Author
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Kinraide, Thomas |
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Submitted to: Journal of Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 10/28/2002 Publication Date: 2/20/2003 Citation: KINRAIDE, T.B. PROTON ALLEVIATION OF GROWTH INHIBITION BY TOXIC METALS (AL, LA, CU) IN RHIZOBIA. JOURNAL OF SOIL BIOLOGY AND BIOCHEMISTRY. 2003. V. 35. P. 199-205. Interpretive Summary: The addition of forage legumes (e.g., clovers) to pastures and hay fields is desirable because of nutritional benefits to animals and because of the incorporation of nitrogen into soils without the addition of fertilizer. Nevertheless, forage legumes often have a low presence in the grasslands of humid, temperate regions in part because of their acidic soils. These soils return to high acidity after liming under most management regimens. High soil acidity appears to adversely affect nodulated and nonnodulated legumes, free-living nodulating bacteria, and the nodulation process. (Nodulation is the process of infection of legume roots by bacteria that allow the plant to use atmospheric nitrogen.) The principal toxicity factors associated with acidic soils are high levels of hydrogen and aluminum, and low levels of calcium, magnesium, and phosphate. The present study considers several toxicity factors for the survival and reproduction of free-living white clover-nodulating bacteria. Specific accomplishments include (1) the establishment of a minimal medium for the culture of nodulating bacteria in which a complete chemical characterization of aluminum is possible and (2) the quantification and interpretation of the intoxicating effects of hydrogen and several toxic metals (aluminum, lanthanum, and copper) in various combinations. This information will aid the development of management practices for acidic soils and a rational program for the development of acidic-resistant nodulating bacteria. Technical Abstract: Free-living rhizobia are sensitive to soils and artificial media that are acidic. Besides the intoxicating effects of excessive H+, the release of Al from acid-soluble minerals in soils appears to play a role in toxicity. The complex, heterotrophic nutrient requirements of rhizobia and the joint occurrence of Al3+ and hydroxo-Al species have prevented a precise attribution of toxicity to the Al species, but at least one study has established the probable toxicity of Al3+ (Wright and Zeto, 1991). In the present study, a medium composed of 0.3 mM MgSO4, 2 mM CaCl2, and 10 mM sucrose (the basal medium) enabled a 1000-fold increase at pH 5 or above. Additions of 1 æM AlCl3 to the basal medium were highly intoxicating, especially at higher pH. At pH 5, cell numbers did not change from the inoculum; at lower pH the cell numbers increased slightly; but at higher pH the cell numbers declined. Similar trends were observed for La3+ and Cu2+ intoxication. Uptake of methylene blue, a positively charged dye, was inhibited by pH reductions between pH 3.5 and 6.0. These results bear a similarity to the responses of higher plant roots factors that decrease cell-surface negativity (such as lower pH) reduced the intoxication by cations and uptake of cationic dyes, but the pH responsiveness of the rhizobia in our system was much greater than the pH responsiveness of higher plants. Although plant-root intoxication by mononuclear hydroxo-Al species has been discounted, rhizobia may be sensitive to those species. These results have implications for the management of rhizobia in soils and the development of acid-soil resistant strains. |
