Light-induced absorbance changes in chloroplasts mediated by Photosystem I and Photosystem II at low temperature

Abstract

Stable light-induced absorbance changes in chloroplasts at −196 °C were measured across the visible spectrum from 370 to 730 nm in an effort to find previously undiscovered absorbance changes that could be related to the primary photochemical activity of Photosystem I or Photosystem II. A Photosystem I mediated absorbance increase of a band at 690 nm and a Photosystem II mediated absorbance increase of a band at 683 nm were found. The 690-nm change accompanied the oxidation of P 700 and the 683-nm increase accompanied the reduction of C-550. No Soret band was detected for P 700. A specific effort was made to measure the difference spectrum for the photooxidation of P 680 under conditions (chloroplasts frozen to −196 °C in the presence of ferricyanide) where a stable, Photosystem II mediated EPR signal, attributed to P 680 + has been reported. The difference spectra, however, did not show that P 680 + was stable at −196 °C under any conditions tested. Absorbance measurements induced by saturating flashes at −196 °C (in the presence or absence of ferricyanide) indicated that all of the P 680 + formed by the flash was reduced in the dark either by a secondary electron donor or by a backreaction with the primary electron acceptor. We conclude that P 680 + is not stable in the dark at −196 °C: if the normal secondary donor at −196 °C is oxidized by ferricyanide prior to freezing, P 680 + will oxidize other substances.