Correlation of ultrastructure with light-induced ion transport in chloroplasts

Abstract

The relation of light-dependent ion transport to light-induced conformations of isolated chloroplasts has been investigated. The following conformational states have been established: ( a) Chloroplasts swell upon illumination in solutions containing completely dissociated monovalent ions, such as sodium chloride. This is interpreted as the inward transport of hydrogen ions accompanied by an influx of the anion present to maintain electrical neutrality. ( b) Chloroplasts also swell if illuminated in solutions containing cations capable of associating with hydrogen ions, for instance ammonium chloride. It is proposed that this reflects an inward movement of the cation (in its uncharged form) in response to a transient lowering of internal pH, accompanied by an influx of the anion to maintain electrical neutrality, ( c) Chloroplasts shrink upon illumination in solutions such as sodium acetate containing anions that can associate with hydrogen ions. This is interpreted as an efflux of the anion (in its uncharged form) driven by the pH gradient produced by the proton pump, ( d) If cations and anions are present—both of which can associate with hydrogen ions—as in ammonium acetate, chloroplasts swell even in the dark, presumably reflecting an inward movement of both cations and anions in their uncharged form. These results are consistent with the view that the ionic environment, defined in terms of the presence or absence of associated or dissociated ions, exerts a direct control over light-induced conformations of isolated chloroplasts by interacting with light-dependent hydrogen ion transport.