Assessment of chromium species dynamics in root solutions using isotope tracers

Background Chromium (Cr) exists in the environment in two chemical forms; CrIII is an essential micronutrient for glucose and lipid metabolism, whereas CrVI is toxic and a recognised carcinogen through inhalation. Numerous studies have attempted to evaluate their transfer mechanisms from soil and solution media into plants, usually with respect to the hyperaccumulation, detoxification and tolerance of the plant to CrVI. Methods Isotopically enriched species of Cr, added as 50CrIII and 53CrVI, were used to investigate transfer from solution into the root systems of Spinacia oleracea. In addition the effect of sulphate (SO42−), as a competitor for CrVI uptake, was investigated. Separation of 50CrIII and 53CrVI was undertaken using HPLC-ICP-QQQ following isolation of root solutions using freeze/thaw centrifugation. Results Irrespective of supplied CrVI concentration (250, 500 or 1000 μg L−1), the dominant species in both apoplastic (routed through cell wall and intercellular space as a passive mechanism) and symplastic (routed through cytoplasm as an active mechanism) root solutions was CrIII. There was evidence for CrVI reduction in the rhizosphere prior to uptake as an additional detoxification mechanism. Sulphate promoted uptake of CrVI through the active pathway, although increases in SO42- concentration did not yield a proportional increase in Cr symplastic solution concentration; CrIII was also the dominant species in these root solutions. Conclusion The results indicate that Spinacia oleracea plants can effectively reduce CrVI to CrIII and that the uptake pathways for both CrIII and CrVI are more complex than previously reported. Further work is required to understand the physiological processes that result in the reduction of CrVI prior to, and during, uptake. The efficacy of sulphate to augment existing agricultural management strategies, such as liming and organic reincorporation, also requires further investigation to establish suitable application rates and applicability to other environmental contaminants.