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Translocation of Manganese, Iron, Cobalt, and Zinc in Tomato

United States Department of Agriculture, ARS, SWC, Mineral Nutrition Laboratory, Beltsville, Maryland 20705

Tomato plants in solution culture were treated with 0 to 50 µM Mn, Co, or Zn in the presence of 5 µM Fe. Stem exudates were analyzed to determine quantities and forms of the metals translocated.

Mn had no effect on exudate volume. Co and Zn at 5 µM and above depressed exudate volumes. Nutrient Mn was concentrated 2 to 5 times in the exudates. Co and Zn concentrations were between 1 and 3.

Tomato roots treated with 0 to 10 µM Mn released into the exudate an average of 29% of the Fe they absorbed. They released 21% of the absorbed Fe when treated with 50 µM Mn. On 50 µM Co or Zn, the roots released only about 1% of the absorbed Fe.

The exudates contained 12 to 47 µM citrate, which was usually considerably in excess of Fe. Electrophoresis of exudate revealed Fe as the only metal in anionic form. Mn, Co, and Zn migrated as cations.

The concentration of Ca was >3 mM and Mg >1.5 mM in the exudates. Estimates based on metal-citrate equilibrium constants and constants of metal displacement caused by Ca and Mg confirmed that Mn, Co, and Zn were transported predominantly as inorganic cations in the stem exudates.