Antenna Sizes of Photosystem I and Photosystem II in Chlorophyll b-Deficient Mutants of Rice. Evidence for an Antenna Function of Photosystem II Centers That are Inactive in Electron Transport

Abstract

Light-harvesting capacities of photosystem I (PSI) and photosystem II (PSII) in a wild-type and three chlorophyll 6-deficient mutant strains of rice were determined by measuring the initial slope of light-respons e curve of PSI and PSII electron transport and kinetics of light-induced redox changes of P-700 and QA, respectively. The light-harvesting capacity of PSI determined by the two methods was only moderately reduced by chlorophyll 6-deficiency. Analysis of the fluorescence induction that monitors time course of QA photoreduction showed that both relative abundance and antenna size of PSIIa decrease with increasing deficiency of chlorophyll b and there is only PSII,, in chlorina 2 which totally lacks chlorophyll b. The numbers of antenna chlorophyll molecules associated with the mutant PSII centers were, therefore, three to five times smaller than that of PSII a in the wild type rice. Rates of PSII electron transport determined on the basis of PSII centers in the three mutants were 60-70% of that in the normal plant at all photon flux densities examined, indicating that substantial portions of the mutant PSII centers are inactive in electron transport. The initial slopes of light-respons e curves of PSII electron transport revealed that the functional antenna sizes of the active populations of PSII centers in the mutants correspond to about half that of PSII a in the wild type rice. Thus, the numbers of chlorophyll molecules that serve as antenna of the oxygen-evolving PSII centers in the mutants are significantly larger than those that are actually associated with each PSII center. It is proposed that the inactive PSII serves as an antenna of the active PSII in the three chlorophyll ^-deficient mutants of rice. In spite of the reduced antenna size of PSII, therefore, the total light-harvesting capacity of PSII approximately matches that of PSI in the mutants.