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 investigate d in isolated spinach thylakoids. Conditions were optimised for reliab le determination of Delta F/Fm' and Phi(s) with methyl viologen or fer ricyanide 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 in terpreted to reflect superoxide reduction by ascorbate. In the absence of ascorbate, the plot of Delta F/Fm' vs. Phi(s) was mostly linear, e xcept for the range of high quantum yields, i.e. at rather low photon flux densities. With ferricyanide as acceptor, use of relatively low c oncentrations (0.1-0.3 mM) was essential for correct Fm'-determination s, particularly under uncoupled conditions. Under coupled and uncouple d conditions the same basic relationship between Delta F/Fm' and Phi(s ) was observed, irrespective of Phi(s) being decreased by increasing l ight intensity or by DCMU-addition. The plots obtained with methyl vio logen and ferricyanide as accepters were almost identical and similar to corresponding plots reported previously by other researchers for in tact leaves. It is concluded that the index Delta F/Fm' can be used wi th isolated chloroplasts for characterisation of such types of electro n 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 discu ssed. An involvement of 'inactive' PS II centers with separate units a nd inefficient Q(A) - Q(B) electron transfer is considered likely.