Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/1996
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 such as malate 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 functions to provide Al tolerance to plants growing in soils. Therefore in this study we looked at the decomposition of fmalate by microbes in 4 different Al-toxic, acid soils. It was found that malate was rapidly taken up and metabolized by soil bacteria. These results indicate that in acid soils where soil microbe populations are high, these microbes might break down a significant portion of the malate released from roots and reduce the efficiency of this process as a mechanism of Al tolerance.
Technical Abstract: The kinetics and characteristics of malate degradation were studied in four acid soils ranging in both pH(4.30 to 5.00) and vegetation type. The breakdown of malate was rapid in all soils with a half life of approximately 1.7 h, Km of 1.7 mM and Vmax of 70 nmol g-1 soil h-1. No relationship was observed between malate decomposition rate and pH. Co- metabolism studies with other C and N substrates (glucose, glycine, glutamate, citrate and succinate) indicated that the microorganisms were not N limited and competitive inhibition of malate breakdown was only observed in the presence of succinate. Studies with isolated mixed bacterial cultures indicated that the bacterial malate uptake was mediated by an energy dependent, dicarboxylate transporter which can be inhibited by succinate and is independent of pH between pH 5.0 and 7.0. The Km and Vmax parameters ranged from 279-955 yM and 0.1-17 ymol mg-1 protein h-1 for the mixed bacterial cultures depending on the bacteria's previous C source. Th results indicate that in acid topsoils where microbial populations are high, the microbes may provide a considerable sink for organic acids. If organic acids are being released by roots in response to an environmental stress (e.g. Al toxicity, P deficiency) it can be expected that the efficiency of these root mediated metal resistance mechanisms will be markedly reduced by rapid microbial degradation.