Mechanisms of light-induced structural changes in chloroplasts I. Light-scattering increments and ultrastructural changes mediated by proton transport

Abstract

Chloroplasts suspended in NaCl solutions undergo structural modifications when illuminated by red actinic light in the presence of electron carriers. These changes are manifested by increments in light scattering and volume, and disruptive alterations of grana structure as demonstrated by electron microscopy. Simultaneously with these modifications protons are transported into the chloroplasts in a light-dependent process. It was postulated that the decrease in internal pH which would result from the proton transport can cause many of the changes observed in chloroplasts during illumination. The following experimental results support this argument: 1. (a) The kinetics of proton transport and light-scattering increments resemble each other both in onset and decay. 2. (b) The pH activation curves are similar for both processes, with optima lying between 5.5 and 6.5. 3. (c) Lowering external pH in the dark to levels which reproduce theoretical internal hydrogen ion concentrations causes alterations in light scattering, volume, and ultrastructure similar to those induced by illumination. Hence light-induced modifications of chloroplast structure in NaCl solutions are satisfactorily explained by a decreased internal pH resulting from proton transport into the chloroplast.