pH mapping in transparent gel using color indicator videodensitometry

The colored pH indicator method introduced by Weisenseel et al. (1979) is particularly useful for localizing the zones along roots where acidification/alkalinization occurs. It can also be used to assess the direction and intensity of the proton fluxes. Because the method has not been quantitatively evaluated, however, it is nowadays little used or used in conjunction with other such as potentiometry. In the present study we examine the theoretical basis underlying this method of colorimetric visualization and show its similarity to spectrodensitometry. It thus becomes possible to quantify the luminous information and express it in terms of environmental pH. We describe the method used, emphasizing in particular the conditions required to achieve maximum accuracy of measurement, and an appropriate experimental device. pH distribution around roots can be mapped with a relative error of 0.03 pH units. The experimental device is easy to use and incorporates a computer-controlled video camera, thanks to which all acquisition and calculation procedures can be automated.