Efficient superoxide scavenging and metal immobilization in roots determines the level of tolerance to Vanadium stress in two contrasting Brassica napus genotypes

Brassica napus also known as Rapeseed is a member of the Brassicaceae family which is mainly cultivated for its oil-rich seeds. Indeed, B. napus is ranked the third-largest source of vegetable oil in the world. Brassica napus growth, development and yield are negatively affected by heavy metals. Vanadium is a heavy metal and presence in high concentrations impact plant growth and development negatively. However, the impact of Vanadium on B. napus growth and development is unknown. Therefore, in this study we assessed the effects of Vanadium stress on leaf physiology and biochemistry response of two B. napus cultivars (namely Agamax and AV Garnet). A randomised pot-experiment under controlled conditions was used to grow B. napus cultivars under control (distilled water) and Vanadium (350 μM NaVO₃) treatments. Results showed that Vanadium caused yellowing of AV Garnet leaves but not Agamax leaves. Furthermore, Vanadium stress caused a more severe decrease in leaf dry and fresh weight of AV Garnet as compared to the decrease in leaf dry and fresh weight of Agamax. We also observed that Vanadium stress only decreased leaf chlorophyll content (a, b and total) of AV Garnet but had no effect on chlorophyll content of Agamax. In addition, Vanadium stress induced an increase in toxic superoxide (O₂⁻) content in leaves of both AV Garnet and Agamax however; we observed more O₂⁻ content in AV Garnet leaves than Agamax leaves. Furthermore, we observed a more drastic increase in leaf lipid peroxidation and leaf cell death (assessed by Evans blue uptake) of AV Garnet when compared to Agamax. In order to investigate whether Vanadium regulates O₂⁻ metabolising enzymes we assessed superoxide dismutase (SOD) activity (total SOD enzymatic activity and SOD isoform activity). Vanadium inhibited the total leaf SOD activity of AV Garnet more than the total leaf SOD activity of Agamax. The SOD isoform analyses displayed that Vanadium treatment did not alter the leaf MnSOD as well as leaf Cu/ZnSOD isoforms of both Agamax and AV Garnet. However, we observed that FeSOD 1 and FeSOD 3 activity was upregulated in Agamax leaves in response to Vanadium treatment but decreased in AV Garnet following Vanadium treatment. Furthermore, we observed that the leaf FeSOD 2 activity was inhibited in both Agamax and AV Garnet. In addition, we also analysed the Vanadium contents in the two cultivars following Vanadium treatment and observed more Vanadium content in Agamax roots than in AV Garnet roots. Furthermore, translocation factor (TF) analysis showed that AV Garnet had a higher Vanadium TF from roots to leaves than Agamax following Vanadium treatment. In conclusion, Vanadium stress tolerance in B. napus is possibly controlled by SOD activity and Vanadium content root immobilisation.