The electrochromic signal, redox reactions in the cytochrome bf complex and photosystem functionality in photoinhibited tobacco leaf segments

Abstract

Photosynthetic electron transport in vivo was investigated in tobacco leaves pre-illuminated with strong light under conditions where Photosystem (PS) II repair was inhibited by lincomycin. Flash-induced redox changes of cytochrome b₅₆₃, cytochrome f and plastocyanin, and the electrochromic (EC) signal (caused by a carotenoid band-shift due to charge separation across thylakoid membranes) from leaf segments were measured by deconvoluting absorbance changes at 520, 554, 564 and 575 nm. The EC signal was composed of easily separable fast and slow components. The fast EC signal decreased linearly with the loss of functional PS II centres, but there was a residual fast EC phase which was attributable to PS I centres alone. Inactivation of PS II centres by photoinhibitory light was also well-correlated with the quenching of variable fluorescence measured as the ratio of variable to maximum fluorescence, F_v/F_m. On complete photoinactivation of PS II centres, the slow rise of the flash-induced EC signal became more prominent, suggesting enhanced electrogenic charge transfer across the cytochrome bf complex as part of a path of electron flow involving PS I. Thus, both PS I and the cytochrome bf complex appeared to be fully functional after treatment of tobacco leaves with photoinhibitory light at room temperature. In totally photoinhibited leaf segments, the rate coefficients of cyt f^III re-reduction increased from 59 s^-1 (+ lincomycin, no photoinhibitory light) to 130 s^-1, and that of cytochrome b₅₆₃ reduction also increased, from 270 s^-1 to 500 s^-1, suggesting that the prevailing plastoquinol concentration was higher after photoinhibitory light treatment. The source of the electrons entering the pool under these conditions was probably a high concentration of NADPH and reduced ferredoxin.