Publications by authors named "Wah Soon Chow"

90 Publications

Inhibition of non-photochemical quenching increases functional absorption cross-section of photosystem II as excitation from closed reaction centres is transferred to open centres, facilitating earlier light saturation of photosynthetic electron transport.

Funct Plant Biol 2021 Mar 12. Epub 2021 Mar 12.

Induction of non-photochemical quenching (NPQ) of chlorophyll fluorescence in leaves affords photoprotection to the photosynthetic apparatus when, for whatever reason, photon capture in the antennae of photosystems exceeds their capacity to utilise this excitation in photochemistry and ultimately in CO2 assimilation. Here we augment traditional monitoring of NPQ using the fast time resolution, remote and relatively non-intrusive light induced fluorescence transient (LIFT) technique (Kolber et al. 2005; Osmond et al. 2017) that allows direct measurement of functional (σ'PSII) and optical cross-sections (a'PSII) of PSII in situ, and calculates the half saturation light intensity for ETR (Ek). These parameters are obtained from the saturation and relaxation phases of fluorescence transients elicited by a sequence of 270, high intensity 1 μs flashlets at controlled time intervals over a period of 30 ms in the QA flash at intervals of a few seconds. We report that although σ'PSII undergoes large transient increases after transfer from dark to strong white light (WL) it declines little in steady-state as NPQ is induced in shade- and sun-grown spinach and Arabidopsis genotypes Col, OEpsbs, pgr5bkg, stn7 and stn7/8. In contrast, σ'PSII increases by ~30% when induction of NPQ in spinach is inhibited by dithiothreitol and by inhibition of NPQ in Arabidopsis npq1, npq4 and pgr5. We propose this increase in σ'PSII arises as some excitation from closed PSII reaction centres is transferred to open centres when excitation partitioning to photochemistry (YII) and NPQ (YNP) declines, and is indicated by an increased excitation dissipation from closed PSII centres (YNO, including fluorescence emission). Although Ek increases following dissipation of excitation as heat when NPQ is engaged, it declines when NPQ is inhibited. Evidently photochemistry becomes more easily light saturated when excitation is transferred from closed RCIIs to open centres with larger σ'PSII. The NPQ mutant pgr5 is an exception; Ek increases markedly in strong light as electron transport QA → PQ and PQ → PSI accelerate and the PQ pool becomes strongly reduced. These novel in situ observations are discussed in the context of contemporary evidence for functional and structural changes in the photosynthetic apparatus during induction of NPQ.
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http://dx.doi.org/10.1071/FP20347DOI Listing
March 2021

My precarious career in photosynthesis: a roller-coaster journey into the fascinating world of chloroplast ultrastructure, composition, function and dysfunction.

Authors:
Wah Soon Chow

Photosynth Res 2021 Aug 4;149(1-2):5-24. Epub 2021 Feb 4.

Division of Plant Sciences, Research School of Biology, The Australian National University, 46 Sullivans Creek Road, Acton, ACT, 2601, Australia.

Despite my humble beginnings in rural China, I had the good fortune of advancing my career and joining an international community of photosynthesis researchers to work on the 'light reactions' that are a fundamental process in Nature. Along with supervisors, mentors, colleagues, students and lab assistants, I worked on ionic redistributions across the photosynthetic membrane in response to illumination, photophosphorylation, forces that regulate the stacking of photosynthetic membranes, the composition of components of the photosynthetic apparatus during acclimation to the light environment, and the failure of the photosynthetic machinery to acclimate to too much light or even to cope with moderate light due to inevitable photodamage. These fascinating underlying mechanisms were investigated in vitro and in vivo. My career path, with its ups and downs, was never secure, but the reward of knowing a little more of the secret of Nature offset the job uncertainty.
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http://dx.doi.org/10.1007/s11120-021-00818-2DOI Listing
August 2021

Mehler reaction plays a role in C and C photosynthesis under shade and low CO.

Photosynth Res 2021 Aug 3;149(1-2):171-185. Epub 2021 Feb 3.

ARC Centre of Excellence for Translational Photosynthesis, Hawkesbury Institute for the Environment, Western Sydney University, Hawkesbury Campus, Locked Bag 1797, Penrith, NSW, 2751, Australia.

Alternative electron fluxes such as the cyclic electron flux (CEF) around photosystem I (PSI) and Mehler reaction (Me) are essential for efficient photosynthesis because they generate additional ATP and protect both photosystems against photoinhibition. The capacity for Me can be estimated by measuring O exchange rate under varying irradiance and CO concentration. In this study, mass spectrometric measurements of O exchange were made using leaves of representative species of C and C grasses grown under natural light (control; PAR ~ 800 µmol quanta m s) and shade (~ 300 µmol quanta m s), and in representative species of gymnosperm, liverwort and fern grown under natural light. For all control grown plants measured at high CO, O uptake rates were similar between the light and dark, and the ratio of Rubisco oxygenation to carboxylation (V/V) was low, which suggests little potential for Me, and that O uptake was mainly due to photorespiration or mitochondrial respiration under these conditions. Low CO stimulated O uptake in the light, V/V and Me in all species. The C species had similar V/V, but Me was highest in the grass and lowest in the fern. Among the C grasses, shade increased O uptake in the light, V/V and the assimilation quotient (AQ), particularly at low CO, whilst Me was only substantial at low CO where it may contribute 20-50% of maximum electron flow under high light.
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http://dx.doi.org/10.1007/s11120-021-00819-1DOI Listing
August 2021

The effect of different spectral light quality on the photoinhibition of Photosystem I in intact leaves.

Photosynth Res 2021 Aug 6;149(1-2):83-92. Epub 2021 Jan 6.

Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT, 2601, Australia.

Light energy causes damage to Photosystem I (PSI) and Photosystem II (PSII). The majority of the previous photoinhibition studies have been conducted with PSII, which shows much larger photoinhibition than PSI; therefore, relatively little is known about the mechanism of PSI photoinhibition so far. A previous report showed that the photoinhibition action spectrum measured with PSI activity of isolated thylakoid is similar to the absorption spectrum of chlorophyll. However, it is known that the extent of PSI photoinhibition is much smaller in vivo compared to in vitro. It is also possible that the different extent of PSII photoinhibition, caused by different spectral light qualities, can affect the photoinhibition of PSI in vivo because PSI receives electrons from PSII. In the present research, to study the effect of light quality and the effect of the extent of PSII photoinhibition on the PSI photoinhibition in vivo, intact leaves were photoinhibited under four different light qualities. The rate coefficient of PSI photoinhibition was significantly higher in blue and red light compared to white light. The rate of PSI photoinhibition at the same photon-exposure was the largest in blue and red light and followed by white and green light. These results support the notion that light absorption by chlorophyll is responsible for the PSI photoinhibition, even in intact leaves. The variation among light colors in the relationships between the extent of photoinhibition of PSII and that of PSI indicate that PSI and PSII are independently photoinhibited with different mechanisms in the early stage of in vivo photoinhibition.
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http://dx.doi.org/10.1007/s11120-020-00805-zDOI Listing
August 2021

A tribute to Robert John Porra (august 7, 1931-may 16, 2019).

Photosynth Res 2021 Feb 2;147(2):125-130. Epub 2021 Jan 2.

Örebro Life Science Center, School of Science and Technology, Örebro University, 70182, Örebro, Sweden.

Robert John Porra (7.8.1931-16.5.2019) is probably best known for his substantial practical contributions to plant physiology and photosynthesis by addressing the problems of both the accurate spectroscopic estimation and the extractability of chlorophylls in many organisms. Physiological data and global productivity estimates, in particular of marine primary productivity, are often quoted on a chlorophyll basis. He also made his impact by work on all stages of tetrapyrrole biosynthesis: he proved the C pathway to chlorophylls, detected an alternative route to protoporphyrin in anaerobes and the different origin of the oxygen atoms in anaerobes and aerobes. A brief review of his work is supplemented by personal memories of the authors.
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http://dx.doi.org/10.1007/s11120-020-00800-4DOI Listing
February 2021

Carbohydrate saving or biomass maintenance: which is the main determinant of the plant's long-term submergence tolerance?

Photosynth Res 2021 Aug 1;149(1-2):155-170. Epub 2020 Nov 1.

College of Forestry, Beijing Forestry University, Beijing, 100083, China.

It is hypothesized that plant submergence tolerance could be assessed from the decline of plant biomass due to submergence, as biomass integrates all eco-physiological processes leading to fitness. An alternative hypothesis stated that the consumption rate of carbohydrate is essential in differing tolerance to submergence. In the present study, the responses of biomass, biomass allocation, and carbohydrate content to simulated long-term winter submergence were assessed in four tolerant and four sensitive perennials. The four tolerant perennials occur in a newly established riparian ecosystem created by The Three Gorges Dam, China. They had 100% survival after 120 days' simulated submergence, and had full photosynthesis recovery after 30 days' re-aeration, and the photosynthetic rate was positively related to the growth during the recovery period. Tolerant perennials were characterized by higher carbohydrate levels, compared with the four sensitive perennials (0% survival) at the end of submergence. Additionally, by using a method which simulates posterior estimates, and bootstraps the confidence interval for the difference between strata means, it was found that the biomass response to post-hypoxia, rather than that to submergence, could be a reliable indicator to assess submergence tolerance. Interestingly, the differences of changes in carbohydrate content between tolerant and sensitive perennials during submergence were significant, which were distinct from the biomass response, supporting the hypothesis that tolerant perennials could sacrifice non-vital components of biomass to prioritize the saving of carbohydrates for later recovery. Our study provides some insight into the underlying mechanism(s) of perennials' tolerance to submergence in ecosystems such as temperate wetland and reservoir riparian.
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http://dx.doi.org/10.1007/s11120-020-00791-2DOI Listing
August 2021

The relationship between anthocyanin accumulation and photoprotection in young leaves of two dominant tree species in subtropical forests in different seasons.

Photosynth Res 2021 Aug 9;149(1-2):41-55. Epub 2020 Sep 9.

Guangzhou Key Laboratory of Subtropical Biodiversity and Biomonitoring, Guangdong Provincial Key Laboratory of Biotechnology for Plant Development, School of Life Sciences, South China Normal University, Guangzhou, 510631, PR China.

Increasing amounts of experimental evidence show that anthocyanins provide physiological protection to plants under stress. However, the difference in photoprotection mediated by anthocyanins and other photoprotective substances in different seasons is still uncertain. To determine the relationship between anthocyanin accumulation and the photoprotective effects in different seasons, Castanopsis chinensis and Acmena acuminatissima, whose anthocyanin accumulation patterns differ in different seasons, were used as materials to explain how plants adapt to different seasons; as such, their physiological and biochemical responses were analyzed. Young leaves of C. chinensis and A. acuminatissima presented different colors in the different seasons. In summer, the young leaves of C. chinensis were purplish red, while those of A. acuminatissima were light green. In winter, the young leaves of C. chinensis were light green, while those of A. acuminatissima were red. Compared with the young red leaves, the young light green leaves that did not accumulate anthocyanins had higher flavonoid and phenolics contents, total antioxidant capacity, non-photochemical quenching (NPQ), and relative membrane leakage, and a slower recovery rate in the maximum photochemical efficiency (F/F) after high-light treatment. In addition, the net photosynthesis rate (P), transpiration rate (T), stomatal conductance (g), and the effective quantum yield of PSII (Φ) of the young leaves in winter were significantly lower than those in summer, while the activities of catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), and superoxide dismutase (SOD, EC 1.15.1.1) were significantly higher than those in summer. These data indicate that to adapt to seasonal changes anthocyanins, other antioxidative substances and antioxidative enzymes, as well as components involved in the safe dissipation of excitation energy as heat need to cooperate with one another.
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http://dx.doi.org/10.1007/s11120-020-00781-4DOI Listing
August 2021

Do rapid photosynthetic responses protect maize leaves against photoinhibition under fluctuating light?

Photosynth Res 2021 Aug 11;149(1-2):57-68. Epub 2020 Aug 11.

Key Laboratory of Plant Resources, Institute of Botany, Chinese Academy of Sciences, Beijing, 100093, People's Republic of China.

Plants in their natural environment are often exposed to fluctuating light because of self-shading and cloud movements. As changing frequency is a key characteristic of fluctuating light, we speculated that rapid light fluctuation may induce rapid photosynthetic responses, which may protect leaves against photoinhibition. To test this hypothesis, maize seedlings were grown under fluctuating light with various frequencies (1, 10, and 100 cycles of fluctuations/10 h), and changes in growth, chlorophyll content, gas exchange, chlorophyll a fluorescence, and P700 were analyzed carefully. Our data show that though the growth and light-saturated photosynthetic rate were depressed by rapidly fluctuating light, photosynthesis induction was clearly speeded up. Furthermore, more rapid fluctuation of light strikingly reduced the chlorophyll content, while thermal dissipation was triggered and enhanced. The chlorophyll a fluorescence induction kinetics and P700 absorption results showed that the activities of both photosystem II and photosystem I decreased as the frequency of the fluctuating light increased. In all treatments, the light intensities of the fluctuating light were kept constant. Therefore, rapid light fluctuation frequency itself induced the acceleration of photosynthetic induction and the enhancement of photoprotection in maize seedlings, which play important roles in protecting photosynthetic apparatus against fluctuating high light to a certain extent.
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http://dx.doi.org/10.1007/s11120-020-00780-5DOI Listing
August 2021

The major photoprotective role of anthocyanins in leaves of Arabidopsis thaliana under long-term high light treatment: antioxidant or light attenuator?

Photosynth Res 2021 Aug 27;149(1-2):25-40. Epub 2020 May 27.

Guangdong Provincial Key Laboratory of Biotechnology for Plant Development, Guangzhou Key Laboratory of Subtropical Biodiversity and Biomonitoring, School of Life Sciences, South China Normal University, Guangzhou, 510631, People's Republic of China.

Anthocyanins are water-soluble pigments in plants known for their photoprotective role against photoinhibitory and photooxidative damage under high light (HL). However, it remains unclear whether light-shielding or antioxidant activity plays a major role in the photoprotection exerted by anthocyanins under HL stress. To shed light on this question, we analyzed the physiological and biochemical responses to HL of three Arabidopsis thaliana lines (Col, chi, ans) with different light absorption and antioxidant characteristics. Under HL, ans had the highest antioxidant capacity, followed by Col, and finally chi; Col had the strongest light attenuation capacity, followed by chi, and finally ans. The line ans had weaker physiological activity of chloroplasts and more severe oxidative damage than chi after HL treatment. Col with highest photoprotection of light absorption capacity had highest resistance to HL among the three lines. The line ans with high antioxidant capacity could not compensate for its disadvantages in HL caused by the absence of the light-shielding function of anthocyanins. In addition, the expression level of the Anthocyanin Synthase (ANS) gene was most upregulated after HL treatment, suggesting that the conversion of colorless into colored anthocyanin precursors was necessary under HL. The contribution of anthocyanins to flavonoids, phenols, and antioxidant capacity increased in the late period of HL, suggesting that plants prefer to synthesize red anthocyanins (a group of colored antioxidants) over other colorless antioxidants to cope with HL. These experimental observations indicate that the light attenuation role of anthocyanins is more important than their antioxidant role in photoprotection.
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http://dx.doi.org/10.1007/s11120-020-00761-8DOI Listing
August 2021

Photosynthetic compensation of non-leaf organ stems of the invasive species Sphagneticola trilobata (L.) Pruski at low temperature.

Photosynth Res 2021 Aug 15;149(1-2):121-134. Epub 2020 Apr 15.

Guangdong Provincial Key Laboratory of Biotechnology for Plant Development, Guangzhou Key Laboratory of Subtropical Biodiversity and Biomonitoring, College of Life Sciences, South China Normal University, Guangzhou, 510631, China.

Biological invasion is a hot topic in ecological research. Most studies on the physiological mechanisms of plants focus on leaves, but few studies focus on stems. To study the tolerance of invasive plant (Sphagneticola trilobata L.) to low temperature, relevant physiological indicators (including anthocyanin and chlorophyll) in different organs (leaves and stems) were analyzed, using a native species (Sphagneticola calendulacea L.) as the control. The results showed that, upon exposure to low temperature for 15 days, the stems of two Sphagneticola species were markedly reddened, their anthocyanin content increased, chlorophyll and chlorophyll fluorescence parameters decreased, and the accumulation of reactive oxygen species in the stem increased. The percentage increases of antioxidants and total antioxidant capacities in stems were significantly higher in S. trilobata than in S. calendulacea. This showed that S. trilobata had higher cold tolerance in stems while leaves were opposite. To further verify the higher cold tolerance of the stem of S. trilobata, a defoliation experiment was designed. We found that the defoliated stem of S. trilobata reduced anthocyanin accumulation and increased chlorophyll content, while alleviating membrane lipid damage and electrical conductivity, and the defoliated stem still showed an increase in stem diameter and biomass under low temperature. The discovery of the physiological and adaptive mechanisms of the stem of S. trilobata to low temperature will provide a theoretical basis for explaining how S. trilobata maintains its annual growth in South China. This is of great significance for predicting the future spread of cloned and propagated invasive plants.
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http://dx.doi.org/10.1007/s11120-020-00748-5DOI Listing
August 2021

Probing functional and optical cross-sections of PSII in leaves during state transitions using fast repetition rate light induced fluorescence transients.

Funct Plant Biol 2019 06;46(6):567-583

Centre for Sustainable Ecosystem Solutions, School of Biological Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia.

Plants adjust the relative sizes of PSII and PSI antennae in response to the spectral composition of weak light favouring either photosystem by processes known as state transitions (ST), attributed to a discrete antenna migration involving phosphorylation of light-harvesting chlorophyll-protein complexes in PSII. Here for the first time we monitored the extent and dynamics of ST in leaves from estimates of optical absorption cross-section (relative PSII antenna size; aPSII). These estimates were obtained from in situ measurements of functional absorption cross-section (σPSII) and maximum photochemical efficiency of PSII (φPSII); i.e. aPSII = σPSII/φPSII (Kolber et al. 1998) and other parameters from a light induced fluorescence transient (LIFT) device (Osmond et al. 2017). The fast repetition rate (FRR) QA flash protocol of this instrument monitors chlorophyll fluorescence yields with reduced QA irrespective of the redox state of plastoquinone (PQ), as well as during strong ~1 s white light pulses that fully reduce the PQ pool. Fitting this transient with the FRR model monitors kinetics of PSII → PQ, PQ → PSI, and the redox state of the PQ pool in the 'PQ pool control loop' that underpins ST, with a time resolution of a few seconds. All LIFT/FRR criteria confirmed the absence of ST in antenna mutant chlorina-f2 of barley and asLhcb2-12 of Arabidopsis, as well as STN7 kinase mutants stn7 and stn7/8. In contrast, wild-type barley and Arabidopsis genotypes Col, npq1, npq4, OEpsbs, pgr5 bkg and pgr5, showed normal ST. However, the extent of ST (and by implication the size of the phosphorylated LHCII pool participating in ST) deduced from changes in a'PSII and other parameters with reduced QA range up to 35%. Estimates from strong WL pulses in the same assay were only ~10%. The larger estimates of ST from the QA flash are discussed in the context of contemporary dynamic structural models of ST involving formation and participation of PSII and PSI megacomplexes in an 'energetically connected lake' of phosphorylated LHCII trimers (Grieco et al. 2015). Despite the absence of ST, asLhcb2-12 displays normal wild-type modulation of electron transport rate (ETR) and the PQ pool during ST assays, reflecting compensatory changes in antenna LHCIIs in this genotype. Impaired LHCII phosphorylation in stn7 and stn7/8 accelerates ETR from PSII →PQ, over-reducing the PQ pool and abolishing the yield difference between the QA flash and WL pulse, with implications for photochemical and thermal phases of the O-J-I-P transient.
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http://dx.doi.org/10.1071/FP18054DOI Listing
June 2019

Two Quenchers Formed During Photodamage of Phostosystem II and The Role of One Quencher in Preemptive Photoprotection.

Sci Rep 2019 11 21;9(1):17275. Epub 2019 Nov 21.

Research School of Biology, College of Science, The Australian National University, Canberra, ACT, 2601, Australia.

The quenching of chlorophyll fluorescence caused by photodamage of Photosystem II (qI) is a well recognized phenomenon, where the nature and physiological role of which are still debatable. Paradoxically, photodamage to the reaction centre of Photosystem II is supposed to be alleviated by excitation quenching mechanisms which manifest as fluorescence quenchers. Here we investigated the time course of PSII photodamage in vivo and in vitro and that of picosecond time-resolved chlorophyll fluorescence (quencher formation). Two long-lived fluorescence quenching processes during photodamage were observed and were formed at different speeds. The slow-developing quenching process exhibited a time course similar to that of the accumulation of photodamaged PSII, while the fast-developing process took place faster than the light-induced PSII damage. We attribute the slow process to the accumulation of photodamaged PSII and the fast process to an independent quenching mechanism that precedes PSII photodamage and that alleviates the inactivation of the PSII reaction centre.
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http://dx.doi.org/10.1038/s41598-019-53030-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6872554PMC
November 2019

Cyclic electron flow and light partitioning between the two photosystems in leaves of plants with different functional types.

Photosynth Res 2019 Dec 13;142(3):321-334. Epub 2019 Sep 13.

ARC Centre of Excellence for Translational Photosynthesis, Hawkesbury Institute for the Environment, Western Sydney University, Hawkesbury Campus, Locked Bag 1797, Penrith, NSW, 2751, Australia.

Cyclic electron flow (CEF) around photosystem I (PSI) is essential for generating additional ATP and enhancing efficient photosynthesis. Accurate estimation of CEF requires knowledge of the fractions of absorbed light by PSI (f) and PSII (f), which are only known for a few model species such as spinach. No measures of f are available for C grasses under different irradiances. We developed a new method to estimate (1) f in vivo by concurrently measuring linear electron flux through both photosystems [Formula: see text] in leaf using membrane inlet mass spectrometry (MIMS) and total electron flux through PSII (ETR2) using chlorophyll fluorescence by a Dual-PAM at low light and (2) CEF as ETR1-[Formula: see text]. For a C grass, f was 0.5 and 0.4 under control (high light) and shade conditions, respectively. C species belonging to NADP-ME and NAD-ME subtypes had f of 0.6 and PCK subtype had 0.5 under control. All shade-grown C species had f of 0.6 except for NADP-ME grass which had 0.7. It was also observed that f ranged between 0.3 and 0.5 for gymnosperm, liverwort and fern species. CEF increased with irradiance and was induced at lower irradiances in C grasses and fern relative to other species. CEF was greater in shade-grown plants relative to control plants except for C NADP-ME species. Our study reveals a range of CEF and f values in different plant functional groups. This variation must be taken into account for improved photosynthetic calculations and modelling.
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http://dx.doi.org/10.1007/s11120-019-00666-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6874625PMC
December 2019

Partially Dissecting Electron Fluxes in Both Photosystems in Spinach Leaf Disks during Photosynthetic Induction.

Plant Cell Physiol 2019 Oct;60(10):2206-2219

ARC Centre of Excellence for Translational Photosynthesis, Research School of Biology, The Australian National University, Acton, ACT, Australia.

Photosynthetic induction, a gradual increase in photosynthetic rate on a transition from darkness or low light to high light, has ecological significance, impact on biomass accumulation in fluctuating light and relevance to photoprotection in strong light. However, the experimental quantification of the component electron fluxes in and around both photosystems during induction has been rare. Combining optimized chlorophyll fluorescence, the redox kinetics of P700 [primary electron donor in Photosystem I (PSI)] and membrane inlet mass spectrometry in the absence/presence of inhibitors/mediator, we partially estimated the components of electron fluxes in spinach leaf disks on transition from darkness to 1,000 �mol photons�m-2�s-1 for up to 10 min, obtaining the following findings: (i) the partitioning of energy between both photosystems did not change noticeably; (ii) in Photosystem II (PSII), the combined cyclic electron flow (CEF2) and charge recombination (CR2) to the ground state decreased gradually toward 0 in steady state; (iii) oxygen reduction by electrons from PSII, partly bypassing PSI, was small but measurable; (iv) cyclic electron flow around PSI (CEF1) peaked before becoming somewhat steady; (v) peak magnitudes of some of the electron fluxes, all probably photoprotective, were in the descending order: CEF1 > CEF2 + CR2 > chloroplast O2 uptake; and (vi) the chloroplast NADH dehydrogenase-like complex appeared to aid the antimycin A-sensitive CEF1. The results are important for fine-tuning in silico simulation of in vivo photosynthetic electron transport processes; such simulation is, in turn, necessary to probe partial processes in a complex network of interactions in response to environmental changes.
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http://dx.doi.org/10.1093/pcp/pcz114DOI Listing
October 2019

Optimising the linear electron transport rate measured by chlorophyll a fluorescence to empirically match the gross rate of oxygen evolution in white light: towards improved estimation of the cyclic electron flux around photosystem I in leaves.

Funct Plant Biol 2018 Oct;45(11):1138-1148

Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT 2601, Australia.

The cyclic electron flux (CEF) around photosystem I (PSI) was discovered in isolated chloroplasts more than six decades ago, but its quantification has been hampered by the absence of net formation of a product or net consumption of a substrate. We estimated in vivo CEF in leaves as the difference (ΔFlux) between the total electron flux through PSI (ETR1) measured by a near infrared signal, and the linear electron flux through both photosystems by optimised measurement of chlorophyll a fluorescence (LEFfl). Chlorophyll fluorescence was excited by modulated green light from a light-emitting diode at an optimal average irradiance, and the fluorescence was detected at wavelengths >710nm. In this way, LEFfl matched the gross rate of oxygen evolution multiplied by 4 (LEFO2) in broad-spectrum white actinic irradiance up to half (spinach, poplar and rice) or one third (cotton) of full sunlight irradiance. This technique of estimating CEF can be applied to leaves attached to a plant.
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http://dx.doi.org/10.1071/FP18039DOI Listing
October 2018

NDH-1 Is Important for Photosystem I Function of sp. Strain PCC 6803 under Environmental Stress Conditions.

Front Plant Sci 2017 17;8:2183. Epub 2018 Jan 17.

Department of Biology, College of Life and Environment Sciences, Shanghai Normal University, Shanghai, China.

Cyanobacterial NDH-1 interacts with photosystem I (PSI) to form an NDH-1-PSI supercomplex. Here, we observed that absence of NDH-1 had little, if any, effect on the functional fractions of PSI under growth conditions, but significantly reduced the functional fractions of PSI when cells of sp. strain PCC 6803 were moved to conditions of multiple stresses. The significant reduction in NDH-1-dependent functional fraction of PSI was initiated after PSII activity was impaired. This finding is consistent with our observation that the functional fraction of PSI under growth conditions was rapidly and significantly decreased with increasing concentrations of DCMU, which rapidly and significantly suppressed PSII activity by blocking the transfer of electrons from to in the PSII reaction center. Furthermore, absence of NDH-1 resulted in the PSI limitation at the functionality of PSI itself but not its donor-side and acceptor-side under conditions of multiple stresses. This was supported by the result of a significant destabilization of the PSI complex in the absence of NDH-1 but the presence of multiple stresses. Based on the above results, we propose that NDH-1 is important for PSI function of sp. strain PCC 6803 mainly via maintaining stabilization of PSI under conditions of environmental stresses.
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http://dx.doi.org/10.3389/fpls.2017.02183DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5776120PMC
January 2018

Changes in activities of both photosystems and the regulatory effect of cyclic electron flow in field-grown cotton (Gossypium hirsutum L) under water deficit.

J Plant Physiol 2018 Jan 10;220:74-82. Epub 2017 Nov 10.

The Key Laboratory of Oasis Eco-agriculture, Xinjiang Production and Construction Group, Shihezi University, Shihezi, 832003, PR China. Electronic address:

To clarify the influence of water deficit on the functionality of the photosynthetic apparatus of cotton plants, leaf gas exchange, chlorophyll a fluorescence, and P700 redox state were examined in field-grown cotton Gossypium hirsutum L. cv. Xinluzao 45. In addition, we measured changes in the P515 signal and analyzed the activity of ATP synthase and the trans-thylakoid proton gradient (ΔpH). With increasing water deficit, the net CO assimilation rate (A) and stomatal conductance (g) significantly decreased, but the maximum quantum efficiency of PSII photochemistry (F/F) did not change. The photochemical activity of photosystem II (PSII) was reflected by the photochemical quenching coefficient (qP), quantum efficiency of photosystem II [Y(II)], and electron transport rate through PSII [ETR(II)], while the activity of photosystem I (PSI) was reflected by the quantum efficiency of photosystem I [Y(I)] and the electron transport rate through PSI [ETR(I)]. Both activities were maintained under mild water deficit, but were slightly decreased under moderate water deficit. Under moderate water deficit, cyclic electron flow (CEF), the fraction of absorbed light dissipated thermally via the ΔpH- and xanthophyll-regulated process [Y(NPQ)], and the fraction of P700 oxidized under a given set of conditions [Y(ND)] increased. Our results suggest that the activities of both photosystems are stable under mild water deficit and decrease only slightly under moderate water deficit. Moderate water deficit stimulates CEF, and the stimulation of CEF is essential for protecting PSI and PSII against photoinhibition.
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http://dx.doi.org/10.1016/j.jplph.2017.10.011DOI Listing
January 2018

Relative functional and optical absorption cross-sections of PSII and other photosynthetic parameters monitored in situ, at a distance with a time resolution of a few seconds, using a prototype light induced fluorescence transient (LIFT) device.

Funct Plant Biol 2017 Oct;44(10):985-1006

Centre for Sustainable Ecosystem Solutions, School of Biological Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia.

The prototype light-induced fluorescence transient (LIFT) instrument provides continuous, minimally intrusive, high time resolution (~2s) assessment of photosynthetic performance in terrestrial plants from up to 2m. It induces a chlorophyll fluorescence transient by a series of short flashes in a saturation sequence (180 ~1μs flashlets in <380μs) to achieve near-full reduction of the primary acceptor QA, followed by a relaxation sequence (RQA; 90 flashlets at exponentially increasing intervals over ~30ms) to observe kinetics of QA re-oxidation. When fitted by the fast repetition rate (FRR) model (Kolber et al. 1998) the QA flash of LIFT/FRR gives smaller values for FmQA from dark adapted leaves than FmPAM from pulse amplitude modulated (PAM) assays. The ratio FmQA/FmPAM resembles the ratio of fluorescence yield at the J/P phases of the classical O-J-I-P transient and we conclude that the difference simply is due to the levels of PQ pool reduction induced by the two techniques. In a strong PAM-analogous WL pulse in the dark monitored by the QA flash of LIFT/FRR φPSIIWL ≈ φPSIIPAM. The QA flash also tracks PQ pool reduction as well as the associated responses of ETR QA → PQ and PQ → PSI, the relative functional (σPSII) and optical absorption (aPSII) cross-sections of PSII in situ with a time resolution of ~2s as they relax after the pulse. It is impractical to deliver strong WL pulses at a distance in the field but a longer PQ flash from LIFT/FRR also achieves full reduction of PQ pool and delivers φPSIIPQ ≈ φPSIIPAM to obtain PAM-equivalent estimates of ETR and NPQ at a distance. In situ values of σPSII and aPSII from the QA flash with smaller antenna barley (chlorina-f2) and Arabidopsis mutants (asLhcb2-12, ch1-3 Lhcb5) are proportionally similar to those previously reported from in vitro assays. These direct measurements are further validated by changes in antenna size in response to growth irradiance. We illustrate how the QA flash facilitates our understanding of photosynthetic regulation during sun flecks in natural environments at a distance, with a time resolution of a few seconds.
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http://dx.doi.org/10.1071/FP17024DOI Listing
October 2017

Mechanism of Photodamage of the Oxygen Evolving Mn Cluster of Photosystem II by Excessive Light Energy.

Sci Rep 2017 08 8;7(1):7604. Epub 2017 Aug 8.

Division of Material Science (Physics), Graduate school of Science, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8602, Japan.

Photodamage to Photosystem II (PSII) has been attributed either to excessive excitation of photosynthetic pigments or by direct of light absorption by MnCaO cluster. Here we investigated the time course of PSII photodamage and release of Mn in PSII-enriched membranes under high light illumination at 460 nm and 660 nm. We found that the loss of PSII activity, assayed by chlorophyll fluorescence, is faster than release of Mn from the MnCaO cluster, assayed by EPR. Loss of PSII activity and Mn release was slower during illumination in the presence of exogenous electron acceptors. Recovery of PSII activity was observed, after 30 min of addition of electron donor post illumination. The same behavior was observed under 460 and 660 nm illumination, suggesting stronger correlation between excessive excitation and photodamage compared to direct light absorption by the cluster. A unified model of PSII photodamage that takes into account present and previous literature reports is presented.
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http://dx.doi.org/10.1038/s41598-017-07671-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5548768PMC
August 2017

The half-life of the cytochrome bf complex in leaves of pea plants after transfer from moderately-high growth light to low light.

Funct Plant Biol 2017 Feb;44(3):351-357

Division of Plant Sciences, Research School of Biology, The Australian National University, Acton, ACT 2601, Australia.

The content of cytochrome (cyt) bf complex is the main rate-limiting factor that determines light- and CO2-saturated photosynthetic capacity. A study of the half-life of the cyt f content in leaves was conducted whereby Pisum sativum L. plants, grown in moderately high light (HL), were transferred to low light (LL). The cyt f content in fully-expanded leaves decreased steadily over the 2 weeks after the HL-to-LL transfer, whereas control leaves in HL retained their high contents. The difference between the time courses of HL-to-LL plants and control HL plants represents the time course of loss of cyt f content, with a half-life of 1.7 days, which is >3-fold shorter than that reported for tobacco leaves at constant growth irradiance using an RNA interference approach (Hojka et al. 2014). After transfer to LL (16h photoperiod), pea plants were re-exposed to HL for 0, 1.5h or 5h during the otherwise LL photoperiod, but the cyt f content of fully-expanded leaves declined practically at the same rate regardless of whether HL was re-introduced for 0, 1.5h or 5h during each 16h LL photoperiod. It appears that fully-expanded leaves, having matured under HL, were unable to increase their cyt f content when re-introduced to HL. These findings are relevant to any attempts to maintain a high photosynthetic capacity when the growth irradiance is temporarily decreased by shading or overcast weather.
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http://dx.doi.org/10.1071/FP16222DOI Listing
February 2017

A magic red coat on the surface of young leaves: anthocyanins distributed in trichome layer protect Castanopsis fissa leaves from photoinhibition.

Tree Physiol 2016 10 10;36(10):1296-1306. Epub 2016 Sep 10.

Guangzhou Key Laboratory of Subtropical Biodiversity and Biomonitoring, Guangdong Provincial Key Laboratory of Biotechnology for Plant Development, School of Life Sciences, South China Normal University, Guangzhou 510631, PR China

The presence of anthocyanins in young leaves plays an important role in mitigation against photodamage and allows leaves to grow and develop normally. Many studies have reported that foliar anthocyanins are distributed within the vacuoles of mesophyll cells, so we explored the novel defence style of anthocyanin-coated young leaves of Castanopsis fissa, a dominant subtropical forest tree species, via removable trichomes. Anthocyanins were distributed in C. fissa leaf trichomes, which produced a red coating for the young leaves. As young leaves developed and then matured, the thickness and density of the anthocyanin trichomes progressively decreased, the coating finally disappearing, allowing greater utilization of light by mature leaves. In addition to anthocyanins, the trichomes contained a remarkably high amount of phenolics, which enable the red coating to be more efficient in screening ultraviolet light. Compared with mature leaves, the young leaves exhibited lower photosynthetic ability, which was attributable to the reduced chlorophyll and Rubisco contents. Removal of the red coating had little effect on the photosynthetic capacity of young leaves. However, the young leaves without the coating suffered greater light-induced photoinhibition due to greater excess light entering the chloroplast and the production of HO Our results suggest that the anthocyanin coating is photoprotective and this anthocyanin defence style may be a metabolically cost-effective way of adjusting the anthocyanin content in response to demand.
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http://dx.doi.org/10.1093/treephys/tpw080DOI Listing
October 2016

Remembering Joan (Jan) Mary Anderson (1932-2015).

Photosynth Res 2016 Aug;129(2):129-46

Division of Plant Sciences, Research School of Biology, The Australian National University, 46 Sullivan's Creek Road, Acton, ACT, 2601, Australia.

Joan Mary Anderson, known to most people as Jan, was born on May 12, 1932 in Dunedin, New Zealand. She died on August 28, 2015 in Canberra, Australia. To celebrate her life, we present here a brief biography, some comments on her discoveries in photosynthesis during a career spanning more than half a century, and reminiscences from family and friends. We remember this wonderful person who had an unflagging curiosity, creative ability to think laterally, enthusiasm, passion, generosity and love of color and culture.
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http://dx.doi.org/10.1007/s11120-016-0287-1DOI Listing
August 2016

Different strategies of acclimation of photosynthesis, electron transport and antioxidative activity in leaves of two cotton species to water deficit.

Funct Plant Biol 2016 May;43(5):448-460

The Key Laboratory of Oasis Eco-agriculture, Xinjiang Production and Construction Group, Shihezi University, Shihezi, 832 003, PR China.

To better understand the adaptation mechanisms of the photosynthetic apparatus of cotton plants to water deficit conditions, the influence of water deficit on photosynthesis, chlorophyll a fluorescence and the activities of antioxidant systems were determined simultaneously in Gossypium hirsutum L. cv. Xinluzao 45 (upland cotton) and Gossypium barbadense L. cv. Xinhai 21 (pima cotton). Water deficit decreased photosynthesis in both cotton species, but did not decrease chlorophyll content or induce any sustained photoinhibition in either cotton species. Water deficit increased ETR/4-AG, where ETR/4 estimates the linear photosynthetic electron flux and AG is the gross rate of carbon assimilation. The increase in ETR/4-AG, which represents an increase in photorespiration and alternative electron fluxes, was particularly pronounced in Xinluzao 45. In Xinluzao 45, water deficit increased the activities of antioxidative enzymes, as well as the contents of reactive oxygen species (ROS), which are related to the Mehler reaction. In contrast, moderate water deficit particularly increased non-photochemical quenching (NPQ) in Xinhai 21. Our results suggest that Xinluzao 45 relied on enhanced electron transport such as photorespiration and the Mehler reaction to dissipate excess light energy under mild and moderate water deficit. Xinhai 21 used enhanced photorespiration for light energy utilisation under mild water deficit but, when subjected to moderate water deficit, possessed a high capacity for dissipating excess light energy via heat dissipation.
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http://dx.doi.org/10.1071/FP15247DOI Listing
May 2016

Physiological and proteomic responses to salt stress in chloroplasts of diploid and tetraploid black locust (Robinia pseudoacacia L.).

Sci Rep 2016 Mar 15;6:23098. Epub 2016 Mar 15.

College of Life Science, Northeast Forestry University, Harbin 150040, P.R. China.

Salinity is an important abiotic stressor that negatively affects plant growth. In this study, we investigated the physiological and molecular mechanisms underlying moderate and high salt tolerance in diploid (2×) and tetraploid (4×) Robinia pseudoacacia L. Our results showed greater H2O2 accumulation and higher levels of important antioxidative enzymes and non-enzymatic antioxidants in 4× plants compared with 2× plants under salt stress. In addition, 4× leaves maintained a relatively intact structure compared to 2× leaves under a corresponding condition. NaCl treatment didn't significantly affect the photosynthetic rate, stomatal conductance or leaf intercellular CO2 concentrations in 4× leaves. Moreover, proteins from control and salt treated 2× and 4× leaf chloroplast samples were extracted and separated by two-dimensional gel electrophoresis. A total of 61 spots in 2× (24) and 4× (27) leaves exhibited reproducible and significant changes under salt stress. In addition, 10 proteins overlapped between 2× and 4× plants under salt stress. These identified proteins were grouped into the following 7 functional categories: photosynthetic Calvin-Benson Cycle (26), photosynthetic electron transfer (7), regulation/defense (5), chaperone (3), energy and metabolism (12), redox homeostasis (1) and unknown function (8). This study provides important information of use in the improvement of salt tolerance in plants.
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http://dx.doi.org/10.1038/srep23098DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4791547PMC
March 2016

Rapid recovery of photosynthetic rate following soil water deficit and re-watering in cotton plants (Gossypium herbaceum L.) is related to the stability of the photosystems.

J Plant Physiol 2016 May 26;194:23-34. Epub 2016 Feb 26.

The Key Laboratory of Oasis Eco-agriculture, Xinjiang Production and Construction Group, Shihezi University, Shihezi 832003, PR China. Electronic address:

The responses of gas exchange, chlorophyll fluorescence and the anti-oxidative system of cotton leaves were studied during water deficit and recovery. The results show that water deficit led to a reversible reduction in the photosynthetic rate. This reduction was mainly accompanied by stomatal limitation. The activity of photosystem II (PSII) and photosystem I (PSI) was relatively stable during water deficit and recovery. Water deficit caused an enhanced production of reactive oxygen species (ROS) and increased lipid peroxidation. Proline accumulation and the anti-oxidative enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX) and peroxidase (POD), along with the antioxidant ascorbate (AsA), increased during water deficit. On re-watering, the ROS generation rate, anti-oxidative enzymes activities and the extent of the lipid peroxidation returned to near control values. Overall, rapid recovery of the photosynthetic rate is related to the stability of the photosystems which appears to be a critical mechanism allowing cotton plants to withstand and survive drought environments.
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http://dx.doi.org/10.1016/j.jplph.2016.01.016DOI Listing
May 2016

Obstacles in the quantification of the cyclic electron flux around Photosystem I in leaves of C3 plants.

Photosynth Res 2016 Sep 4;129(3):239-51. Epub 2016 Feb 4.

Division of Plant Sciences, Research School of Biology, The Australian National University, 46 Sullivans Creek Road, Acton, ACT, 2601, Australia.

Sixty years ago Arnon and co-workers discovered photophosphorylation driven by a cyclic electron flux (CEF) around Photosystem I. Since then understanding the physiological roles and the regulation of CEF has progressed, mainly via genetic approaches. One basic problem remains, however: quantifying CEF in the absence of a net product. Quantification of CEF under physiological conditions is a crucial prerequisite for investigating the physiological roles of CEF. Here we summarize current progress in methods of CEF quantification in leaves and, in some cases, in isolated thylakoids, of C3 plants. Evidently, all present methods have their own shortcomings. We conclude that to quantify CEF in vivo, the best way currently is to measure the electron flux through PS I (ETR1) and that through PS II and PS I in series (ETR2) for the whole leaf tissue under identical conditions. The difference between ETR1 and ETR2 is an upper estimate of CEF, mainly consisting, in C3 plants, of a major PGR5-PGRL1-dependent CEF component and a minor chloroplast NDH-dependent component, where PGR5 stands for Proton Gradient Regulation 5 protein, PGRL1 for PGR5-like photosynthesis phenotype 1, and NDH for Chloroplast NADH dehydrogenase-like complex. These two CEF components can be separated by the use of antimycin A to inhibit the former (major) component. Membrane inlet mass spectrometry utilizing stable oxygen isotopes provides a reliable estimation of ETR2, whilst ETR1 can be estimated from a method based on the photochemical yield of PS I, Y(I). However, some issues for the recommended method remain unresolved.
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http://dx.doi.org/10.1007/s11120-016-0223-4DOI Listing
September 2016

Photoinhibition and photoinhibition-like damage to the photosynthetic apparatus in tobacco leaves induced by pseudomonas syringae pv. Tabaci under light and dark conditions.

BMC Plant Biol 2016 Jan 25;16:29. Epub 2016 Jan 25.

College of Life Science, Northeast Forestry University, Harbin, 150040, China.

Background: Pseudomonas syringae pv. tabaci (Pst), which is the pathogen responsible for tobacco wildfire disease, has received considerable attention in recent years. The objective of this study was to clarify the responses of photosystem I (PSI) and photosystem II (PSII) to Pst infection in tobacco leaves.

Results: The net photosynthetic rate (Pn) and carboxylation efficiency (CE) were inhibited by Pst infection. The normalized relative variable fluorescence at the K step (W k) and the relative variable fluorescence at the J step (V J) increased while the maximal quantum yield of PSII (F v/F m) and the density of Q A-reducing PSII reaction centers per cross section (RC/CSm) decreased, indicating that the reaction centers, and the donor and acceptor sides of PSII were all severely damaged after Pst infection. The PSI activity decreased as the infection progressed. Furthermore, we observed a considerable overall degradation of PsbO, D1, PsaA proteins and an over-accumulation of reactive oxygen species (ROS).

Conclusions: Photoinhibition and photoinhibition-like damage were observed under light and dark conditions, respectively, after Pst infection of tobacco leaves. The damage was greater in the dark. ROS over-accumulation was not the primary cause of the photoinhibition and photoinhibition-like damage. The PsbO, D1 and PsaA proteins appear to be the targets during Pst infection under light and dark conditions.
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http://dx.doi.org/10.1186/s12870-016-0723-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4727333PMC
January 2016

Photodamage to the oxygen evolving complex of photosystem II by visible light.

Sci Rep 2015 Nov 12;5:16363. Epub 2015 Nov 12.

Research School of Biology, The Australian National University, Canberra, Australian Capital Territory, 2601 Australia.

Light damages photosynthetic machinery, primarily photosystem II (PSII), and it results in photoinhibition. A new photodamage model, the two-step photodamage model, suggests that photodamage to PSII initially occurs at the oxygen evolving complex (OEC) by light energy absorbed by manganese and that the PSII reaction center is subsequently damaged by light energy absorbed by photosynthetic pigments due to the limitation of electrons to the PSII reaction center. However, it is still uncertain whether this model is applicable to photodamage to PSII under visible light as manganese absorbs visible light only weakly. In the present study, we identified the initial site of photodamage to PSII upon illumination of visible light using PSII membrane fragments isolated from spinach leaves. When PSII samples were exposed to visible light in the presence of an exogenous electron acceptor, both PSII total activity and the PSII reaction centre activity declined due to photodamage. The supplemental addition of an electron donor to the PSII reaction centre alleviated the decline of the reaction centre activity but not the PSII total activity upon the light exposure. Our results demonstrate that visible light damages OEC prior to photodamage to the PSII reaction center, consistent with two-step photodamage model.
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http://dx.doi.org/10.1038/srep16363DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4642293PMC
November 2015

The action spectrum of Photosystem II photoinactivation in visible light.

J Photochem Photobiol B 2015 Nov 11;152(Pt B):247-60. Epub 2015 Aug 11.

Research School of Biology, College of Medicine, Biology and Environment, The Australian National University, Canberra ACT 2601, Australia. Electronic address:

Photosynthesis is always accompanied by light induced damage to the Photosystem II (PSII) which is compensated by its subsequent repair. Photoinhibition of PSII is a complex process, balancing between photoinactivation, protective and repair mechanisms. Current understanding of photoinactivation is limited with competing hypotheses where the photosensitiser is either photosynthetic pigments or the Mn4CaO5 cluster itself, with little consensus on the mechanisms and consequences of PSII photoinactivation. The mechanism of photoinactivation should be reflected in the action spectrum of PSII photoinactivation, but there is a great diversity of the action spectra reported thus far. The only consensus is that PSII photoinactivation is greatest in the UV region of the electromagnetic spectrum. In this review, the authors revisit the methods, technical constraints and the different action spectra of PSII photoinactivation reported to date and compare them against the diverse mechanisms proposed. Upon critical examination of the reported action spectra, a hybrid mechanism of photoinactivation, sensitised by both photosynthetic pigments and the Mn4CaO5 appears to be the most plausible rationalisation.
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http://dx.doi.org/10.1016/j.jphotobiol.2015.08.007DOI Listing
November 2015

Multiple roles of oxygen in the photoinactivation and dynamic repair of Photosystem II in spinach leaves.

Photosynth Res 2016 Mar 22;127(3):307-19. Epub 2015 Aug 22.

Division of Plant Sciences, Research School of Biology, The Australian National University, 46 Sullivans Creek Road, Acton, ACT, 2601, Australia.

Oxygen effects have long been ambiguous: exacerbating, being indifferent to, or ameliorating the net photoinactivation of Photosystem II (PS II). We scrutinized the time course of PS II photoinactivation (characterized by rate coefficient k i) in the absence of repair, or when recovery (characterized by k r) occurred simultaneously in CO2 ± O2. Oxygen exacerbated photoinactivation per se, but alleviated it by mediating the utilization of electrons. With repair permitted, the gradual net loss of functional PS II during illumination of leaves was better described phenomenologically by introducing τ, the time for an initial k r to decrease by half. At 1500 μmol photons m(-2) s(-1), oxygen decreased the initial k r but increased τ. Similarly, at even higher irradiance in air, there was a further decrease in the initial k r and increase in τ. These observations are consistent with an empirical model that (1) oxygen increased k i via oxidative stress but decreased it by mediating the utilization of electrons; and (2) reactive oxygen species stimulated the degradation of photodamaged D1 protein in PS II (characterized by k d), but inhibited the de novo synthesis of D1 (characterized by k s), and that the balance between these effects determines the net effect of O2 on PS II functionality.
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http://dx.doi.org/10.1007/s11120-015-0185-yDOI Listing
March 2016
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