Comprehensive analysis of hemp growth, physiological responses, and secondary metabolite production under zinc sulfate and melatonin treatments

This study investigated the interactive effects of melatonin (12 ppm) and zinc sulfate (ZnSO₄; 0, 0.1, and 0.5 ppm) on growth, physiological traits, oxidative stress responses, nutrient accumulation, and secondary metabolite production in hemp ( Cannabis sativa L.) under greenhouse conditions. A factorial completely randomized design with three replications was employed. Results showed that melatonin and ZnSO₄, individually and in combination, enhanced shoot biomass, leaf area, chlorophylls, and carotenoids, while reducing oxidative stress indicators (H₂O₂ and MDA). Cannabidiol (CBD) accumulation increased significantly under ZnSO₄ and was further modulated by melatonin in selected treatments, whereas tetrahydrocannabinol (THC) levels consistently decreased. Nutrient analysis revealed altered Zn and Cu uptake patterns, suggesting possible Zn–Cu antagonism. Although some parameters exhibited synergistic responses (e.g., shoot growth, chlorophyll), others (e.g., root elongation, CBD accumulation) responded primarily to ZnSO₄ alone. These results indicate that melatonin can enhance specific physiological effects of ZnSO₄, contributing to stress mitigation and cannabinoid optimization in hemp. However, the short experimental period (21 days) and the lack of molecular analyses limit mechanistic interpretation. Further work integrating gene expression studies of Zn transporters, ROS-scavenging enzymes, and cannabinoid biosynthesis pathways is needed to fully understand the interactions between melatonin and Zn nutrition in hemp. • Melatonin (12 ppm) + ZnSO₄ (0–0.5 ppm) boosted hemp growth/biomass. • ppm ZnSO₄ + melatonin maximized shoot (15 %) & root (21 %) growth. • Melatonin + 0.1 ppm ZnSO₄ enhanced chlorophyll & leaf water status. • Oxidative stress (MDA/H₂O₂) reduced; CBD rose 81 % at 0.5 ppm ZnSO₄. • ZnSO₄ + melatonin synergistically improved stress resilience in hemp.