Abstract

Introduction: Drought stress accounts for over 70% of the potential losses in crop yield and productivity worldwide and is one of the chronic abiotic crop stresses that affect plant growth and development. Quinoa (Chenopodium quinoa Willd.) is rich in vital minerals, vitamins, and health-promoting substances, and its balanced amino acid composition and high protein quality make it a nutritious food. As quinoa is suited to many agroecological zones and resistant to abiotic stresses like salinity and drought, it is a good option for improving food security in the face of climate change. Melatonin (MT) is a common indoleamine compound found in plants that is linked to a number of physiological and metabolic functions, including senescence and the germination, flowering, and fruiting of seeds. Furthermore, melatonin is essential for controlling how plants adjust to stressful situations. Melatonin can encourage root and shoot growth, particularly in drought-stressed environments. A class of plant steroid hormones known as brassinosteroids (BRs) significantly affects a number of plant growth indices, such as the leaf area index, crop growth rate, relative water content, ion leakage, proline accumulation, and protein content. Because of their ability to enhance plant development and resilience to abiotic stressors including salinity and drought, brassinosteroids are a major focus of agricultural research and crop management plans.

Materials and Methods: This study was carried out in split plots using a randomized complete block design with three replications in 2019–2020 and 2020–2021, at the Shahid Bahonar University Research Farm, Kerman, which is situated at an elevation of 1774 meters above sea level, 75 degrees 7 minutes 13 seconds east longitude, and 30 degrees 14 minutes 36 seconds north latitude. In this experiment, the main factor included three irrigation levels (100, 75, and 50 percent of easily available water) and the sub-factor included nine foliar spray levels (control, melatonin 0.5 μM, melatonin 1 μM, brassinosteroid 0.25 μM, brassinosteroid 0.5 μM, combination of melatonin 0.5 μM and brassinosteroid 0.25 μM, combination of melatonin 1 μM and brassinosteroid 0.25 μM, combination of melatonin 0.5 μM and brassinosteroid 0.5 μM, and combination of melatonin 1 μM and brassinosteroid 0.5 μM).

Results and Discussion: Severe drought stress reduced the relative water content (36.95%), carotenoid (43.41%) and grain yield (51.66%) of quinoa compared to the non-stress treatment. Foliar application of melatonin 0.5 μM + brassinosteroid 0.5 μM increased the relative water content, carotenoid and grain yield of quinoa by 26.58, 41.55 and 62.96% respectively compared to the non-foliar application of growth regulators under severe stress conditions. With increasing drought stress levels, the activities of ascorbate peroxidase and phenylalanine ammonia lyase enzymes also increased. Also, the use of growth regulators increased the activities of these two enzymes under moderate and severe stress conditions.

Conclusion: The results of this study showed that drought stress significantly affects the physiological and biochemical traits of quinoa (Chenopodium quinoa Willd.). The use of melatonin and brassinosteroid as stress modulators improved the relative leaf water content, reduced ion leakage, and increased the activity of antioxidant enzymes. These compounds also helped to increase the content of chlorophyll and carotenoids, which in turn could lead to improved plant performance under drought stress conditions.