Effects of NH4+, NO3- and HCO3- on apoplast pH in the outer cortex of root zones of maize, as measured by the fluorescence ratio of fluorescein boronic acid

A fluorimetric ratio technique was elaborated to measure apoplastic pH in the outer root cortex of maize (Zea mays L.) grown hydroponically. A newly synthesized fluorescent probe, fluorescein boronic acid (pK(a) = 5.48), which covalently binds to the cell wall of the outer cell layers, was used. Under conditions of saturating ion concentrations the apoplastic pH was determined along the root axis ranging from I to 30 mm behind the root: tip. Apoplastic pH was recorded for root segment areas (1 mm(2)), and pH values of high statistical significance were obtained. With an external solution of pH 5, the apoplastic pH was about pH 5.1 in the division zone, between pH 4.8 and 4.9 in the elongation region and about pH 4.9 in the root hair zone. At an external pH of 8.6, the difference between the external pH and the apoplastic pH was considerably more, with a pH of 5.2-5.3 in all root zones. Addition of 1 mM NH4+ caused a small apoplastic pH decrease (0.05 of a pH unit) in all root zones. Apoplastic alkalization upon application of 6 mM NO3- was highest (0.3 of a pH unit) in the zone where root hairs emerge; in the division and early elongation zones, apoplastic pH increased only transiently. In the presence of 10 mM HCO3-, NO3- elicited a higher and persistent alkalization (0.06-0.25 of a pH unit) in all root zones. Application of fusicoccin reduced apoplastic pH from 4.85 to 4.75 in the elongation zone, while inhibition of the Ht-ATPase with vanadate alkalized the apoplast in the root hair zone from pH 5.4 to 5.6. The observed pH differences along the root axis upon differential N supply and application of HCO3- provide evidence that this new pH technique is a useful tool with which to measure apoplastic pH, and in future may permit measurements at microsites at the cell level by use of microscope imaging.