A Simple Method for Quantitative Estimation of Rhizosphere pH along Root Axes through Visualization

Abstract

Rhizosphere pH is known to be strongly influenced by nitrogen sources and plant species. We have evaluated whether the exposure of the shoot to light affects the pH changes in the rhizosphere of legumes and cereals in the presence of different forms of nitrogen (ammonium, nitrate or both). The pH changes in the rhizosphere were quantified as apparent proton fluxes by image analysis of agar gel containing a pH indicator, in which the root was embedded. In the presence of ammonium or ammonium nitrate, the rhizosphere was alkalized under dark conditions and acidified under light conditions in both cowpea and sorghum. This implied that, in the presence of ammonium, acidification of the rhizosphere is induced by exposure of shoot to light. In the presence of nitrate, both plants alkalized their rhizospheres in the dark, whereas in the light cowpea acidified its rhizosphere although sorghum alkalized it. Light-induced acidification, in the presence of nitrate, was also found in chickpea and adzuki bean, but not in maize. We found, that a particular part of the root axis strongly acidifies the rhizosphere in response to the exposure of shoot to light, especially in legumes. We conclude that rhizosphere pH is strongly affected by the light conditions encountered by the shoot, and the pH changes in response to the light locally along the root axis.