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Organic Acids and Iron Translocation in Maize Genotypes

^a Plant Stress Laboratory, Plant Physiology Institute and Agricultural Chemicals Management Laboratory, Agricultural Environmental Quality Institute, Northeastern Region, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705

Translocation of Fe was studied in WF9 (Fe-efficient) and ys₁/ys₁ (Fe-inefficient) maize (Zea mays L.) genotypes. Iron-deficient WF9 translocated more Fe to the tops than Fe-deficient ys₁/ys₁. Malate and citrate contents of root saps increased nearly 2-fold and aconitate increased over 4-fold in both genotypes as Fe of nutrient solutions increased from 0.1 to 3 milligrams per liter. Relative acid contents in root saps were as follows: malate > aconitate > citrate. Citric acid concentrations in stem exudates were nearly the same as in root sap. Malic acid concentrations were considerably lower in exudates than in root saps, and only a trace of aconitic acid was detected in the exudates. The concentration of Fe was 7-fold higher in exudate of WF9 than in exudate of ys₁/ys₁ and the concentration of exudate P was about the same for both genotypes.

Electropherograms of WF9 stem exudates showed that ⁵⁹Fe moved toward the anode as ⁵⁹Fe-citrate. Exudates of ys₁/ys₁ contained insufficient ⁵⁹Fe to produce radiographs. When ⁵⁹Fe was added in vitro to ys₁/ys₁ stem exudate, the ⁵⁹Fe moved as ⁵⁹Fe-citrate, indicating that sufficient citric acid was present in the exudate to chelate the Fe. Effectiveness of citric, isocitric, trans-aconitic, and malic acids in moving ⁵⁹Fe electrophoretically in acetate, citrate, isocitrate, trans-aconitate, and malate buffers was studied. Malic, acetic, and trans-aconitic acids were ineffective in moving Fe from the origin. Citric acid moved Fe anodically whenever present on the electropherogram and successfully competed with the other acids for Fe.

Results with ys₁/ys₁ roots indicate an absence of an efficient mechanism for transporting Fe from cortical cells to the xylem. If Fe can reach the xylem stream, the ys₁/ys₁ genotype should be as efficient as WF9 in moving Fe to the leaves.

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