Abstract
<title>Abstract</title> <p> <italic>Amaranthus hybridus</italic> is an important leafy vegetable consumed daily in many Nigerian households because of its high nutritional content and adaptability to local soils. Its ability to bioaccumulate metals, including manganese, presents both an advantage and a risk. While sufficient Mn promotes biomass accumulation, chlorophyll synthesis, and crop quality, excess Mn can reduce productivity and lead to undesirable accumulation in edible tissues, which may affect consumer health. This study investigated the impacts of varying manganese (Mn) concentrations on the growth and development of <italic>Amaranthus hybridus</italic> seeds (variety NGAC-00-3) obtained from the National Horticultural Research Institute (NIHORT), Ibadan. Loamy-sand soil was collected at coordinates 6°27”33.16'N 4°45'53.86°'E and fortified with Mn to desired concentrations designated as treatments S <sub>1</sub> (0.13g), S <sub>2</sub> (0.10g), S <sub>3</sub> (0.07g), and S <sub>4</sub> (control- untreated), each replicated three times and arranged in Randomised Complete Block Design (RCBD). Growth characters, including leaf length, leaf width, petiole length, stem length, and stem width, were measured. Plant proximate analysis was carried out using a standard procedure as described by AOAC (2005. Data obtained was analysed using SPSS version 16, while mean separation was carried out using Duncan's Multiple Range Test at p ≤ 0.05. Control (S4: 6.41 ± 0.33 cm) showed the highest leaf length, while the highest Mn stress (S1: 5.90 ± 0.30 cm) was observed, implying that excessive Mn inhibits leaf elongation. Highest leaf width S4 (6.41 + 0.33 cm), petiole length S4 (5.19 ± 0.16 cm) and stem width S4 (1.24 + 0.06 cm) were observed while Mn-stressed plants exhibited reduced radial growth, which implies Mn toxicity limits both vertical and radial stem expansion. These findings highlight the importance of regulating Mn levels in soils to optimise the growth and biomass production of leafy vegetables. The data provide critical insights for nutrient management and sustainable cultivation practices of <italic>Amaranthus hybridus</italic> in Mn-prone soils. </p>