Oregano (Origanum vulgare), a popular flavoring herb, is widely used throughout the world for flavoring foods and beverages. Water stresses in the arid and semiarid regions of the Middle Eastern countries, however, severely limit growth, production, and survival of oregano. To determine the growth and ecophysiological responses of native Iranian oregano to prolonged water stress, six populations sourced from different bioclimatic zones in Iran were studied. The plants, grown in pots, were subjected to three water stress conditions, no water stress, mild water stress, and moderate water stress, which were continuously maintained throughout the entire plant development and growth period. Relative water content, cellular injury, leaf pigmentation, proline content, leaf area, biomass production, and antioxidant enzymes (peroxidase, ascorbate peroxidase, catalase, and superoxide dismutase) were monitored. The relative plant yield, water content, pigmentation, and leaf area were reduced under water stress, although the ratio of carotenoids to total chlorophyll in the studied populations was increased. Under the prolonged water stress conditions, three oregano populations exhibited higher plant yield along with higher levels of proline accumulation and increased activity of catalase, superoxide dismutase, and peroxidase along with smaller increases in ion leakage and malondialdehye content than other populations. The positive relationship was observed among activity of superoxide dismutase (SOD) and proline content, however, the correlation result showed a negative relationship between ion leakage and activity of SOD. Our results indicated that populations of Baneh, Ardabil, and Namin had a higher tolerance to water deficit conditions than other investigated populations of oregano.
