Requirement of the light-harvesting pigment · protein complex for magnesium ion regulation of excitation energy distribution in chloroplasts

Cation regulation of excitation energy distribution was examined in chloroplasts isolated from (a) pea seedlings, grown in intermittent illumination, which contain no light-harvesting complex, (b) a barley mutant which is deficient in the major polypeptide component of the light-harvesting complex, and (c) a soybean mutant which contains a reduced amount of light-harvesting complex. It was found that: 1. (1) Mg²⁺-induced increase in Photosystem II fluorescence at room temperature is small in the chloroplasts of the soybean mutant, smaller in the barley mutant, and almost absent in the light-harvesting complex-less chloroplasts of pea as compared to their respective controls. 2. (2) Mg²⁺-induced increase in the F₆₈₅ F₇₃₀ emission peak ratio at 77 K is not detected in the isolated chloroplasts of the intermittent light-grown pea and the barley mutant. 3. (3) Pre-illumination induced State 1-State 2 adaptation in vivo is absent in the barley mutant and is less pronounced in the soybean mutant as compared to their respective controls. 4. (4) Increase of slow fluorescence decay upon addition of Mg²⁺ observed in control chloroplasts was not detected in chloroplasts of intermittent-light grown peas. These results confirm earlier conclusions (Armond, P.A., Arntzen, C.J., Briantais, J.-M. and Vernotte, C. (1976) Arch. Biochem. Biophys. 175, 54-63; Davis, D.J., Armond, P.A., Gross, E.L. and Arntzen, C.J. (1976) Arch. Biochem. Biophys. 175, 64-70) that light-harvesting complex is required for the Mg²⁺-induced regulation of the excitation energy distribution between Photosystems I and II. The characteristic P-S decay and I-D dip of the in vivo fluorescence inductions (Kautsky effect) were not significantly altered in the light-harvesting complex-less and the light-harvesting complex-deficient chloroplasts as compared to their respective controls. These results indicate that light-harvesting complex is not obligatorily required to observe the P-S decay or the I-D dip.