On the origin ofglow peaks in euglena cells, spinach chloroplasts and subchloroplast fragments enriched in system I or II

Abstract

The origin of glow peaks (thermoluminescence) was investigated in isolated spinach chloroplasts and Euglena cells by pretreatment with various concentrations of 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU), different light intensities, and after mild heating at various temperatures. Experiments are also reported on subchloroplast fractions enriched in pigment systems I (PSI) or II (PSII) (prepared under conditions to reduce destruction of membranes by excessive detergent contact). These results provide the following, most likely, suggestion for the origin of glow peaks: (1) Z peak originates in metastable states; it is insensitive to DCMU, temperature (320-328 K), and appears only when other peaks are saturated (10 Wm^-2). (2) Peak I involves the use of a reducing entity A (plastoquinone) beyond Q (the primary electron acceptor of pigment system II, PSII), or, of a high "S" state (charge accumulator) of oxygen evolving system; its intensity is dramatically reduced by low concentrations (1 μ M) of DCMU, and, there is more of it in PSII than in PSI particles. (3) Peak II is due to reaction of Q^- with the "S" states of the oxygen evolving system; its intensity increases upon the addition of low concentrations of DCMU, at the expense of peak I; it is most sensitive to mild heating, and there is more of it in PSII than in PSI particles. (4) Peak III was not studied here as it was not resolved in most of our preparations. (5) Peak IV is from both pigment system I and II; it is sensitive to heating (>50° C), is somewhat sensitive to DCMU, and is present in both PSI and PSII particles. (6) Peak V is from PSI; it is least sensitive to mild heating, and it is enriched in PSI particles. The present studies have extended our knowledge regarding the origin of glow peaks in spinach chloroplasts and Euglena cells; in particular, the involvement of the charge accumulating "S" states of oxygen evolution (for peaks I and II) and of system I (for peak V) are emphasized in this paper.