The absorbance increase at 505 nm reflects formation of zeaxanthin via de-epoxidation of violaxanthin induced upon acidification of the thylakoid lumen (Yamamoto et al. 1972; Bilger et al. 1989). Zeaxanthin changes are slow and can be kinetically differentiated from faster 515–520 nm and 535 nm changes. The absorbance increase peaking at 515–520 nm is caused OICR-9429 mw by an electrochromic shift of absorption of various photosynthetic pigments, including carotenoids (Junge and Witt 1968). It has been described by the abbreviated terms P515, carotenoid shift or ECS. In the present communication, the terms ECS and P515 are used interchangeably. The ECS (P515) signal

may be considered an intrinsic optical voltmeter that rapidly responds to changes of the electrical potential across the thylakoid membrane (Witt 1971, 1979;

Joliot MDV3100 and Joliot 1989). Photosynthetic electron transport involves three electrogenic reactions, namely the two photoreactions (PS I and PS II) (Witt 1971) and the Q-cycle of the cyt bf complex (Velthuys 1978; Joliot and Joliot 1986). While the ECS due to PS I and PS II responds without measurable delay to the onset of light, the ECS caused by the Q-cycle responds with a time constant in the order of 10 ms to light. Finally, the absorbance increase around 535 nm for long has been attributed to a light induced increase of light scattering caused by internal acidification of the thylakoids (Heber 1969). It has been used in numerous in vivo studies as a convenient MG-132 concentration semi-quantitative optical probe of “membrane energization” and of

the ΔpH component of the pmf in selleck compound intact leaves. It closely correlates with the fluorescence-based indicators of “energization” qE and NPQ (see e.g., Bilger et al. 1988). While it has been assumed that 535 nm changes are caused by changes in grana stacking, this interpretation recently has been questioned by Ruban et al. (2002) who suggest that the 535 nm increase of absorbance is due to a red shift of the zeaxanthin absorption peak. Therefore, when the 535 nm changes are referred to as “light scattering” changes, this is done with quotation marks. The original Joliot-type kinetic spectrophotometer (Joliot and Delosme 1974; Joliot et al. 1980) was developed for highly sensitive measurements of flash relaxation kinetics in suspensions of algae and thylakoid membranes (i.e., for conditions avoiding the complications resulting from overlapping 535 and 505 nm changes that are characterized by relatively slow kinetics during continuous illumination). Absorption was measured during each of a series of 2 μs monochromatic flashes given at various intervals after the actinic flashes (pump-and-probe method).