Investigation of the absorption changes of the plasto-quinone system in broken chloroplasts. The effect of bicarbonate-depletion

Abstract

The absorption changes at 334, 265, and 703 nm were measured in chloroplast suspensions upon depletion and readdition of bicarbonate. The normal absorption changes in the ultraviolet region are reinterpreted taking into account the existence of an additional plastoquinone molecule R located between the primary electron acceptor Q and the plastoquinone pool PQ. On this basis, three reversible effects of bicarbonate depletion were located in the electron transfer chain: 1. (1) The reaction centers II are partly inactivated as observed by the reduced magnitude of absorption changes of a quinone X-320 (or Q). 2. (2) The formation of R 2− is retarded by a factor of 10–15 (from 500 μs to 7±3 ms), as observed by the dark relaxation of the absorption change at 334 nm. This is consistent with the slowing down of the dark relaxation of the oxygen evolving S states (Sn′ to Sn+1) from a value of approx. 500 μs to 10 ms (observed by Stemler et al. (1974) Proc. Natl. Acad. Sci. U.S. 71, 4679–4683). 3. (3) The reduction of plastoquinone by R 2− is slowed down from approximately a ms to 100–200 ms in agreement with the interpretation of chlorophyll a fluorescence decay after the third flash (150 ms; Govindjee et al. (1976) Biochim. Biophys. Acta 449, 602–605). This slow reaction time is apparent in both the absorption changes of P-700 (reaction center I) and plastoquinone after a 20 μs flash (with an alteration from a predominating approx. 50 ms phase to a predominating approx. 200 ms phase) and also in the plastoquinone signal after an 85 ms flash (from 25 to 100 ms). This deceleration is the major effect of bicarbonate depletion and must be associated with the 4 to 8-fold loss in Hill activity observed by earlier workers.