COLD-RESISTANT AND COLD-SENSITIVE MAIZE LINES DIFFER IN THE PHOSPHORYLATION OF THE PHOTOSYSTEM-II SUBUNIT, CP29

The effects of low temperature on the relative contributions of the re action center and the antenna activities to photosystem II (PSII) elec tron transport were estimated by chlorophyll fluorescence. The inhibit ion of PSII photochemistry resulted from photodamage to the reaction c enter and/or a reduced probability of excitation energy trapping by th e reaction center. Although chill treatment did not modify the proport ion of the dimeric to monomeric PSII, it destabilized its main light-h arvesting complex. Full protection of the reaction center was achieved only in the presence of the phosphorylated PSII subunit, CP29. In a n onphosphorylating genotype the chill treatment led to photoinhibitory damage. The phosphorylation of CP29 modified neither its binding to th e PSII core nor its pigment content. Phosphorylated CP29 was isolated by flat-bed isoelectric focusing. Its spectral characteristics indicat ed a depletion of the chlorophyll spectral forms with the highest exci tation transfer efficiency to the reaction center. It is suggested tha t phosphorylated CP29 performs its regulatory function by an yet undes cribed mechanism based on a shift of the equilibrium for the excitatio n energy toward the antenna.