salinity (200 and 400 Mm NaCl) in <i>A. Macrostachyum</i> and <i>S. fruticosa</i> are accompanied by an increase in Chl a/b ratio, carotenoids, phenolics contents, and esterase activity. Proline content was decreased under high salinity (400 and 600 mM NaCl) in both species compared to <i>S. europaea</i>, while peroxidase showed the lowest activity in both plants under all salt levels except under 600 mM NaCl in <i>Arthrocnemum macrostachyum</i> compared to <i>S. europaea</i>. These results suggest two differential strategies; (1) the salt tolerance is due to activation of antioxidant enzymes and biosynthesis of proline in <i>S. europaea</i>, (2) the salt tolerance in <i>A. macrostachyum</i>, <i>S. fruticosa</i> are due to rearrangement of chlorophyll ratio and biosynthesis of antioxidant compounds (carotenoids, phenolics and flavonoids) which their cost seem to need less energy than activation of antioxidant enzymes. The differential behavior in halophytes of the same habitat confirms that the tolerance mechanism in halophytes is species-specific which provides new insight about the restoration strategy of saline areas.