ASSESSMENT OF PHOTOSYSTEM-II PHOTOCHEMICAL QUANTUM YIELD BY CHLOROPHYLL FLUORESCENCE QUENCHING ANALYSIS

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 Delta F/F-m' as indices for the photosystem II (PSII) photochemical quantum yield, Phi(II), in the dark-adapted or illuminated states, respectively. The relationship between Phi(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 Delta F/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 level 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, I-1 and I-2, in the polyphasic rise from the O to the P level. The I-1 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 I-2 level for assessment of F-m or F-m', on which calculation of F-v/F-m or Delta F/F-m' is based. It is shown that although an assessment based on the I-1 level, as practised by the so-called pump-and-probe method, does lead to a consistent underestimation of Delta F/F-m', in many cases similar information as with 12 determination is obtained.