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Background: Root system architecture (RSA) exhibits significant genetic variability and is closely associated with drought tolerance. However, the evaluation of drought-tolerant cotton cultivars based on RSA in the field conditions is still underexplored. Results: So, this study conducted a comprehensive analysis of drought tolerance based on physiological and morphological traits (i.e., aboveground and RSA, and yield) within a rain-out shelter, with two water treatments: well-watered (75 ± 5% soil relative water content) and drought stress (50 ± 5% soil relative water content). The results showed that principal component analysis identified six principal components, including highlighting the importance of root traits and canopy parameters in influencing drought tolerance. Moreover, the systematic cluster analysis was used to classify 80 cultivars into 5 categories, including drought-tolerant cultivars, relatively drought-tolerant cultivars, intermediate cultivars, relatively drought-sensitive cultivars, and drought-sensitive cultivars. Further validation of the drought tolerance index showed that the yield drought tolerance index and biomass drought tolerance index of the drought-tolerant cultivars were 8.97 and 5.05 times higher than those of the drought-sensitive cultivars, respectively. Conclusions: The RSA of drought-tolerant cultivars was characterised by a significant increase in average length-all lateral roots, a significant decrease in average lateral root emergence angle and a moderate root/shoot ratio. In contrast, the drought-sensitive cultivars showed a significant decrease in average length-all lateral roots and a significant increase in both average lateral root emergence angle and root/shoot ratio. It is therefore more comprehensive and accurate to assess field crop drought tolerance by considering root performance. [ABSTRACT FROM AUTHOR]

Drought stress is the major environmental factor that inhibits the growth and reduces the yield of cotton (Gossypium hirsutum L.). The exogenous application of melatonin has been shown to alleviate drought stress damage, but it is unclear whether melatonin mediates the alleviation by preventing the drought-induced inhibition of root growth. To address this gap, we subjected cotton plants grown in RhizoPotsto treatments of thorough watering, drought stress treatment, and drought stress supplemented with exogenous melatonin. Melatonin promoted the aboveground growth compared with the non-melatonin treatment under drought stress, and increased the plant height, stem diameter, leaf area, SPAD value, and net photosynthetic rate. The application of melatonin significantly prevented the drought-induced yield losses by increasing the boll weight. The drought stress+melatonin treatment significantly increased the root length, root surface area, root volume, and specific root length by 18.36%, 25.46%, 26.07%, and 18.53%, respectively, compared with the drought stress treatment. Under drought stress, melatonin increased the root length by increasing its maximum and average rates of accumulation and increased the taproot diameter by increasing its maximum and average rates of accumulation. The drought stress+melatonin treatment significantly increased the lifespan of lateral roots and root hairs compared with the drought stress treatment. In conclusion, this study indicated that exogenous melatonin could improve the drought resistance and yield of cotton by promoting root growth and delaying root senescence. [Display omitted] • Exogenous melatonin enhances cotton drought resistance by promoting root's development and prolonging root's lifespan. • Exogenous melatonin increases the root length by increasing its maximum and average accumulation rates. • Exogenous melatonin increases the length and lifespan of root hairs under drought stress. [ABSTRACT FROM AUTHOR]