Based on these observations made with the bacterial reaction centre a model was proposed (Fig. Traditionally, oxygen therapy is done by breathing air with high oxygen concentration. In PSI, under reducing conditions when the iron sulphur clusters are prereduced or when vitamin K1 is removed from the reaction centre, charge recombination reactions also occur leading to the triplet state of P700 at room temperature (for a review see Brettel, 1997). Copyright © 2020 Elsevier B.V. or its licensors or contributors. Two of these triplet states are probably generated in the core complex while the third one may be generated in the light-harvesting complex (Santabarbara et al., 2002). Institut für Biologie II, Biochemie der Pflanzen, Universität Freiburg, Schänzlestr. In addition, the very reactive singlet oxygen can be generated by an input of energy. 1O2 can react with carotenoids which act as a catalyst, deactivating 1O2. However, experimental evidence for the production of 1O2 by triplet formation in the antenna and their involvement in the light-induced damage in PSII in vivo in mature leaves is still missing. Production of triplet chlorophyll and 1O2 may play a role during the transition from etioplasts to chloroplasts. The cytochrome b6f complex contains, in addition to the other cofactors, one chlorophyll with an unknown function. Oxygen-derived radicals are generated constantly as part of normal aerobic life. Chl triplet states may be populated in principle either directly by intersystem crossing (changing of the spin) from a singlet excited chlorophyll, or by charge recombination reactions (reversal of the charge separation and electron transfer reactions) in the reaction centres. The change of the potential form of cytochrome b559 was already observed before the process of photoactivation was fully completed (Mizusawa et al., 1997). It has been proposed that the presence of the semiquinone anion $$\mathrm{Q}_{\mathrm{A}}^{{-}}$$ in closed PSII may raise the energy of the primary pair by an electrostatic interaction so that the driving force of the primary charge separation is decreased compared with open reaction centres (state of the centre with the oxidized quinone, QA) (van Gorkom, 1985; Schatz et al., 1988). If the light-induced damage exceeds the controlled D1 degradation and repair of PSII, further protein degradation of chl binding subunits may lead to the production of free chls, which are dangerous photosensitizers. The back reactions between these radical pairs require thermal activation and are thus thermodynamically disfavoured. In the reaction centre, the distance between the carotenes and the triplet chlorophyll is too large to allow a direct triplet quenching. This possibility is given in the antenna system, but not in the reaction centre, although two β-carotene molecules are present in the PSII reaction centre (Telfer, 2002; for the location of the carotenes in the reaction centre, see Kamiya and Shen, 2003; Ferreira et al., 2004). The molecular basis for the shift of the mid-point potential of QA by binding a herbicide to the QB binding pocket is not understood. The triplet state of this radical pair can recombine rapidly, resulting in 3P680. If the water-splitting complex of PSII is inactivated (prior to photoactivation or after Ca2+-depletion), the mid-point potential of QA is shifted and charge recombination reactions are though to occur to the ground state via a safe route (III). FTIR spectra of QA obtained in the presence of a phenolic herbicide compared with DCMU indicate that the protein environment of QA is slightly modified by the phenolic herbicide. the electron orbitals must have some overlap. Some other reactive oxygen species like superoxide or peroxide have been shown to act directly as second messengers in the regulation of expression of the oxidative stress response genes such as gutathione peroxidases, glutathione-S-transferases, and ascorbate peroxidase (for reviews see Mullineaux et al., 2000; Vranová et al., 2002). the light-induced loss of PSII activity and of the D1 protein) of Chlamydomonas reinhardtii cells. These free chls may be bound by ELIP (Early Light Induced Proteins) proteins (Adamska, 1997) or by proteins like WSCP (Water Soluble Chlorophyll Protein) (Schmidt et al., 2003). It was shown by Suh et al. Degradation of LHCII releases a large number of chl. On the one hand chlorophylls are needed for the use of light energy in photosynthesis, on the other hand, the same molecules carry the potential danger of being a singlet oxygen producer (photosensitizer). Keren and coworkers used a series of single turnover flashes, spaced with a dark interval of 32 s, and measured the degree of photoinactivation and loss of the D1 protein. 1O2 can react with proteins, pigments, and lipids and is thought to be the most important species responsible for light-induced loss of PSII activity, the degradation of the D1 protein (protein of the reaction centre of PSII) and for pigment bleaching (for reviews on photoinhibition, see Prasil et al., 1992; Aro et al., 1993). (2003) when estimated from the back reaction rate of $$\mathrm{S}_{2}\mathrm{Q}_{\mathrm{A}}^{{-}}.$$ The effect of the different types of herbicide on the mid-point potential of QA was not only observed for the low potential form but also for the high potential form of QA (Krieger-Liszkay and Rutherford, 1998). Another radical derived from oxygen is singlet oxygen, designated as 1 O 2. On the one hand chlorophylls are needed for the use of light energy in photosynthesis, on the other hand, the same molecules carry the potential danger of being a singlet oxygen producer (photosensitizer). The absolute change in the mid-point potential of QA by these herbicides was much lower (±50 mV) than the shift induced by inactivation of the water-splitting complex (Ca2+- or Mn-depletion), but it has, nevertheless, a big effect on the yield of 1O2 production. At room temperature, the yield of the primary pair is reduced in the presence of $$\mathrm{Q}_{\mathrm{A}}^{{-}},$$ but nevertheless, a significant yield of the primary pair is found in core complexes of Synechococcus (about half of that found in reaction centres with double reduced QA) (Schlodder and Brettel, 1988; van Mieghem et al., 1995). 2) (Johnson et al., 1995; Krieger et al., 1995; Andréasson et al., 1995). By analogy, one can also speculate that a chlorophyll degradation product such as pheophytin, chlorophyllide, or pheophorbide (for chl degradation, see Matile et al., 1999) may act as a signalling molecule. There are recent reports in the literature that, as a response to 1O2 production, genes are specifically up-regulated which are involved in the molecular defence response of the plant against photo-oxidative stress (Leisinger et al., 2001; op den Camp et al., 2003; B Fischer, personal communication). Under such conditions the plastoquinone pool can be in a very reduced state, forward electron transport is very limited, and recombination reactions in PSII can occur. (2002) showed that the yield of 1O2 production in the presence of a phenolic herbicide in active PSII-enriched membrane fragments (with QA in the low potential form) is twice as high as in the presence of DCMU. Explain how oxygen can be fatal to organisms by discussing singlet oxygen, superoxide radical, peroxide anion, and hydroxyl radical and describe how organisms protect themselves from toxic forms of oxygen. Electron transfer reactions in the presence of oxygen can give rise to the production of the reactive intermediates, which themselves can produce different kinds of damage in the cell (Halliwell and Gutteridge, 1998). 3; see also Johnson et al., 1995; Rutherford and Krieger-Liszkay, 2001). in the presence of dithionite and light (van Mieghem et al., 1989) or anaerobiosis and light (Vass et al., 1992), QA becomes doubly reduced, thereby releasing the negative electrostatic effect on the energy of the primary pair, and a high yield of charge separation, recombination, and P680 triplet formation is observed (van Mieghem et al., 1989). 1O2 formation was detected directly by its luminescence (Macpherson et al., 1993) and by EPR spin trapping (Hideg et al., 1994).