Differential cobalt-induced effects on plant growth, ultrastructural modifications, and antioxidative response among four Brassica napus (L.) cultivars

Abstract

Despite the increasing environmental threat of cobalt in the modern era, less is known as its phytotoxicity behavior. Therefore, the present study was undertaken to assess the toxicity effects of cobalt and to understand the associated physio-biochemical response in Brassica napus, an economically important plant crop species. Five-day-old seedlings of four cultivars (Zheda 619, Zheda 622, ZS 758, and ZY 50) were exposed to five different levels of cobalt under hydroponic conditions. Results showed a concentration-dependent inhibition of plant growth, accompanied by notable chlorophyll loss, protein degradation, and accumulation of reactive oxygen species and malondialdehyde. Further, Co contents in different plants parts were found to be higher in Zheda 622 than all other cultivars. In all cultivars, the contents of enzymatic activities (SOD, POD, GR, and GSH) were markedly increased following cobalt exposure; by contrast, catalase and ascorbate peroxidase activities declined with increased cobalt concentration in medium, which was also, echoed by the pattern of enzymes-related mRNA levels. Morphological observations, supported by ultrastructural analysis revealed clear differences in cobalt sensitivity among cultivars, with ZS 758 identified as less sensitive cultivar, and Zheda 622 the most sensitive one. In addition to revealing genotypic differences in cobalt sensitivity in B. napus, findings suggest the mechanisms of cobalt tolerance in this specie could, at least partially, in relation with the ability of plant to sustain the activity of superoxide dismutase and guaicol peroxidase and to maintain glutathione reduced pool through the action of glutathione reductase.