In the reaction centre of PSII, the first detectable radical pair formed after excitation by light is $$\mathrm{P}_{680}^{{+}}\mathrm{Pheo}^{{-}},$$ with P680 being the primary electron donor and pheophytin the primary electron acceptor (for a recent review on PSII see Goussias et al., 2002; for the X-ray structure of PSII, see Zouni et al., 2001; Kamiya and Shen, 2003; Ferreira et al., 2004). A photosynthetic organism is subjected to photo-oxidative stress when more light energy is absorbed than is used in photosynthesis. Damaged D1 protein is degraded and PSII is repaired efficiently by the assembly of newly synthesized D1 in the so-called D1 protein damage–repair cycle (for reviews see Prasil et al., 1992; Aro et al., 1993). This flash-induced charge recombination reaction was exploited to investigate the mechanism of photoinhibition under low light in vivo (Keren et al., 1995) and in vitro (Keren et al., 1997). In the closed reaction centre, however, if primary charge separation occurs, it is followed by recombination of the charges. 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. On the other hand, the decreased mid-point potential of QA induced by phenolic herbicides should make the energy gap between $$\mathrm{P}_{680}^{{+}}\mathrm{Q}_{\mathrm{A}}^{{-}}$$ and $$\mathrm{P}_{680}^{{+}}\mathrm{Pheo}^{{-}}$$ smaller and, therefore, the back reaction via the primary radical pair and the formation of P680 triplet more likely. The yield of primary charge separation was not significantly reduced in Ca2+-depleted PSII, even after several single turnover flashes (Keren et al., 2000), implying that a difference in the charge recombination pathway must be responsible for this phenomenon. Instead reaction products originating either from the D1 protein degradation or products of chlorophyll degradation can be envisaged as signal molecules. When DCMU is bound, then the mid-point potential of $$\mathrm{Q}_{\mathrm{A}}/\mathrm{Q}_{\mathrm{A}}^{{-}}$$ is shifted by 50 mV to a more positive value and this back reaction is disfavoured and direct recombination to the ground state may occur. 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. Institut für Biologie II, Biochemie der Pflanzen, Universität Freiburg, Schänzlestr. Copyright © 1999 Elsevier Science Inc. All rights reserved. This effect is already seen at relatively low light intensities (400 μmol quanta m−2 s−1) and the amount of 1O2 produced increases linearly with increasing light intensities. Op den Camp et al. Sites of production of singlet oxygen in photosynthesis and its potential targets. Singlet oxygen may also play a role in the degradation of light-harvesting proteins. Thank you for submitting a comment on this article. It has been shown that a functional ABC protein is required for the transport of protophorhyrin IX (Møller et al., 2001). (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). Degradation of LHCII releases a large number of chl. Light-induced damage might occur from such chl which is energetically uncoupled from the antenna, and will give a high triplet chl and, therefore, possibly high 1O2 yield. Photosynthetic electron transport. Singlet oxygen is the lowest excited state of the dioxygen molecule. By contrast, other reactive oxygen species like superoxide did not rapidly up-regulate the expression of these genes. 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). The occurrence of double reduced QA, however, has never been shown to occur under physiologically relevant conditions. In this spin exchange reaction, the triplet state of the carotenoid is formed which can either dissipate the excess energy directly as heat or by physical quenching via enhanced intersystem crossing with 3O2 (Edge and Truscott, 1999). 3P680 can react with 3O2 forming 1O2. If the energy is not efficiently used, the spins of the electrons in the excited state can rephase and give rise to a lower energy excited state: the chlorophyll triplet state. In the single turnover flash experiments, the loss of PSII activity was measured and compared with active samples. In the dark, the charges recombine via the formation of the primary radical. An even number of flashes produced the state S3QBH2 which does not recombine, while after an uneven number of flashes, charge recombination between the S2 or S3 state and $$\mathrm{Q}_{\mathrm{B}}^{{-}}$$ occurs leading to singlet and triplet P680. In bacterial reaction centres it has been shown that the free energy gap between the $$\mathrm{P}^{{+}}\mathrm{Q}_{\mathrm{A}}^{{-}}$$ radical pair and the P+BPheo− radical pair has a major influence on the back reaction pathway (Gunner et al., 1982; Gopher et al., 1985; Woodbury et al., 1986; Shopes and Wraight, 1987). Changes of the mid-point potential of the primary quinone acceptor in photosystem II modulate the pathway of charge recombination in photosystem II and influence the yield of singlet oxygen production.