Comparative study of stress generated by osmolytes on the growth, photosynthesis and metabolic responses in Nigella sativa

Several abiotic stresses generate osmotic stress which alters physio-biochemical and metabolic responses in plant. In the present experiment, the impact of different osmolytes on the germination, morphological, and physio-biochemical responses of Nigella sativa L. (black cumin) was investigated. Nigella seeds were subjected to treatments with sodium chloride (NaCl), polyethylene glycol (PEG-6000), mannitol, or sorbitol, resulting in a reduction of the osmotic potential in soil. The osmolyte treatments reduced germination traits (germination percentage, germination index, and vigour index) and seedling growth in treated plants. The decline in seedling establishment and growth is attributed to higher accumulation of various reactive oxygen species like hydrogen peroxide (H₂O₂) and superoxides (O₂⁻) and lipid peroxidation in treated plants. Furthermore, osmolyte stress induced stomatal closure caused decline in water-related parameters (transpiration, stomatal conductance, internal CO₂ concentration) which consequently restrained plant growth and development. In addition, Osmolyte treatments reduced the chlorophyll levels, and photosynthetic activity, while increasing proline and carbohydrate content. Protein, nitrate reductase, and carbonic anhydrase activities were depleted, and root cell death was elevated. To counteract this oxidative damage and mitigate stress, the plant enhances its antioxidant enzymatic defense. Notably, in the present study, the activity of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POX) is upregulated to efficiently scavenge ROS and alleviate the impact of the stress. The current study demonstrated that different osmolytes elicit diverse effects on the morpho-physiological, biochemical, and metabolic responses of Nigella. Among the tested osmolytes, NaCl had the most pronounced and deleterious impact on the plant, followed by mannitol and sorbitol. On the other hand, PEG had the least toxic outcomes on the physiological processes and overall health of plant.