Photoconversion kinetics of chloroplast Photosystems I and II. Effect of Mg2+ |
| |
Authors: | Anastasios Melis Randall A. Ow |
| |
Affiliation: | Division of Molecular Plant Biology, 313 Hilgard Hall, University of California, Berkeley, CA 94720 U.S.A. |
| |
Abstract: | The effect of divalent cations on the primary photoconversion kinetics of chloroplast Photosystems (PS) I and II was investigated by absorbance difference spectrophotometry in the ultraviolet (ΔA320) and red (ΔA700) regions and by fluorescence at room temperature. Three main chlorophyll (Chl) a fluorescence emission components were identified. Addition of 5 mM MgCl2 to unstacked chloroplasts caused a 5–7-fold increase in Fvα, the variable fluorescence yield controlled by the α-centers. The fluorescence yield Fvβ controlled by the β-centers and the nonvariable fluorescence yield F0 were only slightly changed by the treatment. The absolute number of α- and β-centers remained unchanged and independent of divalent cations. The rate constants Kα, Kβ and KP-700 determined from the photoconversion kinetics of Qα, Qβ and P-700 were also unchanged by divalent cations, suggesting a constancy of the respective absorption cross-sections. Evidence is presented that the Mg2+ effect on Chl a fluorescence is not due simply to unstacking. Conclusion: (1) In the absence of divalent cations from the chloroplast suspending medium, the variable fluorescence yield is not complementary to the rate of PS II photochemistry. (2) A spillover of excitation from PS II to PS I in the absence of Mg2+ cannot account for the 7-fold lowering of the variable fluorescence yield Fvα at room temperature. The results are discussed in view of a model of excitation transfer and fluorescence emission in the pigment bed of PS IIα and PS IIβ. |
| |
Keywords: | Photoconversion kinetics Excitation transfer Photosynthesis Chlorophyll fluorescence (Pea chloroplast) PS photosystem P-700 primary donor of PS I Q primary acceptor of PS II Chl chlorophyll LHC light-harvesting complex DCMU 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea Tricine |
本文献已被 ScienceDirect 等数据库收录! |
|