1987; Lavergne and Leci MDV3100 1993; Schansker and Strasser 2005). These instruments can also be used to study the S-states (oxidation states S0, S1, S2, S3 and S4) of

the oxygen evolving complex of PSII. A series of STFs induces period-4 oscillations in the F O-level as a function of the S-states (see Delosme 1972; Delrieu 1998; Ioannidis et al. 2000 for examples of such measurements). To probe the oxidation of reduced Q A following a saturating flash, there are two possible approaches: (1) The easiest method makes use of low-intensity modulated light, which excites only a small find more fraction of the PSII RCs per unit of time. Figure 2 shows an example of such a measurement. For control samples, in which re-oxidation of Q A − via forward electron transport can occur, this approach works well. However, when the sample is inhibited, e.g., by an electron transfer inhibitor such as DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea), which displaces Q B from its binding site (Velthuys 1981; Lavergne 1982b), the low-intensity modulated light leads to the accumulation of a considerable population PR-171 mw of Q A − complicating the analysis of the experiment,

because re-oxidation of Q A − by recombination with the donor side is much slower than forward electron transport to Q B.   (2) The second method uses a combination of a STF followed by a probe flash that probes the redox state of Q A at the time of the probe flash (this is called a pump–probe experiment) (Mauzerall

1972; Robinson and Crofts 1983). The intensity of the probe flash is much lower than that of the STF. In this case, the experiment is repeated many times and each time at a variable time P-type ATPase t after the STF, a probe flash is given to probe the redox state of Q A. In this way, the re-oxidation kinetics are constructed point by point. The actinic light problem, described above for DCMU inhibited samples, does not exist in this case. On the other hand, identical samples do not exist, and therefore, the biological variability between samples will lead to experimental noise and the need for repetitions to obtain smooth kinetics. To make different phases in the re-oxidation kinetics visible, the use of a logarithmic time scale has been introduced (see e.g., Cser and Vass 2007). Commercial equipment to make this type of measurements is the superhead fluorometers (Photon Systems Instruments, Brno, Czech Republic), which can also be used to measure OJIP transients and saturating pulse protocols (see below).   Complementary techniques for flash fluorescence measurements are thermoluminescence (TL) (reviewed by Vass and Govindjee 1996; Misra et al. 2001a, b; Ducruet and Vass 2009) and delayed fluorescence (DF) (recently reviewed by Goltsev et al. 2009) measurements that provide specific information on recombination reactions within PSII RCs. Flash fluorescence measurements are frequently used to study PSII mutants (e.g., Etienne et al. 1990; Nixon et al.