Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/1996
Publication Date: N/A
Interpretive Summary: Aluminum (Al) is a major constituent of the soil and toxic forms of Al are solubilized into the soil solution at low soil pH (pH < 5). Because there are large areas of acid soils throughout the U.S. and the world, Al toxicity is a major agronomic problem limiting crop production on these soils. There has been considerable recent research aimed at understanding the plant-based mechanisms of Al tolerance, and recent progress in this area has identified root release of Al-binding organic acids as a potential Al detoxification mechanism. Most of this research has been conducted on plants grown in nutrient solutions, as basic research on plants growing in acid soils is difficult to carry out due to the complex nature of soils and plant/soil interactions. However, we need to verify that these mechanisms can function to provide Al tolerance to plants growing in soils. Therefore in this study we looked at the interactions of malate, which is an organic acid released from roots and implicated in mechanisms of Al detoxification and tolerance, with acid soils. It was found that malate effectively bound soil Al in preference to other metals in the soil. Also, malate was not strongly bound to soil particles, and thus could remain available to bind Al. These results indicate that if roots release significant quantities of malate into acidic, Al-toxic soils, this could be an effective detoxification strategy for Al tolerant plant species.
Technical Abstract: Malic acid has been implicated as an inducible root mechanism for the detoxification of rhizotoxic Al. The chemical reactions of malate were therefore investigated in four acid soils ranging in pH from 4.30 to 5.00. All the soils behaved similarly, with malate predominantly complexing Al in solution in favor of other metal cations (Fe, Mn, Cu, Zn, Ca and Mg). Comparison of experimental extraction results with theoretical predictions made using Geochem-PC showed a similar agreement, indicating that the behavior of Al and malate in soil was similar to their behavior in an ideal solution. Competitive anion sorption studies with synthetic Fe and Al(OH)3(s) gels showed similar sorption sites for phosphate, citrate and malate, however, malate was weakly sorped in comparison to phosphate and citrate. Sorption was little affected by solution pH. Malate sorption did not appear to be largely influenced by metal complexation in soils; however, sorption onto Fe and Al(OH)3 gels was markedly reduced in the presence of complexing metal cations. The implications of these results in understanding the role of malate exudation from the root in Al detoxification in the rhizosphere are discussed.