Impact of vanadium stress on physiological and biochemical characteristics in heavy metal susceptible and tolerant Brassicaceae
There is an influx in heavy metals into soils and ground water due to activities such as increased mineral mining, improper watering and the use of heavy metal contaminated fertilizers. These heavy metals are able to increase the ROS species within plants which may result in plant metabolism deterioration and tissue damage. Heavy metals may also directly damage plants by rendering important enzymes non-functional through binding in metal binding sites of enzymes. The heavy metal focused on in this study was vanadium due to South Africa being one of the primary produces of this metal. Two related Brassica napus L cultivars namely Agamax and Garnet which are economically and environmentally important to South Africa were exposed to vanadium. Physiological experiments such as cell death, chlorophyll and biomass determination were conducted to understand how these cultivars were affected by vanadium toxicity. A low cost, sensitive and robust vanadium assay was developed to estimate the amount of vanadium in samples such as water, soils and plant material. The oxidative state as well as the antioxidant profile of the two cultivars were also observed under vanadium stress. A chlorophyll assay which was conducted on the two cultivars exposed to vanadium showed a marked decrease in chlorophyll A in the suspected sensitive cultivar which was Garnet. However, the suspected tolerant cultivar Agamax fared better and the decrease in chlorophyll A was much less. A similar trend was observed for the two cultivars when the cell death assay was conducted. The vanadium assay showed that Garnet had higher concentrations of vanadium within its leaves and lower concentrations in its roots when compared to Agamax. This observation displayed that Agamax had inherent mechanisms which it used to localize vanadium in its roots and which assisted in its tolerance to the vanadium stress. The oxidative state was determined by doing assays for the specific reactive oxygen species namely hydrogen peroxide and superoxide. It was observed that vanadium treated Garnet leaves had higher reactive oxygen species (ROS) production when compared to the Agamax treated leaves. In-gel native PAGE activity gels were conducted to determine the antioxidant profile for the two cultivars which were exposed to vanadium. The antioxidant enzymes which were under investigation were ascorbate peroxide (APX), superoxide dismutase (SOD) and glutathione-dependent peroxidases (GPX-like) as these enzymes are known to be responsible for controlling the ROS produced in the plants. The GPX-like profile consisted of three isoforms. No isoforms were inhibited by vanadium treatments but one isoform had increased activity in both the Garnet and Agamax treated samples. The SOD profile for Garnet consisted of six isoforms and Agamax had seven isoforms. One isoform which was visualized in both Agamax as well as Garnet was inhibited by vanadium treatments. Agamax also had two isoforms which were up-regulated however the corresponding isoforms in Garnet showed no change. The Ascorbate peroxidase profile consisted of seven isoforms for both Garnet and Agamax. No isoforms were inhibited by vanadium treatment. Three isoforms were up-regulated in Garnet and Agamax under vanadium treatments. Here, it is illustrated that Garnet lacked certain mechanisms found in Agamax (and thus experienced more cell death, yield and chlorophyll loss) and performed worst under high vanadium concentrations. Although Garnet increased the activity of some of its antioxidant isoforms in response to increasing ROS levels it was not adequate to maintain a normal oxidative homeostasis. This disruption in oxidative homeostasis lead to plant damage. Agamax was observed to produce less ROS than Garnet and was able to control the ROS produced more effectively than Garnet and thus less damage was observed in Agamax.