Growth regulator-mediated modulation of antioxidant and secondary metabolism in coriander under exposure to high...

High temperature poses a serious constraint to coriander (Coriandrum sativum L.) limiting growth, essential oil yield and antioxidant potential. This study provides a novel evaluation of the combined effects of plant growth regulators (salicylic acid, thiosalicylic acid, and thiourea) on antioxidant systems and secondary metabolism across contrasting coriander genotypes (RKD-18 and RCr-728) under variable field temperature conditions. Thiosalicylic acid (TSA) treatment significantly enhanced essential oil content (0.58% in RKD-18) under normal sowing conditions. Among the treatments, foliar application of (TSA, 100 ppm) proved most effective, significantly enhancing osmolyte accumulation (proline: 15.01 µg/g FW), and secondary metabolites, including total phenolics (60.9 mg GAE/g) and flavonoids (18.89 mg QE/g). TSA also markedly enhanced antioxidant enzyme activities, with superoxide dismutase reaching 0.35 µmol min^- 1 g^- 1 FW, catalase up to 0.45 µmol min^- 1 g^- 1 FW, and peroxidase up to 0.37 µmol min^- 1 g^- 1 FW under normal sowing conditions, while reducing lipid peroxidation and improving redox balance. Enhanced antioxidant capacity was further supported by higher ferric reducing antioxidant power (4.54 mg TE/g) and 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (56.1 µmol TE/g) values, particularly in RKD-18. In contrast, thiourea at higher concentration (500 ppm) exhibited inhibitory effects, indicating possible phytotoxicity. Across treatments, RKD-18 consistently outperformed RCr-728, demonstrating superior biochemical and enzymatic resilience. Overall, the study is significant in identifying thiosalicylic acid as an effective and practical foliar elicitor that enhances antioxidant defense and secondary metabolism, thereby improving coriander performance, yield, and quality under variable high-temperature field conditions.