Melatonin and nanocopper synergistically regulate cadmium toxicity in Brassica napus: evidences from photosynthesis phenomics, oxidative metabolism, and multiple defense responses

Abstract

Cadmium (Cd) toxicity severely impedes plant growth and yield. Melatonin (MEL) and copper oxide nanoparticles (CuO NPs) have independently been demonstrated to enhance plant growth and mitigate heavy metal stress. However, their combined effect in managing Cd toxicity in oilseed crops remains largely unexplored. Thus, we evaluated the efficacy of 10 μM MEL and/or 3 mg L⁻¹ CuO NPs to regulate the tolerance of Brassica napus cultivars (ZD 635 and ZD 622) to Cd toxicity. Results revealed that CuO NPs and/or MEL considerably lowered Cd toxicity by minimizing the accumulation of Cd (44–53%), malondialdehyde (29–37%), H₂O₂ (28–35%) and O₂˙⁻ (26–34%) and improving the photosynthesis phenomics (pigments, gas exchanges, PSII and Chl a fluorescence), resulting in higher biomass, membrane integrity and lower oxidative stress. Moreover, CuO NPs and/or MEL significantly boosted the antioxidant enzyme activity and production of total phenolics, total flavonoids, proline and phytohormones (SA, ABA, MEL and JA) in Cd-treated plants, revealing the key roles of CuO NPs and/or MEL in improving plant defense. Additionally, the potential protective roles of CuO NPs and/or MEL significantly recovered the Cd-induced cellular damages, as observed by the improvements in the leaf ultrastructure (chloroplast, mitochondria, and thylakoid membranes), stomatal aperture and guard cells. It was evident that the combined application of CuO NPs and MEL was more effective in alleviating the accumulation and toxicity of Cd in B. napus tissues than the separate treatments. Cd bioaccumulation, photosynthesis and antioxidant defense responses can serve as ideal indicators for oilseed crops grown in Cd-contaminated soils.