Physiological, biochemical, and antioxidant responses of oregano subspecies (Origanum vulgare L. ssp. gracile and ssp. vulgare) to NaCl stress

Salinity is an outstanding barrier against the production of agricultural crops, especially in arid and semi-arid regions. Oregano (Origanum vulgare L.), a valuable herb of the Lamiaceae family, contains various types of biologically active constituents such as essential oils, tannins, resins, sterols, flavonoids, and phenolic glycosides. The present research was carried out to investigate the influence of salinity stress on some physiological and biochemical attributes and antioxidant responses in two oregano subspecies (ssp. vulgare and ssp. gracile). Salt treatments were applied using irrigation with different sodium chloride concentrations (0, 25, 50, and 100 mM NaCl). The results revealed a remarkable decline in relative water content (RWC) and photosynthetic pigments in both subspecies under NaCl stress. Total soluble sugars (TSS) decreased in plants exposed to severe salt stress (100 mM NaCl), whereas H₂O₂ production, electrolyte leakage (EL), malondialdehyde (MDA), and leaf proline contents increased in these plants compared to control plants. A positive relationship was found between H₂O₂ production with EL and MDA. Furthermore, salinity improved phenolic content, antioxidant capacity, and activity of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) enzymes. The highest total flavonoid content (TFC) was achieved at 50 mM NaCl salinity, which increased by 19.33% compared to control plants. A positive relationship between the activity of phenylalanine ammonia-lyase (PAL) and TPC and TFC was observed. Analysis of phenolic compounds by HPLC showed that the amounts of gallic acid, caffeic acid, chlorogenic acid, and quercetin in ssp. gracile and caffeic acid, cinnamic acid, and quercetin in ssp. vulgare significantly increased with increasing salinity stress. In general, the findings of this study demonstrated that oregano subspecies ameliorate salt-induced osmotic and oxidative damages through increasing proline accumulation, antioxidant enzymes activity, and secondary metabolites production.