Oxygen-dependent electron transport and protection from photoinhibition in leaves of tropical tree species

Different photoprotective strategies for white leaves between two co-occurring Actinidia species

At the outer canopy, the white leaves of Actinidia kolomikta can turn pink but they stay white in A. polygama. We hypothesized that the different leaf colors in the two Actinidia species may represent different photoprotection strategies. To test the hypothesis, leaf optical spectra, anatomy, chlorophyll a fluorescence, superoxide (O₂ ˙^- ) concentration, photosystem II photo-susceptibility, and expression of anthocyanin-related genes were investigated. On the adaxial side, light reflectance was the highest for white leaves of A. kolomikta, followed by its pink leaves and white leaves of A. polygama, and the absorptance for white leaves of A. kolomikta was the lowest. Chlorophyll and carotenoid content of white and pink leaves in A. kolomikta were significantly lower than those of A. polygama, while the relative anthocyanin content of pink leaves was the highest. Chloroplasts of palisade cells of white leaves in A. kolomikta were not well developed with a lower maximum quantum efficiency of PSII than the other types of leaves (pink leaves of A. kolomikta and white leaves of A. Polygama at the inner/outer canopy). After high light treatment from the abaxial surface, F_v /F_m decreased to a larger extent for white leaves of A. kolomikta than pink leaf and white leaves of A. polygama, and its non-photochemical quenching was also the lowest. White leaves of A. kolomikta showed higher O₂ ˙^- concentration compared to pink leaves under the same strong irradiance. The expression levels of anthocyanin biosynthetic genes in pink leaves were higher than in white leaves. These results indicate that white leaves of A. kolomikta apply a reflection strategy for photoprotection, while pink leaves resist photoinhibition via anthocyanin accumulation.

Low temperature and high light dependent dynamic photoprotective strategies in Arabidopsis thaliana

Arabidopsis thaliana has been recognized as a chilling tolerant species based on analysis of resistance to low temperature stress, however, the mechanisms involved in this tolerance are not yet clarified. The low temperature-induced effects are exacerbated when plants are exposed to low temperatures in the presence of high light irradiance but the experimental data on the impact of light intensity during cold stress and its influence during recovery from stress are rather limited. The main objective of this study was to re-examine the photosynthetic responses of A. thaliana plants to short term (6 days) low temperature stress (12/10°C) under optimal (150 μmol m^-2 s^-1 ) and high light (500 μmol m^-2 s^-1 ) intensity and the subsequent recovery from the stress. Simultaneous measurements of the in vivo and in vitro functional performance of both photosystem II (PSII) and photosystem I (PSI), as well as, net photosynthesis, low temperature (77 K) chlorophyll fluorescence and immunoblot analysis of the relative abundance of PSII and PSI reaction center proteins were used to evaluate the role of light in the development of possible protective mechanisms during low temperature stress and the consequent recovery from exposure to low temperature and different light intensities. The results presented clearly suggest that Arabidopsis plants can employ a number of highly dynamic photoprotective strategies depending on the light intensity. These strategies include one based on LHCII quenching and two other quenching mechanisms localized within the PSII and PSI reaction centers, which are all expressed to different extent depending on the severity of the photoinhibitory treatments under low temperature stress conditions.

Land plants drive photorespiration as higher electron-sink: comparative study of post-illumination transient O2 -uptake rates from liverworts to angiosperms through ferns and gymnosperms

In higher plants, the electron-sink capacity of photorespiration contributes to alleviation of photoinhibition by dissipating excess energy under conditions when photosynthesis is limited. We addressed the question at which point in the evolution of photosynthetic organisms photorespiration began to function as electron sink and replaced the flavodiiron proteins which catalyze the reduction of O₂ at photosystem I in cyanobacteria. Algae do not have a higher activity of photorespiration when CO₂ assimilation is limited, and it can therefore not act as an electron sink. Using land plants (liverworts, ferns, gymnosperms, and angiosperms) we compared photorespiration activity and estimated the electron flux driven by photorespiration to evaluate its electron-sink capacity at CO₂ -compensation point. In vivo photorespiration activity was estimated by the simultaneous measurement of O₂ -exchange rate and chlorophyll fluorescence yield. All C3-plants leaves showed transient O₂ -uptake after actinic light illumination (post-illumination transient O₂ -uptake), which reflects photorespiration activity. Post-illumination transient O₂ -uptake rates increased in the order from liverworts to angiosperms through ferns and gymnosperms. Furthermore, photorespiration-dependent electron flux in photosynthetic linear electron flow was estimated from post-illumination transient O₂ -uptake rate and compared with the electron flux in photosynthetic linear electron flow in order to evaluate the electron-sink capacity of photorespiration. The electron-sink capacity at the CO₂ -compensation point also increased in the above order. In gymnosperms photorespiration was determined to be the main electron-sink. C3-C4 intermediate species of Flaveria plants showed photorespiration activity, which intermediate between that of C3- and C4-flaveria species. These results indicate that in the first land plants, liverworts, photorespiration started to function as electron sink. According to our hypothesis, the dramatic increase in partial pressure of O₂ in the atmosphere about 0.4 billion years ago made it possible to drive photorespiration with higher activity in liverworts.

The Synergistic Responses of Different Photoprotective Pathways in Dwarf Bamboo (Fargesia rufa) to Drought and Subsequent Rewatering

Dwarf bamboo-dominated forests are often subjected to temporary periods of drought due to rising air temperature and decreasing rainfall. Nevertheless, the relationship among CO2 assimilation, photoprotective pathways and metabolism of reactive oxygen species (ROS) remains unexplored in bamboo species. Changes in leaf gas exchange, chlorophyll fluorescence, energy partitioning, antioxidative system and compounds related to ROS metabolism in Fargesia rufa plants subjected to drought and subsequent rewatering were analyzed. Drought resulted in a reversible inhibition of photochemistry, particularly net CO2 assimilation, and lipid peroxidation due to ROS accumulation. Meanwhile, photoprotective pathways, including the water-water cycle (especially for moderate drought), and adjustment in antenna pigments, thermal dissipation and antioxidative defense capacity at organelle levels (especially for severe drought), were up-regulated at the stress phase. Conversely, photorespiration was down-regulated after drought stress. As a result, rewatering restored most of the photochemical activity under drought, especially moderate drought. Moreover, thermal dissipation under severe drought was still operated for avoiding high ROS levels after rewatering. Therefore, the synergistic function of these photoprotective pathways except photorespiration can protect the photosynthetic apparatus from oxidative damage in response to varying intensities of drought stress when CO2 assimilation is restricted. This is helpful for the gradual recovery of photosynthetic capacity after rewatering. Thus, F. rufa plants can withstand drought and is capable of survival in such environment.

HIGHLIGHTS

1. The effects of drought and subsequent rewatering on Fargesia rufa were studied.2. Drought resulted in a reversible inhibition of photochemistry.3. Photoprotective pathways except photorespiration were up-regulated at the drought phase.4. Rewatering rapidly restored photochemical activity, especially under moderate drought.5. Fargesia rufa plant is capable of resisting and surviving drought environment.

The role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L

This study investigates the role of O2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2 mM DIC and no DIC with different O2 levels (air-equilibrated levels, 3 % of air equilibrium and restored air-equilibrated levels). Lowering O2 did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O2 acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2 mM DIC, air-equilibrated O2; (2) saturating CO2, no O2; and (3) no DIC, air-equilibrated O2. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O2 depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor.

A mechanistic model for the photosynthesis-light response based on the photosynthetic electron transport of photosystem II in C3 and C4 species

A new mechanistic model of the photosynthesis-light response is developed based on photosynthetic electron transport via photosystem II (PSII) to specifically describe light-harvesting characteristics and associated biophysical parameters of photosynthetic pigment molecules. This model parameterizes 'core' characteristics not only of the light response but also of difficult to measure physical parameters of photosynthetic pigment molecules in plants. Application of the model to two C3 and two C4 species grown under the same conditions demonstrated that the model reproduced extremely well (r(2) > 0.992) the light response trends of both electron transport and CO2 uptake. In all cases, the effective absorption cross-section of photosynthetic pigment molecules decreased with increasing light intensity, demonstrating novel operation of a key mechanism for plants to avoid high light damage. In parameterizing these previously difficult to measure characteristics of light harvesting in higher plants, the model provides a new means to understand the mechanistic processes underpinning variability of CO2 uptake, for example, photosynthetic down-regulation or reversible photoinhibition induced by high light and photoprotection. However, an important next step is validating this parameterization, possibly through application to less structurally complex organisms such as single-celled algae.

The response of Hordeum spontaneum desert ecotype to drought and excessive light intensity is characterized by induction of O2 dependent photochemical activity and anthocyanin accumulation

The goal of the current research was to study the role of anthocyanin accumulation, O(2)-related photochemical processes and non-photochemical quenching (NPQ) in the response of desert and Mediterranean plants to drought and excessive light. Plants of Hordeum spontaneum were collected from Mediterranean and desert environments and were subjected to terminal drought for 25 days and then measured for PSII yield at 2 and 21% O(2), NPQ, net carbon assimilation, stomatal conductance, leaf relative water content (LRWC), anthocyanin concentration and leaf absorbance. Under terminal drought, LRWC, carbon assimilation and stomatal conductance decreased similarly and significantly in both the Mediterranean and the desert ecotypes. Anthocyanin accumulated more in the desert ecotype than in the Mediterranean ecotype. NPQ increased more in the Mediterranean ecotype as compared with the desert ecotype. PSII yield decreased significantly in the Mediterranean ecotype under drought and was much lower than in the desert ecotype under drought. The relatively high PSII yield under drought in the desert ecotype was O(2) dependent. The response of the H. spontaneum ecotype from a desert environment to drought stress was characterized by anthocyanin accumulation and induction of O(2) dependent photochemical activity, while the response of the Mediterranean ecotype was based on a higher induction of NPQ.

Implications of alternative electron sinks in increased resistance of PSII and PSI photochemistry to high light stress in cold-acclimated Arabidopsis thaliana

Exposure of control (non-hardened) Arabidopsis leaves to high light stress at 5 °C resulted in a decrease of both photosystem II (PSII) (45 %) and Photosystem I (PSI) (35 %) photochemical efficiencies compared to non-treated plants. In contrast, cold-acclimated (CA) leaves exhibited only 35 and 22 % decrease of PSII and PSI photochemistry, respectively, under the same conditions. This was accompanied by an accelerated rate of P700(+) re-reduction, indicating an up-regulation of PSI-dependent cyclic electron transport (CET). Interestingly, the expression of the NDH-H gene and the relative abundance of the Ndh-H polypeptide, representing the NDH-complex, decreased as a result of exposure to low temperatures. This indicates that the NDH-dependent CET pathway cannot be involved and the overall stimulation of CET in CA plants is due to up-regulation of the ferredoxin-plastoquinone reductase, antimycin A-sensitive CET pathway. The lower abundance of NDH complex also implies lower activity of the chlororespiratory pathway in CA plants, although the expression level and overall abundance of the other well-characterized component involved in chlororespiration, the plastid terminal oxidase (PTOX), was up-regulated at low temperatures. This suggests increased PTOX-mediated alternative electron flow to oxygen in plants exposed to low temperatures. Indeed, the estimated proportion of O(2)-dependent linear electron transport not utilized in carbon assimilation and not directed to photorespiration was twofold higher in CA Arabidopsis. The possible involvement of alternative electron transport pathways in inducing greater resistance of both PSII and PSI to high light stress in CA plants is discussed.

Characteristics of photosynthesis and functions of the water-water cycle in rice (Oryza sativa) leaves in response to potassium deficiency

The mechanisms of photoprotection of photosynthesis and dissipation of excitation energy in rice leaves in response to potassium (K) deficiency were investigated. Net photosynthetic rate and the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase decreased under K deficiency. Compared with the control, non-photochemical quenching of Chl fluorescence increased in K-deficient plant, whereas the efficiency of excitation transfer (F'(v)/F'(m)) and the photochemical quenching coefficient (q(P)) decreased. Thus, thermal dissipation of excitation energy increased as more excess electrons were accumulated in the photosynthetic chain. The electron transport rate through PSII (J(f)) was more sensitive to O2 concentration, and the fraction of electron transport rate required to sustain CO2 assimilation and photorespiration (J(g)/J(f)) was significantly decreased under K deficiency compared with the control. Furthermore, the alternative electron transport (J(a)/J(f)) was increased, indicating that a considerable amount of electrons had been transported to O2 during the water-water cycle in the K-deficient leaves. Although the fraction of electron transport to photorespiration (J(o)/J(f)) was also increased in the K-deficient leaves, it was less sensitive than that of the water-water cycle. With the generation of reactive oxygen species level, the activities of superoxide dismutase and ascorbate peroxidase, two of the key enzymes involved in scavenging of active oxygen species in the water-water cycle, also increased in K-deficient rice. Therefore, it is likely that a series of photoprotective mechanisms were initiated in rice plants in response to K deficiency and the water-water cycle might be critical for protecting photosynthetic apparatus under K deficiency in rice.

The role of antioxidant enzymes in photoprotection

The enzymatic component of the antioxidant system is discussed as one of the defensive mechanisms providing protection against excessive light absorption in plants. We present an analysis of attempts to improve stress tolerance by means of the creation of transgenic plants with elevated antioxidant enzyme activities and conclude that the effect of such transgenic manipulation strongly depends on the manner in which the stress is imposed. The following factors may diminish the differences in photosynthetic performance between transgenic plants and wild type under field conditions: effective functioning of the thermal dissipation mechanisms providing a primary line of defense against excessive light, long-term adjustments of the antioxidant system and other photoprotective mechanisms, the relatively low level of control over electron transport exerted by the Water-Water cycle, especially under warm conditions, and a decrease in the content of the transgenic product during leaf aging.

Cyclic electron flow within PSII protects PSII from its photoinhibition in thylakoid membranes from spinach chloroplasts

Cyclic electron flow within PSII (CEF-PSII) was proven to alleviate the photoinhibition of PSII. We set the conditions where CEF-PSII functioned or did not, by adding nigericin to the reaction mixture for the dissipation of DeltapH across thylakoid membranes, and then the thylakoids were illuminated. When CEF-PSII did not function and the activity of linear electron flow (LEF) was low, light-treated thylakoid membranes largely lost the activity of LEF. The inactivation of LEF was due to the loss of the activity of PSII, but not that of PSI. The inactivation of PSII was suppressed, when CEF-PSII functioned or LEF was enhanced. These results imply that CEF-PSII contributes to the protection of PSII from its photoinhibition with LEF, as an electron sink.

Nitrogen limitation of growth and nutrient dynamics in a disturbed mangrove forest, Indian River Lagoon, Florida

The objectives of this study were to determine effects of nutrient enrichment on plant growth, nutrient dynamics, and photosynthesis in a disturbed mangrove forest in an abandoned mosquito impoundment in Florida. Impounding altered the hydrology and soil chemistry of the site. In 1997, we established a factorial experiment along a tree-height gradient with three zones, i.e., fringe, transition, dwarf, and three fertilizer treatment levels, i.e., nitrogen (N), phosphorus (P), control, in Mosquito Impoundment 23 on the eastern side of Indian River. Transects traversed the forest perpendicular to the shoreline, from a Rhizophora mangle-dominated fringe through an Avicennia germinans stand of intermediate height, and into a scrub or dwarf stand of A. germinans in the hinterland. Growth rates increased significantly in response to N fertilization. Our growth data indicated that this site is N-limited along the tree-height gradient. After 2 years of N addition, dwarf trees resembled vigorously growing saplings. Addition of N also affected internal dynamics of N and P and caused increases in rates of photosynthesis. These findings contrast with results for a R. mangle-dominated forest in Belize where the fringe is N-limited, but the dwarf zone is P-limited and the transition zone is co-limited by N and P. This study demonstrated that patterns of nutrient limitation in mangrove ecosystems are complex, that not all processes respond similarly to the same nutrient, and that similar habitats are not limited by the same nutrient when different mangrove forests are compared.

Cyclic electron flow within PSII functions in intact chloroplasts from spinach leaves

Using thylakoid membranes, we previously demonstrated that accumulated electrons in the photosynthetic electron transport system induces the electron flow from the acceptor side of PSII to its donor side only in the presence of a pH gradient ((Delta)pH) across the thylakoid membranes. This electron flow has been referred to as cyclic electron flow within PSII (CEF-PSII) [Miyake and Yokota (2001) Plant Cell Physiol. 42: 508]. In the present study, we examined whether CEF-PSII operates in isolated intact chloroplasts from spinach leaves, by correlating the quantum yield of PSII [Phi(PSII)] with the activity of the linear electron flow [V(O(2))]. The addition of the protonophore nigericin to the intact chloroplasts decreased Phi(PSII), but increased V(O(2)), and relative electron flux in PSII [Phi(PSII) x PFD] and V(O(2)) were proportional to one another. Phi(PSII) x PFD at a given V(O(2)) was much higher in the presence of (Delta)pH than that in its absence. These effects of nigericin on the relationship between Phi(PSII) x PFD and V(O(2)) are consistent with those previously observed in thylakoid membranes, indicating the occurrence of CEF-PSII also in intact chloroplasts. In the presence of (Delta)pH, CEF-PSII accounted for the excess electron flux in PSII that could not be attributed to photosynthetic linear electron flow. The activity of CEF-PSII increased with increased light intensity and almost corresponded to that of the water-water cycle (WWC), implying that CEF-PSII can dissipate excess photon energy in cooperation with WWC to protect PSII from photoinhibition under limited photosynthesis conditions.

Cyclic flow of electrons within PSII in thylakoid membranes

In photosynthesis, the electrons released from PSII are considered to be shared mainly by carbon metabolism and the water-water cycle. We demonstrated previously that some electrons are utilized in a CO2- and O2-independent manner in leaves of wild watermelon [Miyake and Yokota (2000) Plant Cell Physiol: 41: 335]. In the present study, we examined the mechanism of this alternative flow of electrons in thylakoid membranes, isolated from fresh spinach leaves, by simultaneously measuring the quantum yield of PSII and the flux of the linear flow of electrons. In the presence of the protonophore nigericin, which eliminates the pH gradient across thylakoid membranes, the quantum yield and the flux of the linear flow of electrons were directly proportional to one another. The quantum yield at a given linear flux of electrons was much higher in the absence of nigericin than in its presence, indicating that an additional or alternative flow of electrons can occur independently of the linear flow in the absence of nigericin. In the presence of nigericin, the alternative flux of electrons increased with decreasing pH and with increasing reduction of the plastoquinone pool. Cyclic flow of electrons in PSII appears to be the most plausible candidate for the alternative flow of electrons. The flux reached 280 micromol x e(-) (mg Chl)(-1) x h(-1) and was similar to that of the CO2- and O2-independent alternative flow of electrons that we found in leaves of wild watermelon. The cyclic, alternative flow of electrons in PSII provides a possible explanation for the alternative flow of electrons observed in vivo.

Electron flow to oxygen in higher plants and algae: rates and control of direct photoreduction (Mehler reaction) and rubisco oxygenase

Linear electron transport in chloroplasts produces a number of reduced components associated with photosystem I (PS I) that may subsequently participate in reactions that reduce O2. The two primary reactions that have been extensively studied are: first, the direct reduction of O2 to superoxide by reduced donors associated with PS I (the Mehler reaction), and second, the rubisco oxygenase (ribulose 1,5-bisphosphate carboxylase oxygenase EC 4.1.1.39) reaction and associated peroxisomal and mitochondrial reactions of the photorespiratory pathway. This paper reviews a number of recent and past studies with higher plants, algae and cyanobacteria that have attempted to quantify O2 fluxes under various conditions and their contributions to a number of roles, including photon energy dissipation. In C3 and Crassulacean acid metabolism (CAM) plants, a Mehler O2 uptake reaction is unlikely to support a significant flow of electron transport (probably less than 10%). In addition, if it were present it would appear to scale with photosynthetic carbon oxidation cycle (PCO) and photosynthetic carbon reduction cycle (PCR) activity This is supported by studies with antisense tobacco plants with reduced rubisco at low and high temperatures and high light, as well as studies with potatoes, grapes and madrone during water stress. The lack of significant Mehler in these plants directly argues for a strong control of Mehler reaction in the absence of ATP consumption by the PCR and PCO cycles. The difference between C3 and C4 plants is primarily that the level of light-dependent O2 uptake is generally much lower in C4 plants and is relatively insensitive to the external CO2 concentration. Such a major difference is readily attributed to the operation of the C4 CO2 concentrating mechanism. Algae show a range of light-dependent O2 uptake rates, similar to C4 plants. As in C4 plants, the O2 uptake appears to be largely insensitive to CO2, even in species that lack a CO2 concentrating mechanism and under conditions that are clearly limiting with respect to inorganic carbon supply. A part explanation for this could be that many algal rubsicos have considerably different oxygenase kinetic properties and exhibit far less oxygenase activity in air. This would lead to the conclusion that perhaps a greater proportion of the observed O2 uptake may be due to a Mehler reaction and less to rubisco, compared with C3 plants. In contrast to algae and higher plants, cyanobacteria appear to have a high capacity for Mehler O2 uptake, which appears to be not well coupled or limited by ATP consumption. It is likely that in all higher plants and algae, which have a well-developed non-photochemical quenching mechanism, non-radiative energy dissipation is the major mechanism for dissipating excess photons absorbed by the light-harvesting complexes under stressful conditions. However, for cyanobacteria, with a lack of significant non-photochemical quenching, the situation may well be different.

The water-water cycle as alternative photon and electron sinks

The water-water cycle in chloroplasts is the photoreduction of dioxygen to water in photosystem I (PS I) by the electrons generated in photosystem II (PS II) from water. In the water-water cycle, the rate of photoreduction of dioxygen in PS I is several orders of magnitude lower than those of the disproportionation of superoxide catalysed by superoxide dismutase, the reduction of hydrogen peroxide to water catalysed by ascorbate peroxidase, and the reduction of the resulting oxidized forms of ascorbate by reduced ferredoxin or catalysed by either dehydroascorbate reductase or monodehydroascorbate reductase. The water-water cycle therefore effectively shortens the lifetimes of photoproduced superoxide and hydrogen peroxide to suppress the production of hydroxyl radicals, their interactions with the target molecules in chloroplasts, and resulting photoinhibition. When leaves are exposed to photon intensities of sunlight in excess of that required to support the fixation of CO2, the intersystem electron carriers are over-reduced, resulting in photoinhibition. Under such conditions, the water-water cycle not only scavenges active oxygens, but also safely dissipates excess photon energy and electrons, in addition to downregulation of PS II and photorespiration. The dual functions of the water-water cycle for protection from photoinhibition under photon excess stress are discussed, along with its functional evolution.

Chlorophyll fluorescence--a practical guide

Chlorophyll fluorescence analysis has become one of the most powerful and widely used techniques available to plant physiologists and ecophysiologists. This review aims to provide an introduction for the novice into the methodology and applications of chlorophyll fluorescence. After a brief introduction into the theoretical background of the technique, the methodology and some of the technical pitfalls that can be encountered are explained. A selection of examples is then used to illustrate the types of information that fluorescence can provide.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

PHOTOPROTECTION REVISITED:Genetic and Molecular Approaches

The involvement of excited and highly reactive intermediates in oxygenic photosynthesis poses unique problems for algae and plants in terms of potential oxidative damage to the photosynthetic apparatus. Photoprotective processes prevent or minimize generation of oxidizing molecules, scavenge reactive oxygen species efficiently, and repair damage that inevitably occurs. This review summarizes several photoprotective mechanisms operating within chloroplasts of plants and green algae. The recent use of genetic and molecular biological approaches is providing new insights into photoprotection, especially with respect to thermal dissipation of excess absorbed light energy, alternative electron transport pathways, chloroplast antioxidant systems, and repair of photosystem II.