Enhancing chromium resistance and bulb quality in onion (Allium cepa L.) through copper nanoparticles and possible health risk

Chromium (Cr) is a toxic metal in soil-plant system, hence causing possible health risks prominently in the areas with forgoing industrial activities. Copper nanoparticles (Cu NPs) have been reported as an excellent adsorbent for pollutants. Therefore, this study investigates how copper nanoparticles enhance onion growth while decreasing chromium uptake in onion plants. Additionally, it examines the potential health risks of consuming onion plants with elevated chromium levels. The results demonstrated that the addition of CuNPs at 15 mg kg^-1 significantly improved the plant height (48%), leaf length (37%), fresh weight of root (61%), root dry weight (70%), fresh weight of bulb (52%), bulb dry weight (59%), leaves fresh weight (52%) and dry weight of leaves (59%), leaf area (72%), number of onion leaves per plant (60%), Chl. a (42%), chl. b (36%), carotenoids (40%), total chlorophyll (40%), chlorophyll contents SPAD value (56%), relative water contents (35%), membrane stability index (16%), total sugars (25%), crude protein (21%), ascorbic acid (19%) and ash contents (64%) at 10 mg kg^-1 Cr. Whereas, maximum decline of Cr by 46% in roots, 68% in leaves and 92% in bulb was found with application of 15 mg kg^-1 of Cu NPs in onion plants under 10 mg kg^-1 Cr toxicity. The health risk assessment parameters of onion plants showed minimum values 0.0028 for average daily intake (ADI), 0.001911 for Non-cancer risk (NCR), and 0.001433 for cancer risk (CR) in plants treated with Cu NPs at 15 mg kg^-1 concentration grown in soil spiked with 10 mg kg^-1 chromium. It is concluded that Cu NPs at 15 mg kg^-1 concentration improved growth of plants in control as well as Cr contaminated soil. Therefore, use of Cu NPs at 15 mg kg^-1 concentration is recommended for improving growth of plants under normal and metal contaminated soils.