Chemical Priming with Salt and Urea Improves Germination and Seedling Growth of Black Cumin (Nigella sativa L.) under Osmotic Stress

A common problem with vegetable production in saline areas is poor crop stand, but for black cumin (Nigella sativa L.) germination data are limited and inconsistent. The effects of chemical priming with Urmia lake salt and urea solutions for 16 h at 30 °C on seed germination and seedling growth of black cumin (Nigella sativa L.) were studied under various osmotic stress levels. For a more detailed assessment of chemical priming, the effects of hydropriming for 16 h at 30 °C were also studied. A seed lot that was not exposed to any treatment, except disinfection, was used as control. Osmotic stress levels were − 2, − 4, − 6, and − 8 bar, which were achieved with polyethylene glycol 6000 (PEG 6000). Seed germination of black cumin was reduced by 16.2%, 33.8%, 50.9%, and 74.9% under osmotic potential − 2, − 4, − 6, and − 8 bar, respectively, compared with non-stressed control. Improved germination index values, reduced mean germination time, and increased coefficients of velocity of germination were observed under osmotic stress in primed seeds compared with non-primed control. Averaged over priming treatments, priming improved the final germination percentage by 10.5%, 24.3%, 45.5%, and 74.6% under osmotic potential − 2, − 4, − 6, and − 8 bar, respectively. Post-germination growth was also inhibited under low osmotic potential compared with the non-stressed control. Nevertheless, priming improved length and weight of black cumin seedlings and enhanced peroxidase and catalase activity at all osmotic potential levels compared with non-primed seeds. Higher seedling vigor indices were recorded in seedlings from primed seeds with decreasing osmotic potential levels than non-primed seeds. Urmia lake salt priming had the greatest impact on improving seed germination and vigor indices, especially under osmotic stress conditions. Although seed priming did not completely eliminate the symptoms of osmotic stress in black cumin germination, it is an efficient method to mitigate the impact of osmotic stress on germination of this species.