Labile‐Bound Sulfate in Wheat‐Roots: Localization, Nature and Possible Connection to the Active Absorption Mechanism

The aim of the investigation was to identify a readily exchangeable fraction of sulfate in wheat roots and to study the nature of the fraction and its possible connection to the active mechanism for sulfate uptake. The amount of sulfate absorbed during the initial phase of uptake was higher in plants with a low sulfate status (low sulfate plants) than in plants with a high sulfate status (high sulfate plants). By adding selenate to the test solution, the same amount of sulfate belonging to this phase was obtained both in high sulfate plants and in low sulfate plants. The variations between plants of different sulfate status were believed to be due lo different capacities for labile binding of sulfate to positive sites in the roots. Selenate replaced sulfate at these sites. By leaching experiments, the positive sites were shown to be located on or outside of a very delicate barrier in the cells. The maintenance of the positive sites was demonstrated to be dependent on concurrent metabolism by adding 2.4‐dinitrophenol to the test solution and by varying the root temperature. The amount of sulfate bound to positive sites in labile binding was shown to be dependent on the pH of the test solution. At low sulfate concentrations in the test solutions, there was a quantitative connection between the magnitude of the labile‐bound fraction and the rate of active uptake into the symplasm. At higher sulfate concentrations than 0.1 mM, sulfate positive sites of lower affinity to sulfate were believed lo disturb the quantitative connection mentioned above.