Influence of melatonin on antioxidant machinery, ionic content and biochemical traits of kalonji ( <i>Nigella sativa</i> L.) under chromium stress

This study evaluated the role of melatonin (MT) in mitigating chromium (Cr) stress, focusing on plant growth, antioxidant defense, ionic homeostasis, and secondary metabolites. We hypothesized that foliar applied MT would alleviate Cr induced physiological and biochemical alterations by enhancing antioxidant capacity and stabilizing nutrient uptake in <i>Nigella sativa</i> L. (kalonji). A pot experiment was performed using two kalonji varieties, subjected to 0 and 100 µM Cr in the rooting medium, with foliar MT treatments of 0, 15, and 30 µM. The Cr stress decreased the shoot length, root length (49%, 65.6%), shoot and root fresh weight (39%, 52.8%), shoot potassium (59.3%), and total chlorophyll (54%). While Cr induced an increase in proline (55.3%), glycine betaine (37.7%), total soluble sugar (50.2%), total soluble protein (63.2%), ascorbic acid (AsA;57.1%), peroxidase (POD;72.3%) and superoxide dismutase (SOD;48.8%) levels. Under Cr stress, fortification of 15 μM MT further enhanced total soluble sugars (21.6%), total soluble proteins (23.7%), AsA (25.9%), proline (21.9%), glycine betaine (20.1%), SOD (25.1%), POD (30.3%) and CAT (28%) contents. These results support the hypothesis that MT application (15 µM) mitigates Cr toxicity by modulating antioxidative and metabolic pathways, making it a promising, eco-friendly strategy for managing heavy metal stress in kalonji.