Effect of Brassinosteroid and Melatonin Foliar Application on Biochemical Characteristics and Growth Indices of Quinoa (Chenopodium quinoa Willd) under Drought Stress Condition

IntroductionThis research investigates the effects of brassinosteroid and melatonin foliar applications on quinoa (Chenopodium quinoa Willd.) subjected to drought stress. As a highly nutritious new plant, quinoa is increasingly valued for its tolerance in arid conditions like our country. However, drought stress or irrigation deficit significantly impairs its growth and biochemical composition. Brassinosteroids and melatonin as plant growth regulators are known to enhance plant tolerance to various abiotic stresses, including drought, by modulating physiological and biochemical pathways. This research aims to elucidate how these compounds influence key growth indices, such as plant height, leaf area, biomass, harvest index and as well as biochemical characteristics like soluble carbohydrate, and proline accumulation in quinoa growing under drought stress. By understanding these effects, the study seeks to provide insights into potential agronomic practices to improve quinoa productivity and stress resilience under water-limited conditions.Materials and MethodsThe experiment, conducted at Shahid Bahonar university of Kerman during 2020 and 2021 cropping seasons, used a split-plot layout based on a randomized complete block design with three replications. Main plots consisted of three irrigation levels (100%, 75%, and 50% of field capacity), and sub-plots included five foliar application treatments (control, 0.25 and 0.5 micromolar brassinosteroid, and 0.5 and 1 micromolar melatonin). Soil sampling and nutrient adjustments were made before planting. Trifluralin herbicide was used for weed control, and planting occurred on August 1st with 40 cm row spacing and 3-meter row lengths, followed by immediate irrigation. Foliar applications were conducted at the start of flowering and a week later, early in the morning before sunrise. Water stress treatments began after the first foliar application. Leaf area index (LAI) and Crop Growth Rate (CGR) were measured at the beginning of flowering and one week later, using Gardner's equations. Chlorophyll fluorescence was assessed via the saturation pulse method to determine the maximum quantum yield of photosystem II. Proline and soluble sugars measurements were taken from young leaves post-stress treatment. For morphological trait assessment, including plant height, leaf area, dry weight of aerial parts, and yield components, five plants were randomly sampled from each plot. Biological yield, harvest index, and grain yield were calculated by harvesting plants from the middle four rows, each 2 meters long. Data analysis was performed using SAS v. 9.1, with mean comparisons conducted using Duncan's multiple range test at the 5% probability level.Results and DiscussionThe results showed that foliar application of plant growth regulators (PGR) increased plant height across all irrigation levels. The greatest height, 150.5 cm, was observed with 0.5 micromolar melatonin and 100% field capacity irrigation. Additionally, 0.5 micromolar brassinosteroid with irrigation with 75% of field capacity increased plant height by 11.1% compared to the control at the same irrigation level, with a similar trend under severe drought stress. Increased drought stress reduced the leaf area index (LAI) by 8.37% and 31.6% compared to normal irrigation. The highest concentration of brassinosteroids increased the crop growth rate (CGR) at all irrigation levels (4.8, 4.4 and %16.8 in normal, mild and severe drought stress conditions respectively, but thousand grain weight decreased to %93.3 and %62 of control with rising drought stress. Brassinosteroids were more effective than melatonin in enhancing thousand grain weight. Both brassinosteroid and melatonin applications reduced chlorophyll fluorescence under mild and severe drought stress compared to controls. Soluble carbohydrates content increased by 50.4% at 50% field capacity irrigation compared to normal irrigation. Under mild stress, only brassinosteroid significantly increased proline content, with the highest level (3.19 µmol g-1) at the highest concentration. The highest dry weight, harvest index, and grain yield were achieved with 0.5 micromolar brassinosteroid under mild and severe stress. Brassinosteroids, as new phytohormones, are promising for enhancing crop productivity under stress and non-stress conditions.ConclusionReduction in irrigation from 100% field capacity or increased drought stress led to decreased plant height, growth rate, leaf area index, thousand grain weight, and harvest index, while chlorophyll fluorescence, proline content, and soluble carbohydrates increased. Despite the increases in these two latter mentioned traits, plant dry weight and grain yield decreased under drought stress. The application of brassinosteroids and melatonin improved plant traits, with 0.5 micromolar brassinosteroid being particularly more effective, especially under mild and severe stress. This treatment mitigated drought's negative effects, suggesting its potential to enhance agricultural productivity in arid and semi-arid regions.