Absorption of Ionic Species of Orthophosphate by Barley Roots: Effects of 2,4-Dinitrophenol and Oxygen Tension

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Abstract
The effect of DNP and oxygen tension on absorption of ionic species of orthophosphate by excised barley roots was studied. Oxygen tensions ranged from 0.002%) to 100%, DNP concentrations from 10-6M to 10-4M with pH adjusted to 4 or 7. Radioactive phosphate was used as tracer. In all cases, the inhibited absorption reactions were shown to be reversible in 60 min. experiments. Phosphate absorption reactions are altered irreversibly after 75 min. exposure in solns. in equilibrium with an oxygen tension of 0.05%, and after 60 minutes in solns. containing 10-5 M DNP. Each of the two first order reactions through which H2PO4- and HPO4= ion species are absorbed has an oxygen requirement and is inhibited competitively by DNP. Over-all absorption is independent of oxygen tension over the range from 3% to 100% oxygen. In the reactions through which phosphate is absorbed, one mole of oxygen combines with both carriers; half-maximal absorption rate occurs at 0.3% oxygen tension. The relation between oxygen tension and phosphate absorption is a hyperbolic function over the range from 0-3% oxygen. These results suggest that cytochrome oxidase is the terminal oxidase in six-day-old barley roots.