Absorption of Fluoride and Chloride by Barley Roots

Abstract

The rates of absorption of labeled chloride and fluoride by excised barley roots were studied. Other experiments were carried out to discover the quantity of absorbed halogens that could be desorbed from the roots by water and solutions of chloride and fluoride. Barley roots discriminated markedly against fluoride and in favor of chloride absorption. A 100-fold difference in amounts of chloride and fluoride were found to be absorbed from solutions of initial equal fluoride and chloride concentrations in 3-hour periods. Chloride was absorbed against apparent concentration gradients of up to 500 to 1, but fluoride was not absorbed against concentration gradients. Anaerobic conditions and the metabolic inhibitors 2,4-dinitrophenol and sodium azide decreased the amounts of chloride absorbed. Anaerobic conditions had no effect on fluoride absorption which was in direct proportion to the fluoride content of the ambient solutions. Equilibrium conditions with respect to fluoride absorption were obtained in minutes, but chloride absorption continued for longer time periods. These findings indicate that fluoride absorption by barley roots is a diffusion process and that oxidative metabolic mechanisms are required for the full extent of chloride absorption. Absorbed fluoride was essentially completely desorbed from barley roots by water and inert salt solutions, and 20% of absorbed chloride was removed by these desorption media under the same conditions. Barley root water is 97% exchangeable.