Assessing the Potential Role of Zinc Oxide Nanoparticles for Mitigating Cadmium Toxicity in Capsicumannuum L. Under In Vitro Conditions

The application of metal and metal-oxide nanoparticles (NPs) in agriculture to mitigate the abiotic environmental stress factors and pollutants has gained extensive interests in research, particularly in studying their efficiency and their mechanistic effects. In this study, we aim to assess the potential of Zinc Oxide nanoparticles (ZnONPs) in monitoring Cadmium (Cd) toxicity on pepper (Capsicumannuum L.) in vitro cultured plants, by modulating their physiological and biochemical traits. In vitro culture was carried on Murasnige and Skoog (MS) medium, containing required amounts of macro- and micronutrients, vitamins, sucrose with pH 5.8 and agar. MS and NPs were autoclaved at 120 °C during 20 min. Our results showed that Cd exposure inhibited in vitro germination of seeds (by 17% at day 7, and 7% at day 14), and the growth of shoots (around 49% decrease in length) and roots (around 55% increase in length). However, the application of ZnONPs could recover 70–85% of shoots length, 40–50% of shoots fresh weights, also 60–80% of roots length and 70% of roots fresh weights. In shoots, ZnONPs act positively by boosting the antioxidant activities of enzymes; Catalase increased 30–75% with concentrations of 2.5, 5 and 10 mg L−1. Glutathione peroxidase increased around 13% and 63%, respectively, with concentrations of 2.5, 5 and 10 mg L−1. Guaicol peroxidase increased 40% with ZnONPs 5 mg L−1 but decreased with ZnONPs 10 mg L−1. ZnONPs also increased the activities of ascorbate peroxidase and glutathione reductase, respectively, fourfolds–tenfolds and twofolds–threefolds with increasing concentrations. This improved the elimination of H2O2 by around 70% and 80% with ZnONPs 2.5, 5 and 10 mg L−1, respectively. However, the ameliorating role of ZnONPs in roots response was revealed via reversing the oxidative stress symptoms induced by Cd. Overall, these defense mechanisms depicted that ZnONPs can improve Cd tolerance of plants. In addition, our study contains novelty in using in vitro culture to provide physiological and metabolic-scale insights for understanding the action mechanism of ZnONPs-induced alleviation of Cd toxicity in pepper. Hence, application of ZnONPs can be used as a promising approach in the remediation of Cd-contaminated environment.