Heavy metal contamination has adversely affected crop productivity around the world. Vanadium (V) toxicity is a significant environmental factor that lowers agricultural productivity by inhibiting plant development, diminishing nutrient uptake, and damaging root morphology. Melatonin (ME) is a well-known powerful antioxidant and multifunctional molecule that improves the resilience of plants against environmental stressors. In this work, we investigated the response of different pepper species [Capsicum annuum L. (CA), C. baccatum L. (CB), C. pubescens Ruiz & Pav. (CP)] to V toxicity as well as the possible roles of ME in enhancing V stress tolerance. Pepper roots were pretreated with ME (5 mmol · L^-1) for three days, then V (30 mg · L^-1) was applied for 2 weeks as a stress treatment. The findings showed that plants pretreated with ME exposed to V stress had improved root activity and biomass production. The root morphology and mineral nutrient accumulation of the pepper species were greatly improved by ME application. In addition, ME treatments lowered leaf V concentrations by lowering V translocation from root to shoot. In a nutshell, ME treatment modified root architecture system, increased concentration of mineral nutrient, and reduced V accumulation of pepper species under V stress.

This study assessed the influence of exogenous ME in the mitigation of cold damage in pepper seedlings. Melatonin (ME) is a dynamic molecule that helps plants cope with stress in several ways. Cold stress (CS) is one of the most important environmental factors that restrict plant growth and yield. Pepper (Capsicum annuum L.) is a valuable commercial crop, highly sensitive to CS. Thus, identifying an efficient strategy to mitigate cold damage is critical for long-term pepper production. For this purpose, the roots of pepper seedlings were pretreated with ME (5 μmol · L⁻¹) and exposed to CS for 7 d. The results indicated that CS suppressed pepper growth, hampered photosynthetic capacity, and damaged root architecture in pepper plants. In contrast, the production of reactive oxygen species (ROS), malondialdehyde (MDA), electrolyte leakage (EL), proline, and soluble sugars were enhanced in plants under CS. ME (5 μmol · L⁻¹) pretreatment reduced the negative effects of CS by recovering plant growth, root traits, gas exchange elements, and pigment molecules compared to CS control treatment. Furthermore, ME application efficiently reduced oxidative stress markers [hydrogen peroxide (H₂O₂), superoxide ion (O₂^·–), EL, and MDA] while increasing proline and soluble sugar content in pepper leaves. ME application combined with CS further increased antioxidant enzymes and related gene expression. Collectively, our results confirmed the mitigating potential of ME supplementation for CS by maintaining pepper seedling growth, improving the photosynthesis apparatus, regulating pigments, and osmolyte content.