Fluorescence Emission as a Tool for Monitoring PS II Function
The radiative emission from Chl a offers an invaluable tool to study the dynamics and the efficiencies of the primary processes of photosynthesis described here. Important information can be gathered from various techniques of fluorescence measurements depending on the mode of excitation and detection (Belyaeva et al., 2008, 2011, 2014, 2015). Infor?mation on excitation energy transfer among antenna pigments, trapping, charge separation and charge stabilization within the reaction centers (according to Chemical equation 3), forming of channels for nonphotochemical quenching, pigment-pigment or pigment-protein coupling can be gathered from analyses of the time decay and wavelength dependence of the emitted fluorescence (Schatz et al., 1988; Roelofs et al., 1992; Renger et al., 1995). Often short (typical ps) light pulses are used and time resolved analysis of prompt fluorescence decay after excitation is often performed in the mode of TWCSPC (Schmitt, 2011).
Another technique is to monitor light-induced transients of fluorescence quantum yield, often referred to as fluorescence induction. A great variety of methods is applied for monitoring fluorescence induction curves: samples are excited either by continuous light at different intensities (Govindjee and Jursinic, 1979; Renger and Schulze, 1985; Bulychev et al., 1987; Neubauer and Schreiber, 1987; Baake and Shloeder, 1992; Strasser et al., 1995, 2004; Bulychev and Vredenberg, 2001 Chemeris et al., 2004; Lazar, 2006) or by light pulses of different duration and intensities (Schreiber et al., 1986; Schreiber and Krieger, 1996; Christen et al., 1999, 2000; Goh et al., 1999; Steffen et al., 2001, 2005a, 2005b).
In general, the prompt fluorescence decay and the transient changes of the fluorescence yield are complicated functions depending on many parameters. Therefore model-based data analyses are required to deconvolute both, the fluorescence decay (Schatz et al., 1988; Roelofs et al., 1992; Renger et al., 1995), fluorescence induction (Shinkarev and Govindjee, 1993; Strasser et al., 2004; Lazar, 2006) and the transient changes of the fluorescence yield (Belyaeva et al., 2008, 2011, 2014, 2015) into a set of parameters for individual reactions. Numerous models have been developed to simulate the experimentally determined fluorescence curves. One essential feature for data analyses is the finding that fluorescence emission of oxygen-evolving photosynthetic organisms is dominated by processes connected with photosystem II (PS II) (see Belyaeva et al., 2008). For a review of different models see (Lazar, 2006).
It has been shown that the prompt fluorescence decay is mostly determined by the kinetics of EET in antenna complexes, charge separation and charge stabilisation as shown in Chemical equation 3 (Schatz et al., 1988; Roelofs et al., 1992).
A much greater variety of models exists for the simulation of transient fluorescence change from ns-s range since these are monitored under very different excitation conditions and depend, in a complicated manner, on very different processes that take place on different time scales. The models typically cover a time range of up to about 10 s. At time periods longer than 500 ms, formation and decay of an electric potential and the pH difference across the thylakoid membrane have to be taken into account for an accurate description of transient fluorescence changes (Van Kooten et al., 1986; Bulychev et al., 1987; Bulychev and Vreden- berg, 1999, 2001 Leibl et al., 1989; Dau and Sauer, 1992; Gibasiewicz et al., 2001 Lebedeva et al., 2002; Belyaeva et al., 2003; Vredenberg and Bulychev, 2003; Belyaeva, 2004).