Groupe de Recherche en Biologie Vé gé tale (GRBV), Université du Qué bec à Trois-Riviè res, Qué bec, Canada Measurement of chlorophyll (Chl) a fluorescence constitutes one of the oldest approaches to investigate photosynthesis, the first Chl fluorescence experiments being reported more than 70 years ago [1,2]. Monitoring fluorescence induction (FI) has become a widespread method for probing photosystem II (PSII), mostly because it is noninvasive, easy, fast, and reliable, and requires relatively inexpensive equipment [3]. When dark-adapted photosynthetic samples are excited with actinic light, FI is characterized by the initial fluorescence level (F 0 or O), which represents excitation energy dissipated as photons before it reaches open reaction centers, and a subsequent rise from F 0 to maximal level (F m or P), related to a series of successive events that lead to the progressive reduction of the quinone molecules located on the acceptor side of PSII [3].The progressive reduction of the acceptor side of PSII leads to three distinct major phases of fluorescence rise from O to P with two intermediate steps, J (I 1 ) and I (I 2 ) [4][5][6]. Whereas it is generally accepted that the O-J phase is related to the PSII primary electron acceptor (Q A ) reduction [6][7][8], the origin of the J-I and I-P phases is still a matter of debate [3,[9][10][11] Fluorescence induction has been studied for a long time, but there are still questions concerning what the O-J-I-P kinetic steps represent. Most studies agree that the O-J rise is related to photosystem II primary acceptor (Q A ) reduction, but several contradictory theories exist for the J-I and I-P rises. One problem with fluorescence induction analysis is that most work done to date has used only qualitative or semiquantitative data analysis by visually comparing traces to observe the effects of different chemicals or treatments. Although this method is useful to observe major changes, a quantitative method must be used to detect more subtle, yet important, differences in the fluorescence induction trace. To achieve this, we used a relatively simple mathematical approach to extract the amplitudes and half-times of the three major fluorescence induction phases obtained from traces measured in thylakoid membranes kept at various temperatures. Apparent activation energies (E A ) were also obtained for each kinetic step. Our results show that each phase has a different E A , with E A O-J < E A J-I < E A I-P , and thus a different origin. The effects of two well-known chemicals, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, which blocks electron transfer to the photosystem II secondary electron acceptor (Q B ), and decylplastoquinone, which acts similarly to endogenous reducible plastoquinones, on the quantitative parameters are discussed in terms of the origin of each kinetic phase.Abbreviations A O-J , A J-I and A I-P , amplitude of O-J, J-I and I-P phases, respectively; Chl, chlorophyll; DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea; dPQ, decylplastoquinone; E A , activation ...
The toxic effect of Ni(2+) on photosynthetic electron transport was studied in a photosystem II submembrane fraction. It was shown that Ni(2+) strongly inhibits oxygen evolution in the millimolar range of concentration. The inhibition was insensitive to NaCl but significantly decreased in the presence of CaCl(2). Maximal chlorophyll fluorescence, together with variable fluorescence, maximal quantum yield of photosystem II, and flash-induced fluorescence decays were all significantly declined by Ni(2+). Further, the extrinsic polypeptides of 16 and 24 kDa associated with the oxygen-evolving complex of photosystem II were depleted following Ni(2+) treatment. It was deduced that interaction of Ni(2+) with these polypeptides caused a conformational change that induced their release together with Ca(2+) from the oxygen-evolving complex of photosystem II with consequent inhibition of the electron transport activity.
Polyamines are implicated in plant growth and stress response. However, the polyamines spermine and spermidine were shown to elicit strong inhibitory effects in photosystem II (PSII) submembrane fractions. We have studied the mechanism of this inhibitory action in detail. The inhibition of electron transport in PSII submembrane fractions treated with millimolar concentrations of spermine or spermidine led to the decline of plastoquinone reduction, which was reversed by the artificial electron donor diphenylcarbazide. The above inhibition was due to the loss of the extrinsic polypeptides associated with the oxygen evolving complex. Thermoluminescence measurements revealed that charge recombination between the quinone acceptors of PSII, QA and QB, and the S2 state of the Mn-cluster was abolished. Also, the dark decay of chlorophyll fluorescence after a single turn-over white flash was greatly retarded indicating a slower rate of QA- reoxidation.
Photosystem II (PSII), a multiprotein complex mainly coded by the chloroplast genome in higher plants and algae, contains the oxygen-evolving complex with four manganese atoms responsible for the oxidation of water. After each absorption of a light quantum by pigment molecules in the light harvesting complexes of PSII, the Mn cluster advances in its oxidation states denoted from S(0) to S(4) . The S(4) state decays to S(0) in the dark with the concurrent release of molecular oxygen. Therefore, the oxygen production in PSII exposed to successive single turnover excitations follows a period-four oscillation pattern. The intensity of chlorophyll a fluorescence of PSII is also known to be influenced by the oxidation state of the Mn cluster. In the present work, fluorescence induction kinetics was measured in isolated thylakoids with various initial S-state populations settled by preflashes. The shape of the fluorescence induction traces was strongly affected by preflashes. O-J and J-I phases of the induction followed a period-four oscillation pattern. The results indicate that these changes reflect the influence of the oxidation rate of the Mn cluster on the reduction/oxidation kinetics of the primary quinone acceptor (Q(A) ) of PSII.
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