Assessment of Photosystem II Photochemical Quantum Yield by Chlorophyll Fluorescence Quenching Analysis

Abstract

The general principles involved in chlorophyll fluorescence quenching analysis by the saturation pulse method are presented, outlining the rationale for using the empirical fluorescence parameters F_v/F_m and F_v/F_m' as indices for the photosystem II (PSII) photochemical quantum yield, Φ_II, in the dark-adapted or illuminated states, respectively. The relationship between Φ_II and the quantum yield of photosynthetic electron transport is linear over a wide range of quantum flux densities. However, there is a fraction of PSII contributing approximately 30% to maximal quantum yield, which is closed at rather low quantum flux densities, while at the same time there is only a small drop in ΔF/F_m'. The details of F_m and F_m' determination by application of saturating light are critically examined, with emphasis on the situation in algae where the fluorescence rise to the peak leLel is followed by a rapid decline. For this purpose, the rapid induction kinetics upon onset of strong continuous illumination are investigated. Dark-adapted samples show two distinct intermediate fluorescence levels, I₁ and I₂, in the polyphasic rise from the O to the P level. The I₁ level separates a biphasic 'photochemical' rise, which also can be induced by a saturating single turnover flash, from several 'thermal' phases, induction of which requires multiple turnovers at PSII. Arguments are put forward favouring the I₂ level for assessment of F_m or F_m', on which calculation of F_v/F_m or ΔF/F_m' is based. It is shown that although an assessment based on the I1 level, as practised by the so-called pump- and-probe method, does lead to a consistent underestimation of ΔF/F_m, in many cases similar information as with I₂ determination is obtained.