Abstract
Paper describes principles and application of a novel routine that enables the quantitative analysis of the photochemical O–J phase of the variable fluorescence F v associated with the reversible photo-reduction of the secondary electron acceptor QA of photosystem II (PSII) in algae and intact leaves. The kinetic parameters that determine the variable fluorescence F PP(t) associated with the release of photochemical quenching are estimated from 10 µs time-resolved light-on and light-off responses of F v induced by two subsequent light pulses of 0.25 (default) and 1000 ms duration, respectively. Application of these pulses allows estimations of (i) the actual value of the rate constants k L and k AB of the light excitation (photoreduction of QA) and of the dark re-oxidation of photoreduced QA ({text{Q}}_{text{A}}^{ - }), respectively, (ii) the actual maximal normalized variable fluorescence [nF v] associated with 100 % photoreduction of QA of open RCs, and (iii) the actual size β of RCs in which the re-oxidation of {text{Q}}_{text{A}}^{ - } is largely suppressed (QB-nonreducing RC with k AB ~ 0). The rate constants of the dark reversion of Fv associated with the release of photo-electrochemical quenching F PE and photo-electric stimulation F CET in the successive J–I and I–P parts of the thermal phase are in the range of (100 ms)−1 and (1 s)−1, respectively. The kinetics of fluorescence changes during and after the I–P phase are given special attention in relation to the hypothesis on the involvement of a Δµ H+-dependent effect during this phase and thereafter. Paper closes with author’s personal view on the demands that should be fulfilled for chlorophyll fluorescence methods being a correct and unchallenged signature of photosynthesis in algae and plants.
Highlights
The time pattern of variable chlorophyll a fluorescence of alga and plant leaves in an actinic light pulse provides valuable information on properties and characteristics of the photosynthetic processes that are initiated by the light
The fluorescence induction algorithm (FIA) methodology is conceptually different from the alternate approaches in a sense that it is based on the concept (Vredenberg 2000) of the three-state trapping model (TSTM) and as such not limited by the disputable constraint (Stirbet and Govindjee 2012) that 100 % reduction of the primary quinone acceptor QA is required and sufficient for reaching the maximum fluorescence Fm
The values of amplitudes and rate constants of each of the components are given in the insert box
Summary
The time pattern of variable chlorophyll a (chla) fluorescence of alga and plant leaves (chla fluorescence induction) in an actinic light pulse provides valuable information on properties and characteristics of the photosynthetic processes that are initiated by the light. Amongst those are (i) generation and decay of trans- and inner membrane electric fields associated with primary charge separation in the photochemical systems PSI and PSII, (ii) photochemical reduction of the primary electron acceptor pair [PheQA] with pheophytin (Phe) and QA acting as primary and secondary electron acceptors and fluorescence quenchers, respectively, (iii) secondary processes that are coupled to electron transport in the photosynthetic transport chains, that among others lead to generation and dissipation of a trans-thylakoid electrochemical proton gradient (DlH) which powers ATP synthesis and transmembrane ion fluxes. The FIA methodology is conceptually different from the alternate approaches in a sense that it is based on the concept (Vredenberg 2000) of the three-state trapping model (TSTM) and as such not limited by the disputable constraint (Stirbet and Govindjee 2012) that 100 % reduction of the primary quinone acceptor QA is required and sufficient for reaching the maximum fluorescence Fm
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