Photo-induced pH Changes in the Vicinity of Isolated<i>Peperomia metallica</i>Chloroplasts

Abstract

Photo-induced pH changes of the external medium in a region immediately adjacent to the surface of individual isolated Peperomia metallica chloroplasts have been measured using antimony pH-micro-electrodcs. The pH changes induced by continuous illumination were composed of two phases: an initial alkalization (phase I) and a subsequent acidification (phase II) of the medium. Both phases were severely suppressed by DCMU and protonophorous uncouplers but they showed different sensitivity towards DCCD1 NH4CI and some other agents. Phase I was selectively inhibited by DCCD and was partially restored upon addition of ATP to DCCD-poisoned chloroplasts. Phase II was inhibited by 1.0 mol m−3NH4CI. It appears from these data that phases I and II of light-induced pH changes are determined by different processes. It is suggested that the pH increase (phase I) is due to a photosynthetic CO2 fixation and the pH decrease (phase II) is caused by a light-dependent extrusion of protons from intact chloroplasts.