Other processes that have been associated with the qT are some slowly relaxing component(s) of qE (Lokstein et al. 1993; Joliot and Finazzi 2010) and light-dependent movements of chloroplasts (Cazzaniga et al. 2013). In practice, there are several arguments making it doubtful that the qT is a reliable measure for state transitions. The slowest relaxation phase, the qI, which may last several find more hours can consist of several processes: photoinhibition of PSII and XC related changes (reviewed by Krause and Jahns 2004) and possibly also state II to state I transitions (Schansker et al. 2006) if a change in the JI amplitude is related to state transitions as suggested by Schreiber et al. (1995) for cyanobacteria.

It should be noted that the rate with which these processes reverse in darkness is not necessarily the same in all photosynthetic organisms. For example, the regeneration of the IP phase parallels the qT phase in pea leaves (Schansker et al. 2006), and it is complete within 15 min, whereas the same process in needles of Pinus halepensis takes 1 h (Schansker et al. 2008). Question 16. Why is far-red light used to determine Trichostatin A mw the F O and F O′ values? For leaves, it is reasonable to assume that under most conditions, nearly all PSII RCs are in the open state (Q A oxidized) following dark adaptation. However, the assumption is not true for heat-stressed

leaves (Ducruet 1999; Tóth et al. 2007b) and leaves that show a high GABA Receptor rate of chlororespiration. Chlororespiration refers to the non-photochemical reduction of the plastoquinone

pool by reducing equivalents derived from Fdred or NADPH in the stroma (Bennoun 2002). Feild et al. (1998) showed a high chlororespiratory activity in light acclimated sunflower leaves following a light-to-dark transition leading to considerably higher F O′ values. This F O′ increase is due to a population of reduced Q A associated with a more reduced PQ pool. There is redox interaction between the PQ-pool and Q A leading to a redox-equilibrium (Diner 1977); for pea leaves, it was shown that a completely reduced PQ-pool (induced by anaerobiosis) is in equilibrium with reduced Q A in 20 % of the PSII RCs (Tóth et al. 2007a). To assure maximum oxidation of the PQ pool, the leaf can be pre-illuminated with FR light. For this purpose, FR light in the 720–735 nm range is normally used. FR light preferentially excites PSI and thereby causes an oxidation of the PQ pool. We note that FR light can induce charge separations in PSII (Pettai et al. 2005; Schansker and check details Strasser 2005). Pettai et al. (2005) demonstrated that FR light at 740 nm still induces a low level of oxygen evolution even though the activity is three times less than that induced by FR light at 720 nm. In practice, FR light induces about 2.5 % of F V associated with Q B − in 50 % of the RCs (Schansker and Strasser 2005).