Chitosan-Copper Nanocomposites Exterminate Cd Toxicity in Capsicum annuum L. through Improving Photosynthetic Attributes, Antioxidant Defense, and Reduced Cd Uptake

Plants can readily absorb Cd and distribute it to various parts of the body. In this study, various agronomic, physiological, and biochemical features of Capsicum annuum L. were analyzed under Cd stress along with the potential of chitosan-copper (CTS-Cu) nanocomposites in the mitigation of Cd stress. A two-factor (Cd stress and CTS-Cu nanocomposite) completely randomized design pot experiment with three replicates was conducted under natural conditions. The following treatments were administered: control: normal (untreated) plants, Cd stress: 5 mM CdCl₂ (500 mL per pot), NC1: foliar application of CTS-Cu nanocomposite at 100 mg L^-1, and NC2: foliar application of CTS-Cu nanocomposite at 200 mg L^-1. Results showed that Cd stress reduced the performance index (PI; 52%) and quantum efficiency of photosystem II (10%). This also caused lipid peroxidation in the form of elevated malondialdehyde (33%), relative membrane permeability (31%), and excessive release of H₂O₂ (70%). Cd stress also disrupted the ionic balance through reducing the uptake of K⁺ (41%) and Ca²⁺ (44%). Stressed plants also had high levels of Cd metal ions in their root and shoot. Both levels of CTS-Cu nanocomposite (NC1 and NC2) mitigated oxidative stress caused by Cd toxicity. But the NC2 more efficiently reduced Cd toxicity in C. annuum plants. It improved chlorophyll formation (30%) and chlorophyll fluorescence, as indicated by increased PI (372%). NC2 also increased the activities of antioxidants such as catalase (29%) and peroxidase (30%). It also improved net photosynthesis rate (64%), transpiration rate (17%), and stomatal conductance (54%). NC2 maintained reduced Cd uptake through the root (36%) and its accumulation in the shoot (41%). These findings suggest that CTS-Cu nanocomposite can be used to eliminate abiotic stresses in other crop plants as well.