Electrons are transferred from PS II to the cytochrome bf complex and then to plastocyanin (PC), a blue copper protein and electron carrier. The PC complex carries the electron that will neutralize an electron pair in the reaction center of PS I. As with PS II, a pair of Chl molecules initiates photoinduced charge separation. This pair is referred to as P700 in reference to the wavelength at which the Chl molecules absorb light maximally. The P700 lies in the centre of the protein. Once photoinduced charge separation has been initiated, the electron travels down a pathway through a Chl molecule situated directly above the P700, and then continues through a quinone molecule situated directly above that, through three 4Fe-4S clusters and finally to an interchangeable ferredoxin (Fd) complex. Fd is a soluble protein containing a 2Fe-2S cluster coordinated by four cysteine residues. The positive charge left on the P700 is neutralized by the transfer of an electron from PC. The cooperation between PS II and I creates an electron flow from H2O to NADP+. This pathway is called the 'Z-scheme' because the redox diagram from P680 to P700 resembles the letter z (Nishiyama, Allakhverdiev and Murata 2006).
3. The photoinhibition of photosynthesis
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Photoinhibition can decrease productivity and plant growth, therefore avoidance of photoinhibition is critical for the fitness and survival of plants in natural habitats (Kitao et al. 2000). Photoinhibition is the reduction of the photosynthetic rate in response to light exposure at high irradiance (Fig. 3). Although a reduction in photosynthetic rate can also result from photo-oxidation and other causes, photoinhibition refers to the reduction of photosynthetic capacity independent of changes in pigment concentration induced by exposure to high irradiance. The most recent explanation of the mechanisms underlying photoinhibition is a hypothesis known as the manganese mechanism of photoinhibition proposed by the group of Esa Tyystjärvi (Hakala et al. 2005, Ohnishi et al. 2005). Photoinhibition readily and often occurs in photosynthetic organisms, from higher plants to cyanobacteria. In higher plants, ultraviolet light (UV) causes photoinhibition more efficient than wavelengths of visible light, and blue light is also more efficiently than other wavelengths of visible light (Tyystjärvi et al. 2002). Photoinhibition is a series of chemical reactions that inhibit different activities of PS II, but there is no consensus on what these reactions consist of. The activity of the oxygen-evolving complex of PS II is often lost before the rest of the reaction center loses activity (Hakala et al. 2005, Ohnishi et al. 2005, Tyystjarvi 2008). The photoinhibition of PS II membranes under anaerobic conditions however leads primarily to inhibition of electron transfer on the acceptor side of PSII. The PS I is less susceptible to light-induced damage than PS II, but slow inhibition of this photosystem has been observed (Sonoike 1996). Photoinhibition of PSII is easily caused by singlet oxygen produced either by weakly coupled Chl molecules (Santabarbara et al. 2002). Reactive oxygen species (ROS) have then the crucial role in the electron acceptor-side for the photoinhibition (Tyystjärvi 2008). Strong light causes the reduction of the plastoquinone (Pq) pool, which leads to protonation and double reduction (and double protonation) of the Pq electron acceptor of PS II. The protonated and double reduced forms of Pq do not function in electron transport. Furthermore, charge recombination reactions occurring in the inhibited PS II are more likely to lead to the triplet state of the primary donor (P680) than the same reactions in an active PS II. Triplet P680 can react with oxygen to produce harmful singlet oxygen (Tyystäjrvi 2008). Chemical inactivation of the oxygen-evolving complex causes the remaining electron transfer activity of PS II to become very sensitive to light. It has been considered that, even in a healthy leaf, the oxygen-evolving complex does not always function in all PS II centers, and these reaction centers are prone to rapid irreversible photoinhibition (Anderson et al. 1998). Absorption of a photon by the manganese ions of the oxygen-evolving complex triggers its inactivation as well. Further inhibition of the remaining electron transport reactions occurs by a similar mechanism to that of donor-side photoinhibition of PS II and it is supported by the action spectrum of photoinhibition (Hakala et al. 2005). In the other manner, photoinihbition has been observed to occur by the metabolic energy consumption in guard cells in normal light conditions for stomatal opening (Fig. 4). It is probably due to massive ATP consumption by the plasma membrane K+-channels in light (Goh et al. 2002) and H+-ATPase and anaerobiosis suppresses oxidative phosphorylation in the mitochondria (Goh et al. 1999).
4. Mechanisms of photoinhibition by light and other stress factors
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The dynamics of photosynthesis are associated with three different time scales, namely a rapid photoresponse (minutes), photoinhibition (hours), and photoadaptation (days). A light-induced adverse effect on a biomolecule is termed photodynamic damage. Photodynamic damage usually occurs via the formation of a triplet state of the sensitizing chromophore (Halliwell and Gutteridge 1999, Tyystjärvi 2004). PS II is a multiprotein complex that catalyzes the oxidation of H2O and reduction of Pq during photosynthesis (Iwata and Barber 2004) and susceptible to photoinhibition under visible and UV light (Melis 1999).
Several mechanisms have been suggested to explain the photoinhibition of PS II, including the over-reduction of the acceptor side of PS II, which was proposed to lead to the double reduction of the Pq electron acceptor, oxidative damage caused by sporadic inactivity of the donor-side of PSII (Anderson et al. 1998), and singlet oxygen production by uncoupled antenna Chls (Santabarbara et al. 2001, Adir et al. 2003). Photoinhibition of PS II had been proposed to be triggered by light-induced damage to the oxygen-evolving manganese (Mn) cluster (Onishi et al. 2005, Hakala et al. 2005). It is hypothesized that light absorption by the Mn (III) and/or Mn (IV) ions of the oxygen-evolving complex (OEC) leads to the release of a Mn ion from the OEC, which inhibits electron flow to the oxidized primary donor of PS II. Because photoinhibition mainly involves the photochemical inactivation of PS II, all photosynthesizing organisms are potentially susceptible to damage by irradiation. However, the degree of susceptibility is influenced by several types of factors: environmental (light, temperature, water, CO2, O2, and soil fertility), genotypical (sun or shade plants), phenotypical (bending of leaves), and physiological (carbon metabolism) factors. The imposition of additional stress factors during exposure to high irradiance light exacerbates the adverse effects.
Interaction between light and ambient CO2
Lack of CO2 and low CO2 partial pressure causes photoinhibition in C3 plants that depends on the intensity and length of light exposure. Photoinhibition was also found to be prevented to a great extent by partial O2 pressure in C3 plants; photoinhibition of PS II differs from typical reactions of types I and II, as PS II is readily inactivated in the absence of oxygen (Hakala et al. 2005). As O2 participates in carbon metabolism through the photorespiratory carbon oxidation (PCO) cycle, both O2 and CO2 can prevent photoinhibition by maintaining a low activity of carbon metabolism in light. Under CO2-limiting conditions, Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) catalyzes the oxygenation of ribulose-1,5-biphosphate (RuBP) to produce glycolate-2-P to the Calvin cycle intermediate glycerate-3-P and is proposed to be important for avoiding photoinhibition of PS II, especially in C3 plants (Takahashi et al. 2007). During this PCO cycle, ammonia and CO2 are produced by the mitochondrial Gly decarboxylase. Ammonia is subsequently refixed into Glu by plastidic isozymes of Gln synthetase and Fd-dependent Glu synthase in the photorespiratory nitrogen cycle (Linka and Weber 2005). Thus, the photorespiratory pathway consists of the photorespiratory carbon and nitrogen cycles (Takahashi and Badger 2010). Impairment of the photorespiratory pathway interrupts photosynthetic CO2 fixation (owing to the lack of Calvin cycle metabolites and accumulation of photorespiratory pathway intermediates that inhibit the Calvin cycle) and accelerates photoinhibition (Kozaki and Takeba 1996, Melis 1999 and Takahashi et al. 2007). On the other hand, Li et al. (2008) suggested that elevated [CO2] could increase photochemical efficiency under drought stresses and be beneficial to cucumber seedling in drought environment, thus alleviate or offset the negative consequences of global environmental changes.
Interaction between light and temperature stress
Photosynthesis is one of the first processes affected by low temperatures. Exposure of plants to cool (12°C) or freezing (<0°C) temperatures sometimes produces adverse effects on their metabolic functions. Remarkably, the symptoms of low-temperature damage to the photosynthetic apparatus are particularly evident when substantial light intensity follows exposure to low temperature. In chill-sensitive plants, the main site of photoinhibition at low temperatures is PS I (Sonoike 1999). Hydroxyl-radicals formed by the reaction between hydrogen peroxide and reduced FeS centers destroy the FeS centers and a PS I reaction center subunit, the PsaB protein (Terashima et al. 1998). However, the interactive effect of cool temperatures and moderate irradiances on photosynthetic activity has not been yet fully elucidated. Venema et al. (2000) subjected two Lycopersicon species to 1000 ïmol m-2s-1 at 5°C for two days, and showed that the degradation of leaf pigments was slower in plants grown at suboptimal temperatures than in plants grown at optimal temperatures. Non-photochemical quenching of Chl fluorescence was higher in leaves of sub-optimally grown plants, which presented a larger pool of xanthophyll cycle pigments. These results show that acclimation to suboptimal temperatures increases the capacity to resist chill-induced photodamage. On the other hand, Havaux and Tardy (1996) noticed that treatment of Lycopersicon esculentum leaves at 35°C for two hours induced a rapid increase in heat and tolerance of PS II to light as well as to temperatures above 40°C. These authors confirmed that a short exposure of L. esculentum leaves to a radiance of 1000 ïmol m-2s-1 during four minutes significantly increased the thermostability of PS II, as evaluated by subsequent measurements of fluorescence as a function of temperature.
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Researchers have proposed that particular lipid structures have important roles in ensuring proper photosynthetic function (Siegenthaler and Murata 1998). Several lines of evidence indicate that thylakoid fatty-acid composition influences photoinhibition. Damage to D1 is directly proportional to light intensity (Tyystjärvi and Aro, 1996), and inactivated protein molecules must be replaced by newly synthesized D1 proteins to restore PS II activity (Kanervo et al. 1997, Kettunen et al. 1997, van Wijk et al. 1997, Zhang et al. 1999, 2000). In higher plants, the extent of low-temperature photoinhibition has been correlated with the level of saturated fatty acids (16:0 and 18:0) and of 16:1 (3-trans) in phosphatidylglycerol (PG) (Moon et al. 1995). Wild-type tobacco (Nicotiana tabacum) plants contain 67% of these fatty acids in leaf PG, but in a transgenic line expressing a squash (Cucurbita pepo) acyl-ACP:glycerol-3-P acyltransferase (Rbcs-SQ), the proportion was increased to 88% (Moon et al. 1995). Additional experiments demonstrated that the recovery process (rather than D1 damage) was affected in Rbcs-SQ plants at 17°C and 25°C. The implication is that delayed recovery from photoinhibition may be directly linked to the fatty acid composition of the chloroplast PG (Somerville, 1995). A series of lipid mutants available in Arabidopsis have provided important information about the relationship between lipid structure and membrane function (McConn and Browse 1998), and the effect of altering thylakoid fatty acid composition on damage and recovery processes during photoinhibition.