The analysis of PS II photochemical activity using single and multi-turnover excitations.

Paper describes chlorophyll a fluorescence measurements in algal cells, and intact plant leaves and isolated chloroplasts. It focuses on amplitude and 10 μs-resolved kinetics of variable fluorescence responses upon excitation with fluorescence-saturating pulses (SP) and with 25 μs saturating single turnover flashes (STF) which are exposed before, during and after a 100 s actinic illumination (AL) of low and high intensity. In addition to the amply documented suppression of the maximal variable fluorescence from F(m) to F(m)('), the relative proportion of the distinguished O-J-, J-I- and I-P-phases of an SP-induced response is shown to be distinctly different in dark- and light-adapted leaves. The O-J-phase in the 0.01-1 ms time range is much less sensitive to light adaptation than the other phases in the 1-200 ms range. In algae and chloroplasts, the amplitude F(m)(STF) of the STF-induced response is hardly affected by a shift from the dark- to the light-activated steady state. The results support the hypothesis that the maximal variable fluorescence F(m) induced by a multiple-turnover, fluorescence-saturating pulse (SP), is associated with the release of photochemical and photoelectrochemical quenching. It is argued that the OJIPMT- or Kautsky induction curve of variable chlorophyll fluorescence in the 0-100 s time range is the reflection of the release of photochemical quenching supplemented with a temporary Photosystem I (PSI)-dependent photoelectric stimulation and transient release of photoelectrochemical quenching of radiative energy loss in the Photosystem II (PSII) antennas, rather than solely of a decrease in PSII photochemical activity as is usually concluded.