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 Fm to Fm′, 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 FmSTF 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 Fm 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.

► Fv/Fm is an incorrect measure of the maximal photochemical efficiency ϕp of PSII.
► Decrease in Fm to Fm′ during light adaptation from is not caused by decrease in ϕp.
► ϕp calculation requires estimation of variable fluorescence in single turnover flash (STF).