ON THE RELATIONSHIP BETWEEN CHLOROPHYLL FLUORESCENCE QUENCHING AND THE QUANTUM YIELD OF ELECTRON-TRANSPORT IN ISOLATED THYLAKOIDS

The relationship between the empirical fluorescence index Delta F/Fm' and the quantum yield of linear electron flow, Phi(s), was investigated in isolated spinach thylakoids. Conditions were optimised for reliable determination of Delta F/Fm' and Phi(s) with methyl viologen or ferricyanide as electron accepters under coupled and uncoupled conditions. Ascorbate in combination with methyl viologen was found to stimulate light-induced O-2-uptake which is not reflected in Delta F/Fm' and interpreted to reflect superoxide reduction by ascorbate. In the absence of ascorbate, the plot of Delta F/Fm' vs. Phi(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 Delta F/Fm' and Phi(s) was observed, irrespective of Phi(s) being decreased by increasing light intensity or by DCMU-addition. The plots obtained with methyl viologen and ferricyanide as accepters were almost identical and similar to corresponding plots reported previously by other researchers for intact leaves. It is concluded that the index Delta F/Fm' can be used with isolated chloroplasts for characterisation of such types of electron flow which are difficult to assess otherwise, as e.g. O-2 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 Q(A) - Q(B) electron transfer is considered likely.