Alterations in leaf anatomy, quality, and quantity of flavonols and photosynthetic pigments in Nigella sativa L. subjected to drought and salinity stresses

Nigella sativa was widely used for nutritional and medicinal purposes. The present study investigated the effect of drought and salinity stresses on anatomical leaves structure and some biochemical properties to increase the secondary metabolites. For salt stress plants were treated with NaCl (30, 60 mM), and for drought stress plants were irrigated daily (control), once every two days (2DI) and once every three days (3DI). Compared to control plants, 2DI, 3DI, and NaC160 mM treatments increase significantly leaf rutin content, while the amount of rutin in seeds of NaCI 60 mM treated plants showed a significant decrease. 3DI treatment also significantly increased rutin content in seeds compared to NaC130 mM and control plants. The maximum level of quercetin (0.58 mg g(-1) DW), kaempferol (0.16 mg g(-1) DW), and myricetin (0.04 mg g(-1) DW) in leaves were gained in both NaCI treatments. However, the flavonol components were affected more at salinity conditions rather than drought. In all treated plants, the amount of these compounds in leaves was more than in seeds. The highest amount of total phenol (130 mg g(-1) DW), flavonoids (11.4 mg g(-1) DW), and carotenoid content (1.55 mg g(-1) DW) of leaves were observed under 2DI stress. Treated plants probably encountered different changes in the anatomical structure of leaves, including the decrease of phloem area, reducing vascular bundles and diameters, decreasing the number, and increasing the volume of cortex cells. The study also corroborates the cooperation between increasing the antioxidant capacity with the total flavonoid, rutin, and quercetin. Results indicated a higher sensitivity of N. sativa to drought stress than salinity stress and indicated that moderate salinity and drought could enhance secondary metabolites of seeds in this plant. The formation of potent antioxidants via the treatments could be worthy for pharmaceutical industries.