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Vanadium Uptake by Plants

Absorption Kinetics and the Effects of pH, Metabolic Inhibitors, and Other Anions and Cations ¹

United States Plant, Soil and Nutrition Laboratory, Agricultural Research Service, United States Department of Agriculture, Tower Road, Ithaca, New York 14850, Department of Agronomy, Cornell University, Ithaca, New York 14850

The kinetics of vanadium absorption by excised barley (Hordeum vulgare L., cv. Eire) roots were investigated with respect to ionic species of V in solution, time and concentration dependence, Ca sensitivity, and interaction with various anions, cations, and pH levels. The role of metabolism in V absorption was also studied using anaerobic treatment (N₂ gas) and chemical inhibitors (NaN₃, KCN, or 2,4-dinitrophenol). Approximately one-third of the labeled V initially taken up by excised roots was desorbed to a constant level after 45 min in unlabeled V solutions. The rate of absorption of labeled V from 5 µM NH₄VO₃ solutions containing 0.5 mM CaSO₄ was constant for at least 3 hours. Omission of Ca resulted in a 72% reduction in V uptake when compared to controls with 0.5 mM CaSO₄. The rate of uptake of V was highest at pH 4 but dropped to a very low level at pH 10. It was relatively constant between the pH levels of 5 and 8 at which the VO₃^– ion is the predominant ionic species in solution. The rate of absorption of V was followed as a function of concentrations from 0.5 to 100 µM NH₄VO₃. It was found to be a linear function of concentration and did not follow saturation kinetics. Absorption experiments carried out with labeled V from either N_aVO₃ or NH₄VO₃ sources gave similar results. No anion studied (i.e. HPO₄^2–, HAsO₄^2–, MoO₄^2–, SeO₄^2–, SeO₃^2–, CrO₄^2–, BO₃^3–, No₃^–, and Cl^–) interfered appreciably (i.e. less than 30% inhibition) with the absorption of labeled V. Anaerobic treatment of absorption solution with N₂ gas did not inhibit V absorption by excised roots. The results obtained using chemical inhibitors were not consistent. It was concluded that V is not actively absorbed by excised barley roots.