A measurement on dark adapted (closed symbols) which has an oxidized PQ-pool and a low J-step and a measurement made 5 s later (open symbols) where Q A had become re-oxidized in part of the PSII RCs due to recombination (O level considerably below P), the PQ-pool is still almost completely reduced (J level near P), and the acceptor side of PSI is almost completely re-oxidized (I level close to that of the dark-adapted state) (G. SAHA HDAC ic50 Schansker, unpublished data)   [3] Instruments designed to study the

steady state (relatively stable photosynthetic activity after 5–10 min of illumination). With such instruments, light-induced regulatory mechanisms, interaction between ETC,

Calvin–Benson cycle, stomatal opening, and photorespiration learn more (the process initiated when the enzyme Rubisco reacts with O2 instead of CO2) are studied (see Fig. 4). Fig. 4 Slow Chlorophyll a fluorescence kinetics (in arbitrary units) using a PAM-2100 fluorometer. The dark-adapted leaf is illuminated with weak modulated measuring light to give the zero fluorescence level F 0. Application of a saturation pulse (SP) allows measurement of the maximum fluorescence level in the dark F M. Photosynthesis MK-2206 is then activated by an actinic light source (in this case 250 μmol photons m−2 s−1). SPs during the light phase were triggered spaced 1 min apart (indicated by arrows) to determine the maximum fluorescence intensity in the light (F M′), and for each SP, qP, Φ PSII, and

NPQ parameters were calculated, and these are indicated in the figure (Penella et al. unpublished data)   Flash fluorescence measurements Figure 2 shows an example of a typical flash fluorescence experiment. These measurements are based on the concept of a single turnover flash (STF). An STF has to meet two requirements: (1) The intensity of a STF must be high enough to excite the antennae of all PSII reaction centers (RCs) followed by a charge separation in all PSII RCs leading to a reduction of essentially all Q A; (2) A STF must be short enough to induce only one charge separation in each PSII RC. In practice, this situation is never completely reached, and either misses or double 4-Aminobutyrate aminotransferase hits are induced in a small fraction of PSII RCs (see e.g., Kok et al. 1970; Shinkarev 2005). The re-oxidation of Q A − can then be followed: in active RCs, most electrons will be transferred to Q B and following a second flash to Q B − (see Fig. 2). The first reaction has a half-time of 100–200 μs, and the second reaction has a half-time of 400–600 μs (reviewed by Petrouleas and Crofts 2005). If no PQ is bound to the Q B-site, the electron on Q A − has to wait, till a PQ molecule binds to the Q B-site, and this process can take a few ms (Crofts and Wraight 1983).