University of Birjand-Research Group of Drought and Climate Change Journal of Drought and Climate change Research 3092-6076 Articles in Press 2025 09 02 Effect of brassinosteroid and melatonin on antioxidant enzymes and grain yield of quinoa (Chenopodium quinoa Willed) under drought stress condition Effect of brassinosteroid and melatonin on antioxidant enzymes and grain yield of quinoa (Chenopodium quinoa Willed) under drought stress condition 3595 10.22077/jdcr.2025.9942.1163 FA Neda Pakgohar Shahid Bahonar University of Kerman Ali Akbar Maghsoudi Mood Shahid Bahonar university of Kerman 0000-0003-2731-9057 Hassan Farahbakhsh Head of Genetic and Plant Production Dep. 0000-0001-6172-3825 Amin Pasandi Pur Shahid Bahonar University of Kerman Journal Article 2025 08 18 To investigate the effects of brassinosteroid and melatonin on antioxidant enzymes and yield of quinoa under drought stress, a split-plot experiment based on a randomized complete block design was conducted during the 2019–2020 and 2020–2021 growing seasons at the Research Farm of Shahid Bahonar University of Kerman. Treatments included three irrigation levels (100, 75, and 50% of field capacity) as the main plots, and nine foliar applications as subplots. Drought stress, foliar application, and their interactions significantly affected physiological traits and yield. Increasing drought enhanced malondialdehyde (MDA) content and polyphenol oxidase, while reducing leaf protein. The highest MDA obtained under no foliar application with 50% field capacity irrigation (36.52% increase), whereas foliar application under 75 and 50% irrigation significantly reduced it. The lowest leaf protein was also observed under no foliar application at 50% field capacity (22% reduction). The maximum catalase occurred under 75% irrigation with foliar application of 0.5µM melatonin × 0.5µM brassinosteroid, while the minimum was under 50% irrigation without foliar application. Combined foliar spraying increased peroxidase activity (up to 0.83%) compared with the control. Drought stress significantly enhanced polyphenol oxidase, with the highest activity (25% increase) recorded at 50% field capacity. Grain yield was strongly affected, with the highest yield under 100% irrigation and the lowest under 50% in the second year. Combined foliar application of melatonin and brassinosteroid, particularly under stress (75 and 50%), significantly improved yield by up to 65% compared with the control, with higher concentration being more effective under severe drought. To investigate the effects of brassinosteroid and melatonin on antioxidant enzymes and yield of quinoa under drought stress, a split-plot experiment based on a randomized complete block design was conducted during the 2019–2020 and 2020–2021 growing seasons at the Research Farm of Shahid Bahonar University of Kerman. Treatments included three irrigation levels (100, 75, and 50% of field capacity) as the main plots, and nine foliar applications as subplots. Drought stress, foliar application, and their interactions significantly affected physiological traits and yield. Increasing drought enhanced malondialdehyde (MDA) content and polyphenol oxidase, while reducing leaf protein. The highest MDA obtained under no foliar application with 50% field capacity irrigation (36.52% increase), whereas foliar application under 75 and 50% irrigation significantly reduced it. The lowest leaf protein was also observed under no foliar application at 50% field capacity (22% reduction). The maximum catalase occurred under 75% irrigation with foliar application of 0.5µM melatonin × 0.5µM brassinosteroid, while the minimum was under 50% irrigation without foliar application. Combined foliar spraying increased peroxidase activity (up to 0.83%) compared with the control. Drought stress significantly enhanced polyphenol oxidase, with the highest activity (25% increase) recorded at 50% field capacity. Grain yield was strongly affected, with the highest yield under 100% irrigation and the lowest under 50% in the second year. Combined foliar application of melatonin and brassinosteroid, particularly under stress (75 and 50%), significantly improved yield by up to 65% compared with the control, with higher concentration being more effective under severe drought.