Oxygen-evolving system and secondary quinonic acceptors are highly reduced in dark adapted Euglena cells: A thermoluminescence study

Abstract

Characteristics of thermoluminescence glow curves were compared in three types of Euglena cells: (i) strictly autotrophic, Cramer and Myers cells; (ii) photoheterotrophic cells sampled from an exponentially growing culture containing lactate as substrate repressing the photosynthetic activity; (iii) semiautotrophic cells, sampled when the lactate being totally exhausted, the photosynthesis was enhanced.In autotrophic and semiautotrophic cells, composite curves were observed after series of two or more actinic flashes fired at -10°C, which can be deconvoluted into a large band peaking in the range 12-22°C and a smaller one near 40°C, This second band presents the characteristics of a typical B band (due to S2/3QB (-) recombination), whereas the first one resembled the band, shifted by -15-20°C, which is observed in herbicide resistant plants. The amplitude of this major band, which was in all cases very low after one flash, exhibited oscillations of period four but rapidly damping, with maxima after two and six flashes. In contrast, photoheterotrophic Euglena displayed single, non-oscillating curves with maxima in the range 5-10°C.In autotrophic and semiautotrophic cells, oxidizing pretreatments by either a preillumination with one or more (up to twenty-five) flashes, or a far-red preillumination in the presence of methylviologen, followed by a short dark period, induced thermoluminescence bands almost single and shifted by +3-5°C, or +12°C, respectively. In autotrophic cells, far-red light plus methyl viologen treatment induced a band peaking at 31°C, as in isolated thylakoids from Euglena or higher plants, while it had barely any effect in photoheterotrophic cells.Due to metabolic activities in dark-adapted cells, a reduction of redox groups at the donor and acceptor sides of PS II dark-adapted cells is supposed to occur. Two different explanations can be proposed to explain such a shift in the position of the main band in dark-adapted autotrophic control. The first explanation would be that in these reducing conditions a decreasing value of the equilibrium constant for the reaction: SnQA (-)QB⇌SnQAQB (-), would determine the shift of the main TL band towards low temperatures, as observed in herbicide resistant material. The second explanation would be that the main band would correspond to 'peak III' already observed in vivo and assigned to S2/3QB (2-) recombinations.

Compared thermoluminescence characteristics of pea thylakoids studied in vitro and in situ (in leaves). The effect of photoinhibitory treatments

“…In order to explain why such a band was not observed in vitro, it can be proposed that after a prolonged illumination, the reoxidation of the reduced PQ pool in the thylakoids is slow in vivo but would be much faster in vitro. An explanation would be that in vitro the reduced PQ pool is directly reoxidized by oxygen of air whereas in vivo and in the dark, a special mechanism would contribute to poise the PQ pool in a moderately reduced state as it was already stated for spinach leaves (Rutherford et al 1984b) and Euglena (Farineau and Laval-Martin 1992); after a strong illumination, a highly reduced state is attained in vivo and maintained for several minutes in a subsequent period of darkness. In our opinion, a similar process could be operating in other photosynthetic material submitted to high light, such as Chlamydomonas (Ohad et al 1988(Ohad et al , 1990; the modified B bands (observed after 1 fl), with maximum shifted to about 10-15 °C, which were described in the 2 previous papers, would be complex bands containing a component with a size decreasing with time, resulting from S2QA recombinations.…”

“…The TL device has been described in previous publications (Briantais et al 1992, Farineau andLaval-Martin 1992). TL curves were recorded in the range -40 to +80°C with heating rate of 0.5°C/s in the case of charging bands by flashes (flashes fired at -10°C, spaced 0.5s apart).…”

“…For all experiments and especially for deconvolution of curves, the recording of a TL curve for a dark-adapted sample having never received any flash was used as a baseline. A simulation procedure was used to analyze the TL curves as explained in previous papers: Briantais et al (1992), Ducruet and Miranda (1992), Farineau and Laval-Martin (1992). Complex curves were tentatively decomposed into 2 (or more) TL bands, the signal being considered as a sum.…”

“…In this respect, it permits study of the periodic cyclic functioning of PS II when charged by a series of flashes fired in a range of temperature -1 0°C to 20°C (Rutherford et al 1984a). Degree of intactness of the electron transfer chain in the environment of PS II centers, or changes in their redox state after some treatments can be checked; thus, TL experiments performed in vivo with untreated dark-adapted material: disks of spinach (Rutherford et al 1984b) or Euglena (Farineau and Laval-Martin 1992), have demonstrated occurrence of a reduced plastoquinone pool in thylakoids. Several studies were published concerning the TL properties of photoinhibited thylakoids.…”

The mechanism of the ozone-induced changes in thermoluminescence glow curves of barley leaves

“…The down-shift of the Bband under stress conditions reflects changes of the donor side and/or the acceptor side of PS2. A decrease of pH in the lumen of thylakoids and/or an increase of the reduction of the plastoquinone (PQ) pool was suggested as reasons for the down-shift of the B-band (Farineau andLaval-Martin 1992, Miranda andDucruet 1995). A similar effect due to an increased reduction of the PQ pool could be induced by a decreased ability in PQ binding or a changed protein conformation at the acceptor side.…”