Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/2/1997
Publication Date: N/A
Citation: 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. Thus, in this study, we looked at the effect of citric acid, which is released from roots of some Al tolerant plants, on solubilization and chelation of soil Al in an Al-toxic, acid soil. It was found that citrate could effectively bind and detoxify both soluble Al in the soil solution and also Al in the solid soil phase. These findings are important, for they provide experimental evidence that root release of citric acid could be an effective Al tolerance mechanism for plants growing in acid soils.
Technical Abstract: A study was undertaken to quantify the rates of aluminum release in an acid soil (pH 4.40) which was known to produce differential growth responses to Al in Al-resistant and sensitive wheat cultivars which are characterized by differences in root organic acid exudation. Soil columns were leached with artificial soil solutions containing no Al in the presence or absence of citrate for periods of up to 12 days. The Al release rates could be resolved into two dissolution phases: A fast release phase for Al was attributed to the cation exchangeable pool while a second slower phase was attributable to the dissolution of readily weatherable minerals. Citrate increased the dissolution rates two fold in comparison to experiments performed without citrate. It was concluded that for rhizosphere considerations, the total releasable Al pool was finite in size and constituted approximately 2 % of the soil's total Al reserves. This pool was not increased markedly in the presence of citrate. It was concluded that citrate not only complexed Al in solution but also complexed Al directly from the mineral phase. From experimental Al release rates, it was deduced that only the soil solution and exchangeable Al pools were responsible for Al rhizotoxicity and that organic acids exuded from the root probably provide an efficient mechanism for excluding Al from the root. Empirical equations were also constructed to describe Al dissolution from the two release pools for use in soil Al flux models.