Sorghum Detoxification Mechanisms

ABSTRACT

The influence of cadmium (Cd) on internal and external defense mechanisms of Sorghum bicolor (L.) Moench. x Sorghum sudanense was studied by electrochemical methods to infer the type of detoxification processes developed by the plant under environmental stress conditions. Hydroponic experiments with sorghum were conducted in nutrient solutions in the absence and presence of 0.1 mg Cd L⁻¹. Plant exposure to Cd stimulated the release of root exudates with metal-binding affinity. However, their presence should not affect significantly the bioavailability of Cd, since the complex was dissociated within a very short period (≤ 50 milliseconds) in response to the consumption of the free metal ion at the root interface. The presence of Cd in the solution also stimulated a very significant increase of thiolic groups inside the plant, even at the very low concentration of 0.1 mg Cd L⁻¹. These results suggest that the main defense mechanism developed by sorghum against metal toxicity is an internal process, i.e., the synthesis of phytochelatins.

Keywords:

• cadmium
• exudates
• phytochelatins
• sorghum
• toxicity

ACKNOWLEDGMENTS

This work is part of the Research Project PRAXIS/P/QUI/10034/98 of the Science and Technology Foundation, Lisbon, Portugal. It also has financial support from the Portuguese Education Ministry through the PRODEP II—Ph.D. fellowship.

Notes

^e —excluded value due to a foliar disease.