On the relationship between chlorophyll fluorescence quenching and the quantum yield of electron transport in isolated thylakoids.

The relationship between the empirical fluorescence index ΔF/Fm' and the quantum yield of linear electron flow, Φ(s), was investigated in isolated spinach thylakoids. Conditions were optimised for reliable determination of ΔF/Fm' and Φ(s) with methyl viologen or ferricyanide as electron acceptors under coupled and uncoupled conditions. Ascorbate in combination with methyl viologen was found to stimulate light-induced O2-uptake which is not reflected in ΔF/Fm' and interpreted to reflect superoxide reduction by ascorbate. In the absence of ascorbate, the plot of ΔF/Fm' vs. Φ(s) was mostly linear, except for the range of high quantum yields, i.e. at rather low photon flux densities. With ferricyanide as acceptor, use of relatively low concentrations (0.1-0.3 mM) was essential for correct Fm'-determinations, particularly under uncoupled conditions. Under coupled and uncoupled conditions the same basic relationship between ΔF/Fm' and Φ(s) was observed, irrespective of Φ(s) being decreased by increasing light intensity or by DCMU-addition. The plots obtained with methyl viologen and ferricyanide as acceptors were almost identical and similar to corresponding plots reported previously by other researchers for intact leaves. It is concluded that the index ΔF/Fm' can be used with isolated chloroplasts for characterisation of such types of electron flow which are difficult to assess otherwise, as e.g. O2 dependent flux. The origin of the 'non-linear' part of the relationship is discussed. An involvement of 'inactive' PS II centers with separate units and inefficient QA-QB electron transfer is considered likely.