Effects of humic acid and iron nanochelate on osmolytes content of black cumin (Nigella sativa L.) under drought stress conditions

Black cumin (Nigella sativa L.) seeds are used in the traditional medicine in many countries to prevent and treat many disorders and diseases including cough, asthma, nasal congestion, headache, toothache, intestinal worms, menstrual disorders, gastrointestinal diseases, and impotence. To study the effects of humic acid and iron nanochelate on the content of osmotic protective osmolites including glucose, fructose, sucrose, proline, superoxide dismutase, catalase, and peroxidase of medicinal plant black cumin under the drought stress conditions, an experiment was conducted as a split factorial based on the randomized complete block design with three treatments in three replications at the Agricultural and Natural Resources Research Station of Eqlid city during two crop years of 2018 and 2019. The main plot consisted of three levels of irrigation (50, 75, and 100% of available water) and sub-plots included humic acid (0, 250, and 500 mg l-1) and iron nanochelate (0, 1, and 2 g l-1). The results showed that the effects of irrigation, humic acid, and iron chelate treatments on the catalase, superoxide dismutase, glucose, and peroxidase content, irrigation treatments on the proline content, and irrigation and humic acid treatments on the sucrose and fructose content were significant. The content of all protective osmolites increased in the 50% drought stress treatment. The highest content of glucose, fructose, and sucrose was obtained in the 50% drought stress, 500 mg l-1 humic acid, and 2 g l-1 iron nanochelate treatment. Overall, based on the results of this experiment, the application of humic acid fertilizer (500 mg l-1) and iron nanochelate (2 g l-1) could be recommended to reduce the effects of drought stress on black cumin.