Localization and quantification of net fluxes of H+ along maize roots by combined use of pH-indicator dye videodensitometry and H+-selective microelectrodes

Two methods for measuring proton fluxes along intact maize roots grown with NH4+ or NO3- at pH 6.5 were compared. Videodensitometric measurement of ch anges in a pH-indicator dye by video camera was used to map pH around roots and determine the amounts of protons released by various root regions. Thi s method was compared with potentiometric determination of the concentratio n of H+ in the unstirred layer at the root surface using ion-selective micr oelectrodes. With NH4+ the roots released large amounts of H+ in preferenti al regions where the rate of flux can reach 1.4 or even 2.5 nmol m(-1) s(-1 ). Videodensitometry indicated a first region of root acidification in the subapical zone, but this was more difficult to localize with microelectrode s. With NO3- both methods showed that the roots released small amounts of H + and that the apical region took up H+ in the first 10 mm then sometimes r eleased H+ over the following 10 mm of root. The H+ flux profiles obtained by both methods were in good agreement in terms of both order of magnitude of the fluxes and spatial differences along the root. These results suggest that videodensitometry, which is easier to use than potentiometry, can be used to screen different plant species or cultivars under various experimen tal conditions. The microelectrode technique is indispensable, however, for studying the underlying mechanisms of net H+ fluxes.