Assessment of chromium species dynamics in root solutions using isotope tracers

Chromium (Cr) exists in the environment in two chemical forms; Cr^III is an essential micronutrient for glucose and lipid metabolism, whereas Cr^VI 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 Cr^VI.

Isotopically enriched species of Cr, added as ⁵⁰Cr^III and ⁵³Cr^VI, were used to investigate transfer from solution into the root systems of Spinacia oleracea. In addition the effect of sulphate (SO₄^2-), as a competitor for Cr^VI uptake, was investigated. Separation of ⁵⁰Cr^III and ⁵³Cr^VI was undertaken using HPLC-ICP-QQQ following isolation of root solutions using freeze/thaw centrifugation.

Irrespective of supplied Cr^VI 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 Cr^III. There was evidence for Cr^VI reduction in the rhizosphere prior to uptake as an additional detoxification mechanism. Sulphate promoted uptake of Cr^VI through the active pathway, although increases in SO₄^2- concentration did not yield a proportional increase in Cr symplastic solution concentration; Cr^III was also the dominant species in these root solutions.

The results indicate that Spinacia oleracea plants can effectively reduce Cr^VI to Cr^III and that the uptake pathways for both Cr^III and Cr^VI are more complex than previously reported. Further work is required to understand the physiological processes that result in the reduction of Cr^VI 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.