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Photochemistry of the reaction centers of system ii under repetitive flash group excitation in isolated chloroplasts






Photochemistry & Photobiology 31(5): 501-512

Photochemistry of the reaction centers of system ii under repetitive flash group excitation in isolated chloroplasts

Absorption changes induced in isolated spinach chloroplasts by excitation with repetitive flash groups have been measured at 690 nm, indicating the photochemical turnover of chlorophyll-aII (Chl-aII), and at 480 nm and 513 nm, respectively, reflecting via electrochromic effect the formation of a transmembrane electric field. The data are compared with measurements of O2 evolution. The initial amplitude of the 690 nm absorption change induced by the 2nd flash as a function of the time tV between the 1st and 2nd flash of a group increases with a half life of about 35 .mu.s. On the other hand, the average O2 yield due to the 2nd flash as a function of the time tV rises with a half life of about 600 .mu.s (and a kinetics in the ms-range of minor extent), confirming the data of Vater et al. (1968). Under far red background illumination, where contributions due to PS I in the .mu.s-range can be excluded, the difference spectrum in the red of the absorption changes induced by the first flash corresponds with that of the absorption changes induced by the 2nd flash fired 200 .mu.s after the first flash. The pattern of the absorption changes at 690 nm induced by repetitive double flash groups at tV = 200 .mu.s does not markedly change in normal chloroplasts by the presence of [2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone]. Similar 690 nm absorption changes occur in trypsin treated chloroplasts, independent of the presence of [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. The fast regeneration in the .mu.s-range of Chl-aII is also observed in the 3rd flash of a triple flash group at a time tV ; 200 .mu.s between the flashes of the group. The initial amplitudes of the absorption changes with a decay kinetics slower than 100 .mu.s induced by the 2nd flash at 480 nm and 513 nm, respectively, as a function of the time tV between the first and second flash of a group, are characterized by a recovery half-time of about 600 .mu.s, confirming earlier measurements at 520 nm (Witt and Zickler, 1974). On the basis of these results it is inferred that there does exist a photoreaction of Chl-aII with an electron acceptor, referred to as Xa, other than the primary plastoquinone acceptor X320, if X320 persists in its reduced state. Under conditons of X320 being in the reduced state, this photochemical reaction was highly dissipative with respect to charging up the watersplitting enzyme system Y. This Chl-aII-photoreaction with Xa does not lead to a vectorial transmembrane charge separation, which is stable for more than a few microseconds. Different models for the functional and structural organization of PS II are discussed.

Accession: 006110389

DOI: 10.1111/j.1751-1097.1980.tb03736.x

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