Compensatory Alterations in the Photochemical Apparatus of a Photoregulatory, Chlorophyll <i>b</i>-Deficient Mutant of Maize

Characterization of the functional organization of the photochemical apparatus in the light sensitive chlorophyll b-deficient oil yellow-yellow green (OY-YG) mutant of maize (Zea mays) is presented. Spectrophotometric and kinetic analysis revealed substantially lower amounts of the light harvesting complex of photosystem II (LHCII-peripheral) in high light-grown OY-YG thylakoids. However, accumulation of a tightly bound LHCII appears unaffected by the lesion. Changes in photosystem (PS) stoichiometry include lower amounts of PSII with characteristic fast kinetics (PSII(alpha)) and a substantial accumulation of PSII centers with characteristic slow kinetics (PSII(beta)) in the thylakoid membrane of the OY-YG mutant. Thus, PSII(beta) is the dominant photosystem in the mutant chloroplasts. In contrast to wild type, roughly 80% of the mutant PSII(beta) centers are functionally coupled to the plastoquinone pool and are probably localized in the appressed regions of the thylakoid membrane. These centers, designated PSII(beta)-Q(B)-reducing (Q(B) being the secondary electron quinone acceptor of PSII), are clearly distinct from the typical PSII(beta)-Q(B)-nonreducing centers found in the stroma lamellae of wild-type chloroplasts. It is concluded that the observed changes in the stoichiometry of electron-transport complexes reflect the existence of a regulatory mechanism for the adjustment of photosystem stoichiometry in chloroplasts designed to correct any imbalance in light absorption by the two photosystems.