The role of onion as the second most-consumed and cultivated vegetable around the world and its renowned qualities that lead it to be called the “queen of the kitchen” have positioned it as a vital source of nutritional and economic contributions around the world. Polyploidy serves as a groundbreaking innovation in plant breeding, improving the yield and vigor of plants. This study was conducted to determine the effects of applying different concentrations of colchicine to onion seedlings on their morphology, antioxidant activity, and dissolved sugars. The mutagen was applied to the onion seeds at three different concentrations (0.05, 0.1, and 0.2% w/v) for an exposure period of 24 h. A chromosomal analysis confirmed the induction of polyploidy, which led to the successful duplication of the chromosome number from diploid (2n = 16) to tetraploid (4n = 32). The control recorded a survival rate of 91.57%, while 83.33%, 3.33%, and 0.00% survival rates were recorded for seedlings treated with 0.05%, 0.1%, and 0.2% (w/v) concentrations of colchicine, respectively. Furthermore, the tetraploids showed significant differences in morphology, producing the tallest seedlings (reaching up to 73.6 cm tall) and the greatest average bulb diameter (of 5.64 cm) after 14 weeks. The tetraploids also showed significant differences in antioxidant activity and the amount of dissolved sugars, recording the highest DPPH scavenging percentage of 72.58% and refractive index of 1.369. Successful induction of polyploidy was achieved with the application of 0.05% (w/v) colchicine, which produced tetraploids that are morphologically and biochemically superior to other treated and control plants at a significance level of p < 0.05.

To mitigate food insecurity and the low economic status of amaranth farmers in south-west Nigeria, it is imperative to evaluate the predominant commercial A. hybridus cultivars using key yield and nutrient components to identify promising lines with optimum yield and nutritional quality. The current study evaluated five cultivars of Amaranthus hybridus in south-west Nigeria farms using key morphological, biochemical, and seed-proximate component parameters. The results revealed significant differences in the cultivars’ growth and yield indices. The number of leaves ranged from 16.40 to 29.48 ($\stackrel{-}{\mathrm{x}}$ = 22.10) with plant height ($\stackrel{-}{\mathrm{x}}$ = 50.28 cm), while other leaf metrics varied at maturity. Days to 50% flowering ranged from 49 to 90 days. Seed proximate composition also varied significantly. Carbohydrate content ranged from 65.01–76.28%, protein content 6.57–12.42%, ash content 2.73–3.59%, total polyphenols 23.36–29.64 mg/100 g GAE, and total antioxidants 31.6–181.59 mg/100 g AAE. The Pearson correlation matrix and path analysis showed significant relationships among the yield components. The phenotypic variance of the yield components mainly stemmed from environmental factors, indicating their crucial role in the traits’ expression. Careful evaluation of the cultivars revealed that AM-1 and AM-3 had the best growth and yield attributes. Additionally, the two cultivars showed a significant degree of relatedness, suggesting a common ancestor. They have the potential to increase productivity, farmers’ profitability, and consumers’ nutrient gain compared to other cultivars. They are recommended for cultivation and can be further crossed in breeding programs to develop superior lines.