Effect of pH on the Absorption of Boron by Excised Barley Roots

In order to obtain information on the mechanism of B uptake and on the relative importance of the two forms H3BO3 and H2BO3− for net B absorption, the uptake of B by excised barley (Hordeum vulgure var. Briggs) was investigated using solutions having pH values from 6 to 10 and B concentrations from 1 to 10 ppm. The B uptake and final B contents after 1 hour absorption periods increased nearly linearly with increasing B supply. The relative B uptake (uptake at pH 6 = 100%) at all supply levels decreased with increasing pH similar to the decrease of the fraction of undissociated H3BO3. At pH 6 final B contents in roots and treatment solutions were similar. From this it is concluded that B in root tissues and external solution tends to approach a diffusion equilibrium which is governed by the undissociated acid. Boron thus differs fundamentally from most other nutrients which are present in an ionized form and are transported actively.