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

• Published in: Planta Vol. 209; no. 4; pp. 444 - 452
• Main Authors: Kosegarten, H, Grolig, F, Esch, A, Glusenkamp, K.H, Mengel, K
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01-10-1999
• Subjects: Absorption. Translocation of ions and substances. Permeability; adenosinetriphosphatase; Agronomy. Soil science and plant productions; alkalinization; ammonium compounds; anions; bicarbonates; binding; Biological and medical sciences; Cell biochemistry; cell division; Cell physiology; cell walls; cortex; Economic plant physiology; enzyme inhibitors; fluorescein; fluorescence; fluorescent tracers; Fundamental and applied biological sciences. Psychology; fusicoccin; histochemistry; nitrates; Nutrition. Photosynthesis. Respiration. Metabolism; Plant physiology and development; root hairs; root tips; roots; Zea mays; Monocotyledones; Ammonium; Zea mays; Root; Measurement sensor; Fluorescence; Nitrates; Apoplast; Cereal crop; Hydrogencarbonates; Rhizosphere; Calcareous soils; Gramineae; Angiospermae; Fluorescein; pH; Spermatophyta; Application method; Soil solution
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s004250050747

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 1 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 H(+)-ATPase with vanadate alkalized the apoplast in the root hair zone from pH 5.4 to 5.6. The observed pH differences alone 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 ai microsites at the cell level by use of microscope imaging.