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 individually demonstrated to enhance plant growth and mitigate heavy metals stress. But, their interactive effects in managing Cd-toxicity in oilseed crops remain largely unexplored. Thus, we evaluated the efficacy of 10 µM MEL and/or 3 mg/L 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 greatly lowered the 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), correlated with higher biomass, membrane integrity and lowered oxidative stress. Moreover, CuO NPs and/or MEL significantly boosted the antioxidants enzyme activity, total phenolics, total flavonoids, proline and phytohormones (SA, ABA, MEL and JA) production in Cd-treated plants, revealed 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 indulged cellular damages as observed by the improvements in leaf ultrastructure (chloroplast, mitochondria, and thylakoids membrane etc.), stomatal aperture and guard cells. It was obvious that the combined applications of CuO NPs and MEL were more prominent in alleviating the accumulation and toxicity of Cd in B. napus tissues than alone treatment. The Cd-bioaccumulation, photosynthesis and antioxidant defense responses can serve as ideal indicators for oilseed crops grown in Cd-contaminated soils.