[en] electron acceptors ; greening ; Hordeum vulgare ; Photosystem II ; salt effect ; variable fluorescence
[en] The polyphasic variable fluorescence in saturating light (O-J-I-P transient, Strasser et al. (1995) Photochem. Photobiol. 61: 21-42) has been investigated in etiochloroplasts during the greening of etiolated leaves of Hordeum vulgare. The initial photochemical phase (O-J) due to reduction of the primary quinone acceptor Q(A) was found to represent a constant proportion (65-70%) of total variable fluorescence independent of greening time. The partial fluorescence quenching in the Q(A)-reduced state seems, therefore, to represent a basic property of PSII electron transport. The biphasic character of the slower J-I-P transient due to reduction of the plastoquinone pool developed progressively during the first hours of greening. in the same period of lime the proportion and rate constant of rapid PSII alpha sub-population increased, as calculated from the induction curve in the presence of DCMU. Etiochloroplasts or chloroplasts resuspended in low salt medium showed a low I level, which was restored upon readdition of 5 mM MgCl2 and NaCl. Salts also increased the apparent proportion of PSII alpha. These results suggest that the J-I and I-P phases of the induction curve are related to different rates of plastoquinone photoreduction by two distinct PSII populations. The effects of DMQ and of DCBQ on the O-J-I-P transient were also studied in (etio-)chloroplasts, In addition to the already reported quenching of the initial (F-0) and variable fluorescence by DCBQ, a slow fluorescence increase phase was found to appear upon the addition of DCBQ but not of DMQ. The latter observations confirm that DCBQ differs from DMQ by its higher efficiency as PSII electron acceptor. (C) 1997 Elsevier Science S.A.