Foliar application of zinc oxide nanoparticles ameliorated the adverse effects of cobalt toxicity in Raphanus sativus by regulating growth, photosynthesis and antioxidant defense machinery

Abstract

Heavy metals toxicity poses oxidative stress in plants by generating reactive oxygen species (ROS), which in turn cause damage to proteins, lipids, nucleic acids, and disrupt the structural integrity of mitochondria and chloroplasts. The aim of present investigation was to examine the phytotoxic effects of cobalt (Co) on growth, physio-biochemical and oxidative stress parameters related in Raphanus sativus. On the other hand, potential of zinc oxide nanoparticles (ZnO NPs) for alleviation of Co toxicity (100 mg kg^-1) in radish plants was explored. The current findings demonstrated that the foliar spray of ZnO NPs at varying concentrations (20, 40, and 80 mg L^-1) significantly enhanced the growth attributes such as shoot fresh and dry weight, shoot and root length, chlorophyll a, chlorophyll b, and carotenoid contents of R. sativus during non-stressed conditions. The ZnO NPs (80 mg L^-1) augmented the root and shoot fresh biomass, carotenoids, chlorophyll a, and chlorophyll b levels by 49%, 56, 29%, 45%, and 61%, respectively of R. sativus exposed to non-stress regime in contrast with untreated control. Nevertheless, the Co toxicity remarkably reduced the growth parameters and photosynthetic attributes while enhanced the levels of antioxidant enzymes, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), proline, phenol and flavonoids in R. sativus. On the other hand, foliar supply of ZnO NPs enhanced the growth, photosynthetic parameters, proline, phenol, flavonoid, superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) amounts of radish under Co stress with respect to relevant control. The exogenous ZnO NPs (80 mg L^-1), escalated the quantities of SOD, CAT, POD, proline, phenol and flavonoids by 23%, 42%, 34, 29%, 25% and 39%, respectively, in R. sativus subjected to Co stress. Additionally, ZnO NPs (80 mg L^-1) declined the MDA, H₂O₂, and Co uptake in the root and shoot by 28%, 17%, 48%, and 58%, respectively, which might have enabled R. sativus plants to cope with Co toxicity. The outcomes of the present study advocate the promising potential of ZnO NPs as an innovative and ecofriendly approach for sustainable cultivation of vegetables in cobalt-contaminated soils.

Keywords; Cobalt; exogenous, lipid peroxidation; oxidative stress; phytostabiliztion, radish; zinc oxide nanoparticles