At room temperature the delayed fluorescence (luminescence) of spinach chloroplasts, in which the acceptor Q is prereduced, consists of a component with a lifetime of 0.7 As and a more rapid component, presumably with a lifetime of Since the discovery of delayed chlorophyll a fluorescence (in the following we use the term "luminescence" for the sake of simplicity) by Strehler and Arnold in 1951 (1) in the green alga Chlorella pyrenoidosa, it has been suggested by many workers that the emission of luminescence is caused by back reactions, which result in the reversal of the primary photochemical reaction, albeit with a low efficiency, and thus in the reexcitation of chlorophyll a and in light emission. For photosystem II (PS II) reexcitation of chlorophyll a takes place by the recombination of the oxidized primary donor P+ and reduced acceptor. For a discussion of the so-called recombination hypothesis and evidence that PS II is the source of luminescence in algae and higher plants, we may refer to reviews of Lavorel (2), Malkin (3), and Amesz and van Gorkom (4).For spinach chloroplasts and the green alga Chlorella vuilgaris a I-As component was reported (5, 6), which mainly seemed to originate from reaction centers which were in the so-called "closed" state P Q. Q is an acceptor, a plastoquinone, formerly believed to be the primary acceptor. In the presence of reduced Q, Q-, the fluorescence yield of chlorophyll a is high (7). In the light of the recombination hypothesis another acceptor W different from Q was postulated and the luminescence was postulated to be due to the charge recombination P+ W-Q-P1 P*W Q-P W Q-+ hv. W might be an intermediary acceptor between P and Q analogous to I in photosynthetic bacteria, or W might be on a side path. In bacterial species in which the noniron complex X is prereduced, a 10-ns 5889 luminescence component has been found due to the recombination of P-870+ and I-, the oxidized bacteriochlorophyll dimer and the reduced bacteriopheophytin, respectively (8-10). This luminescence slightly but progressively decreases from 300 K down to 77 K (8, 9, 11). A magnetic field of the order of 1-100 mT decreased the reaction center triplet yield and induced a luminescence increase, which was determined by measuring the fractional increase of the emission (the sum of fluorescence and luminescence) (12)(13)(14). This phenomenon was explained in terms of the radical pair mechanism (see ref. 15 for an introduction to the theory of this mechanism).We have investigated the temperature dependence of the (sub) microsecond luminescence of PS II of spinach chloroplasts in which the acceptor Q was prereduced. Lowering the temperature revealed a strong luminescence component with a lifetime of 100-200 ns. Surprisingly, upon cooling we found, in contrast to bacteria mentioned above, an increase of the total luminescence. We also studied the influence of magnetic fields up to 0.3 T on the chlorophyll a luminescence emission yield. These experiments were carried out in two different ways. Fi...

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