Physiological and Biochemical Responses of Three Echinacea Species to Salinity Stress
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To determine the level of salt tolerance of the medicinal plant Echinacea, the physiological and biochemical characteristics of E. purpurea, E. pallida and E. angustifolia exposed to different NaCl levels (0, 50, 75, and 100 mM) were evaluated under hydroponic culture. Dry weights of shoots and roots were not affected by salinity; however E. purpurea and E. pallida exhibited higher survival rate than E. angustifolia, which also showed high salt injury index and electrolyte leakage compared to the other two species. Gas exchange (photosynthetic rate, stomatal conductance, and transpiration rate) showed a decline with increasing salt concentrations in all species with a more pronounced reduction in E. angustifolia. E. purpurea was able to retain more Na+ in the roots than the other two species showing its capacity to regulate Na+ translocation to shoots (Na+ exclusion). Moreover, the activities of two major antioxidant enzymes; superoxide dismutase (SOD) and ascorbate peroxidase (APX) were increased by salinity in E purperea, while the activities were decreased in E. angustifolia. The characteristic phytochemical profiles of caffeic acid derivatives (CADs) and alkamides/ketones were obtained for the three species, and quantitative changes were determined. Cichoric acid, the major CAD in E. purpurea, was increased with salinity up to 75 mM NaCl. A relative increase in alkamides and CADs was recorded in E. angustifolia, while in E. pallida, the level of echinacoside and major ketones (22 and 24) decreased, suggesting that the medicinal value of this species was compromised by salt stress. First evidence of salt-induced changes in alkamides and ketones in Echinacea was demonstrated in this study. Activity of phenylalanine ammonia-lyase (PAL), the major enzyme involved in the biosynthesis of CADs, was increased only in the roots of E. purpurea, further reflecting the differences in salt tolerance between species. It can be concluded that Echinacea species exhibited a limited degree of salt tolerance; however, E. purpurea showed a higher tolerance than E. pallida and E. angustifolia. This tolerance was mainly attributed to the increase in Na+ exclusion capacity, antioxidant activities and PAL activity.