LIGHT‐INDUCED ABSORBANCE CHANGES IN CHLOROPLASTS AT ‐196°C*

Abstract

Abstract— Absorbance changes induced by irradiating chloroplasts at — 196°C were measured in the region of 525–575 nm with a single‐beam spectrophotometer. Irradiation at low temperature caused a bleaching at 556 nm due to oxidation of cytochrome b559 but little or no change of cytochrome f. There occurred in addition a loss of absorbance at 547 nm and an increase at 543 nm. The bleaching at 547 nrn (and possibly the increase at 543 nm) could be induced chemically with dithionite or borohydride but not ascorbate. Subchloroplast particles with only Photosystem I activity showed no light‐induced absorbance changes, while particles containing combined Photosystem I and Photosystem II activities showed the same changes as whole chloroplasts. Scenedesmus mutant No. 11 cells showed no absorbance changes while mutant No. 8 and wild‐type cells showed the normal changes. It is concluded that the photooxidation of cytochrome b559 and the photoreduction causing the bleaching at 547 nm are both mediated by Photosystem II.