Photo-induced pH Changes in the Vicinity of Isolated Peperomia metallica Chloroplasts

Photo-induced pH changes of the external medium in a regionimmediately adjacent to the surface of individual isolated Peperomiametallica chloroplasts have been measured using antimony pH-micro-electrodcs.The pH changes induced by continuous illumination were composedof two phases: an initial alkalization (phase I) and a subsequentacidification (phase II) of the medium. Both phases were severelysuppressed by DCMU and protonophorous uncouplers but they showeddifferent sensitivity towards DCCD₁ NH₄CI and some other agents.Phase I was selectively inhibited by DCCD and was partiallyrestored upon addition of ATP to DCCD-poisoned chloroplasts.Phase II was inhibited by 1.0 mol m^–3NH₄CI. It appearsfrom these data that phases I and II of light-induced pH changesare determined by different processes. It is suggested thatthe pH increase (phase I) is due to a photosynthetic CO₂ fixationand the pH decrease (phase II) is caused by a light-dependentextrusion of protons from intact chloroplasts. Key words: Transport of protons, intact chloroplasts, Peperomia metallica