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The acclimation of the green algae Chlamydomoas reinhardtii to high light (HL) has been studied predominantly under continuous illumination of the cells. Here, we investigated the impact of fluctuating HL in alternation with either low light (LL) or darkness on photosynthetic performance and on photoprotective responses. Compared to intervening LL phases, dark phases led to (1) more pronounced reduction of the photosystem II quantum efficiency, (2) reduced degradation of the PsbS protein, (3) lower energy dissipation capacity and (4) an increased pool size of the xanthophyll cycle pigments. These characteristics indicate increased photo‐oxidative stress when HL periods are interrupted by dark phases instead of LL phases. This overall trend was similar when comparing long (8 h) and short (30 min) HL phases being interrupted by long (16 h) and short (60 min) phases of dark or low light, respectively. Only the degradation of PsbS was clearly more efficient during long (16 h) LL phases when compared to short (60 min) LL phases. Summary Statement: The performance of the unicellular green alga Chlamydomonas reinhardtii has been studied so far, mostly under continuous illumination conditions. We investigated the impact of fluctuating high light (HL) on the photosynthetic performance and the photoprotective response in C. reinhardtii. Our data indicate that intervening dark phases negatively affect cell growth and lead to a delay of HL acclimation processes, including the activation of energy dissipation. [ABSTRACT FROM AUTHOR]

The rice, like other plants, undergoes photoprotection mode by increasing nonphotochemical quenching (NPQ) in high light intensity (> 1200 µmol m− 2s− 1 PPFD), which attenuates photosystem II yield (φPSII) drastically. The plant remains in photoprotection mode even after light intensity becomes not stressful for an extended period. While there are significant differences in the time it takes for photoprotection to recover among different genotypes, its use is limited in plant breeding because measuring the chlorophyll fluorescence parameters in progressive actinic light after dark adaptation takes more than forty-five minutes per genotype. The study finds that instantly measured A400/A1800 ratio by five minutes in flag leaves of 25 diverse genotypes strongly associated with the φPSII400 differences between theoretical and actual, qPd400 and NPQ400 with R2 values 0.74, 0.65 and 0.60, respectively. In two consecutive years, GWAS of A400/A1800 ratio identified the regions with genes reported earlier for plant photoprotection recovery. Additionally, QTL analysis in a RIL population also identified the regions carrying known genes related to photoprotection. Thus, the A400/A1800 ratio can quickly phenotype many plants for easier introgression of the traits in popular cultivars. The identified genotypes, genes, and QTLs can be used to improve yield potential and allele mining. [ABSTRACT FROM AUTHOR]

Plants have evolved various photosynthetic adaptations and photoprotective mechanisms to survive in fluctuating and extreme light environments. Many light-activated photosynthetic proteins and enzymes adjust to plant leaf anatomy and leaf pigments to facilitate these processes. Under excessive amounts of light, plants use non-photochemical quenching (NPQ) mechanisms to dissipate excess absorbed light energy as heat to prevent photoinhibition and, therefore, mitigate damage to the plant's photosystems. In this study, we examined photosynthetic adaptations to the light environment in common beans using representative genotypes of the Andean (Calima) and the Mesoamerican (Jamapa) gene pools. We estimated their leaf chlorophyll fluorescence characteristics using dark- and light-adapted mature leaves from three-week-old plants. Our results indicated a higher chlorophyll fluorescence of the light-adapted leaves in the Mesoamerican genotype. NPQ induction was early and extended in the Andean genotype. A similar response in the Mesoamerican counterpart required high light intensity (≥1500 PAR). The NPQ relaxation was rapid in the Mesoamerican genotype (t1/2: 6.76 min) but sluggish in the Andean genotype (t1/2: 9.17 min). These results indicated variable adaptation to light environments between the two common bean genotypes and suggested different strategies for surviving fluctuating light environments that can be exploited for developing plants with environmentally efficient photosynthesis under light limitations. [ABSTRACT FROM AUTHOR]

SUMMARY: Enhancing the efficiency of photosynthesis represents a promising strategy to improve crop yields, with keeping the steady state of PSII being key to determining the photosynthetic performance. However, the mechanisms whereby the stability of PSII is maintained in oxygenic organisms remain to be explored. Here, we report that the Psb28 protein functions in regulating the homeostasis of PSII under different light conditions in Arabidopsis thaliana. The psb28 mutant is much smaller than the wild‐type plants under normal growth light, which is due to its significantly reduced PSII activity. Similar defects were seen under low light and became more pronounced under photoinhibitory light. Notably, the amounts of PSII core complexes and core subunits are specifically decreased in psb28, whereas the abundance of other representative components of photosynthetic complexes remains largely unaltered. Although the PSII activity of psb28 was severely reduced when subjected to high light, its recovery from photoinactivation was not affected. By contrast, the degradation of PSII core protein subunits is dramatically accelerated in the presence of lincomycin. These results indicate that psb28 is defective in the photoprotection of PSII, which is consistent with the observation that the overall NPQ is much lower in psb28 compared to the wild type. Moreover, the Psb28 protein is associated with PSII core complexes and interacts mainly with the CP47 subunit of PSII core. Taken together, these findings reveal an important role for Psb28 in the protection and stabilization of PSII core in response to changes in light environments. Significance Statement: The small protein Psb28 plays a crucial role in the protection and stabilization of PSII reaction center via direct interaction with the CP47 subunit of PSII core complexes in the model plant Arabidopsis thaliana, providing novel insights into the maintenance of PSII function for boosting photosynthetic capacity in response to changing light environments. [ABSTRACT FROM AUTHOR]

Plants have evolved multiple regulatory mechanisms to cope with natural light fluctuations. The interplay between these mechanisms leads presumably to the resilience of plants in diverse light patterns. We investigated the energy‐dependent nonphotochemical quenching (qE) and cyclic electron transports (CET) in light that oscillated with a 60‐s period with three different amplitudes. The photosystem I (PSI) and photosystem II (PSII) function‐related quantum yields and redox changes of plastocyanin and ferredoxin were measured in Arabidopsis thaliana wild types and mutants with partial defects in qE or CET. The decrease in quantum yield of qE due to the lack of either PsbS‐ or violaxanthin de‐epoxidase was compensated by an increase in the quantum yield of the constitutive nonphotochemical quenching. The mutant lacking NAD(P)H dehydrogenase (NDH)‐like‐dependent CET had a transient significant PSI acceptor side limitation during the light rising phase under high amplitude of light oscillations. The mutant lacking PGR5/PGRL1‐CET restricted electron flows and failed to induce effective photosynthesis control, regardless of oscillation amplitudes. This suggests that PGR5/PGRL1‐CET is important for the regulation of PSI function in various amplitudes of light oscillation, while NDH‐like‐CET acts' as a safety valve under fluctuating light with high amplitude. The results also bespeak interplays among multiple photosynthetic regulatory mechanisms. Summary statement: PGR5/PGRL1‐dependent cyclic electron transports (CET) protect photosystems against light fluctuations of varying amplitudes, while NDH‐like‐dependent CET acts as a safety valve to optimise photosynthesis in high amplitude light fluctuations. Interplays may exist among multiple regulatory mechanisms, including quenching and CET. [ABSTRACT FROM AUTHOR]

In nature, photosynthetic organisms are exposed to fluctuating light, and their physiological systems must adapt to this fluctuation. To maintain homeostasis, these organisms have a light fluctuation photoprotective mechanism, which functions in both photosystems and metabolism. Although the photoprotective mechanisms functioning in the photosystem have been studied, it is unclear how metabolism responds to light fluctuations within a few seconds. In the present study, we investigated the metabolic response of Synechocystis sp. PCC 6803 to light fluctuations using 13C‐metabolic flux analysis. The light intensity and duty ratio were adjusted such that the total number of photons or the light intensity during the low‐light phase was equal. Light fluctuations affected cell growth and photosynthetic activity under the experimental conditions. However, metabolic flux distributions and cofactor production rates were not affected by the light fluctuations. Furthermore, the estimated ATP and NADPH production rates in the photosystems suggest that NADPH‐consuming electron dissipation occurs under fluctuating light conditions. Although we focused on the water–water cycle as the electron dissipation path, no growth effect was observed in an flv3‐disrupted strain under fluctuating light, suggesting that another path contributes to electron dissipation under these conditions. [ABSTRACT FROM AUTHOR]

Fluctuating light (FL) and drought stress usually occur concomitantly. However, whether drought stress affects photosynthetic performance under FL remains unknown. Here, we measured gas exchange, chlorophyll fluorescence, and P700 redox state under FL in drought-stressed tomato (Solanum lycopersicum) seedlings. Drought stress significantly delayed the induction kinetics of stomatal and mesophyll conductances after transition from low to high light and thus delayed photosynthetic induction under FL. Therefore, drought stress exacerbated the loss of carbon gain under FL. Furthermore, restriction of CO 2 fixation under drought stress aggravated the over-reduction of photosystem I (PSI) upon transition from low to high light. The resulting stronger FL-induced PSI photoinhibition significantly suppressed linear electron flow and PSI photoprotection. These results indicated that drought stress not only caused a larger loss of carbon gain under FL but also accelerated FL-induced photoinhibition of PSI. Furthermore, drought stress enhanced relative cyclic electron flow in FL, which partially compensated for restricted CO 2 fixation and thus favored PSI photoprotection under FL. To our knowledge, we here show new insight into how drought stress affects photosynthetic performance under FL. • We examine the effect of drought stress on dynamic photosynthesis in tomato. • Drought stress decreases mesophyll conductance during light induction. • Drought stress causes photosynthetic depression under fluctuating light. • Drought stress aggravates photoinhibition of photosystem I under fluctuating light. • Drought stress largely affects crop physiology under fluctuating light. [ABSTRACT FROM AUTHOR]

With the high variability of natural growth environments, plants exhibit flexibility and resilience in regard to the strategies they employ to maintain overall fitness, including maximizing light use for photosynthesis, while simultaneously limiting light‐associated damage. We measured distinct parameters of photosynthetic performance of Arabidopsis thaliana plants under dynamic light regimes. Plants were grown to maturity then subjected to the following 5‐day (16 h light, 8 h dark) regime: Day 1 at constant light (CL) intensity during light period, representative of a common lab growth condition; Day 2 under sinusoidal variation in light intensity (SL) during the light period that is representative of changes occurring during a clear sunny day; Day 3 under fluctuating light (FL) intensity during the light period that simulates sudden changes that might occur with the movements of clouds in and out of the view of the sun; Day 4, repeat of CL; and Day 5, repeat of FL. We also examined the global transcriptome profile in these growth conditions based on obtaining RNA‐sequencing (RNA‐seq) data for whole plant rosettes. Our transcriptomic analyses indicated downregulation of photosystem I (PSI) and II (PSII) associated genes, which were correlated with elevated levels of photoinhibition as indicated by measurements of nonphotochemical quenching (NPQ), energy‐dependent quenching (qE), and inhibitory quenching (qI) under both SL and FL conditions. Furthermore, our transcriptomic results indicated downregulation of tetrapyrrole biosynthesis associated genes, coupled with reduced levels of chlorophyll under both SL and FL compared with CL, as well as downregulation of photorespiration‐associated genes under SL. We also noticed an enrichment of the stress response gene ontology (GO) terms for genes differentially regulated under FL when compared with SL. Collectively, our phenotypic and transcriptome analyses serve as useful resources for probing the underlying molecular mechanisms associated with plant acclimation to rapid light intensity changes in the natural environment. [ABSTRACT FROM AUTHOR]

Cymbidium sinense is a well-known traditional orchid in China, and has been widely cultivated for a long time. This species is typically a shade species under the forest, but habitat destruction and tree canopy structure change make it subject to the dual fluctuation of light and nutrients. To explore the photosynthetic response of C. sinense to the combined stress of nitrogen and fluctuating light intensity, the leaf nitrogen content, chlorophyll content, and the responses of photosystem I (PS I) and photosystem Ⅱ(PS Ⅱ)to fluctuating light intensity were investigated under different nitrogen treatments. The results were as follows: (1)The C. sinense under 0 mmol·L-1 nitrogen treatment had lower values for leaf nitrogen content, chlorophyll content, PS Ⅱ maximum quantum efficiency (Fv/Fm) and the value of maximum oxidizable P700 of PS I(Pm), but motivated a large amount of non-photochemical quenching and PS Ⅱ non-regulatory energy dissipation. (2) When the light intensity suddenly increased, the PS I reaction center showed over-reduction firstly, and then the over-reduction state was gradually released under 1.25 mmol·L-1, 5 mmol·L-1 and 10 mmol·L-1 nitrogen treatments. Meanwhile, the excitation degree of cyclic electron flow increased first and then gradually decreased, indicating that the dynamic adjustment of cyclic electron flow was closely linked to the redox state of PS I.(3)Under fluctuating light intensity, the excessive reduction of PS I was not observed in C. sinense under 0 mmol·L-1 nitrogen treatment. This was mainly because the few electrons were released by PS Ⅱ, thus avoiding the transfer of excess electrons to PS I. These results suggest that nitrogen fluctuation can affect significantly the response of C. sinense to fluctuating light intensity. These findings provide a scientific basis for the cultivation and conservation of C. sinense, and are helpful to explore how photosynthesis of shade plant responds to the combined stress of nitrogen and fluctuating light intensity. [ABSTRACT FROM AUTHOR]

Fluctuating light (FL) can cause the selective photoinhibition of photosystem I (PSI) in angiosperms. In nature, leaves usually experience FL conditions with the same low light and different high light intensities, but the effects of different FL conditions on PSI redox state and PSI photoinhibition are not well known. In this study, we found that PSI was highly reduced within the first 10 s after transition from 59 to 1809 μmol photons m−2 s−1 in tomato (Solanum lycopersicum). However, such transient PSI over-reduction was not observed by transitioning from 59 to 501 or 923 μmol photons m−2 s−1. Consequently, FL (59-1809) induced a significantly stronger PSI photoinhibition than FL (59-501) and FL (59-923). Compared with the proton gradient (∆pH) level after transition to high light for 60 s, tomato leaves almost formed a sufficient ∆pH after light transition for 10 s in FL (59-501) but did not in FL (59-923) or FL (59-1809). The difference in ∆pH between 10 s and 60 s was tightly correlated to the extent of PSI over-reduction and PSI photoinhibition induced by FL. Furthermore, the difference in PSI photoinhibition between (59-923) and FL (59-1809) was accompanied by the same level of linear electron flow. Therefore, PSI photoinhibition induced by different intensities of FL is more related to the kinetics of ∆pH formation rather than linear electron flow. [ABSTRACT FROM AUTHOR]

Photosynthesis involves a series of redox reactions and is the major source of reactive oxygen species in plant cells. Fluctuating light (FL) levels, which occur commonly in natural environments, affect photosynthesis; however, little is known about the specific effects of FL on the redox regulation of photosynthesis. Here, we performed global quantitative mapping of the Arabidopsis thaliana cysteine thiol redox proteome under constant light and FL conditions. We identified 8857 redox‐switched thiols in 4350 proteins, and 1501 proteins that are differentially modified depending on light conditions. Notably, proteins related to photosynthesis, especially photosystem I (PSI), are operational thiol‐switching hotspots. Exposure of wild‐type A. thaliana to FL resulted in decreased PSI abundance, stability, and activity. Interestingly, in response to PSI photodamage, more of the PSI assembly factor PSA3 dynamically switches to the reduced state. Furthermore, the Cys199 and Cys200 sites in PSA3 are necessary for its full function. Moreover, thioredoxin m (Trx m) proteins play roles in redox switching of PSA3, and are required for PSI activity and photosynthesis. This study thus reveals a mechanism for redox‐based regulation of PSI under FL, and provides insight into the dynamic acclimation of photosynthesis in a changing environment. [ABSTRACT FROM AUTHOR]

Plants growing in nature often experience fluctuating irradiance. However, in the laboratory, the dynamics of photosynthesis are usually explored by instantaneously exposing dark-adapted plants to constant light and examining the dark-to-light transition, which is a poor approximation of natural phenomena. With the aim creating a better approximation, we exposed leaves of pea (Pisum sativum) to oscillating light and measured changes in the functioning of PSI and PSII, and of the proton motive force at the thylakoid membrane. We found that the dynamics depended on the oscillation period, revealing information about the underlying regulatory networks. As demonstrated for a selected oscillation period of 60 s, the regulation tries to keep the reaction centers of PSI and PSII open. We present an evaluation of the data obtained, and discuss the involvement of particular processes in the regulation of photosynthesis. The forced oscillations provided an information-rich fingerprint of complex regulatory networks. We expect future progress in understanding these networks from experiments involving chemical interventions and plant mutants, and by using mathematical modeling and systems identification and control tools. [ABSTRACT FROM AUTHOR]

Background: Diatoms contribute 20% of the global primary production and are adaptable in dynamic environments. Diatoms always bloom earlier in the annual phytoplankton succession instead of dinoflagellates. However, how diatoms acclimate to a dynamic environment, especially under changing light conditions, remains unclear. Results: We compared the growth and photosynthesis under fluctuating light conditions of red tide diatom Skeletonema costatum, red tide dinoflagellate Amphidinium carterae, Prorocentrum donghaiense, Karenia mikimotoi, model diatom Phaeodactylum tricornutum, Thalassiosira pseudonana and model dinoflagellate Dinophycae Symbiodinium. Diatoms grew faster and maintained a consistently higher level of photosynthesis. Diatoms were sensitive to the specific inhibitor of Proton Gradient Regulation 5 (PGR5) depending photosynthetic electron flow, which is a crucial mechanism to protect their photosynthetic apparatus under fluctuating light. In contrast, the dinoflagellates were not sensitive to this inhibitor. Therefore, we investigate how PGR5 functions under light fluctuations in the model diatom P. tricornutum by knocking down and overexpressing PGR5. Overexpression of PGR5 reduced the photosystem I acceptor side limitation (Y (NA)) and increased growth rate under severely fluctuating light in contrast to the knockdown of PGR5. Conclusion: Diatoms acclimatize to fluctuating light conditions better than dinoflagellates. PGR5 in diatoms can regulate their photosynthetic electron flow and accelerate their growth under severe light fluctuation, supporting fast biomass accumulation under dynamic environments in pioneer blooms. [ABSTRACT FROM AUTHOR]

SUMMARY: C4 plants can fix CO2 efficiently using CO2‐concentrating mechanisms (CCMs), but they require additional ATP. To supply the additional ATP, C4 plants operate at higher rates of cyclic electron transport around photosystem I (PSI), in which electrons are transferred from ferredoxin to plastoquinone. Recently, it has been reported that the NAD(P)H dehydrogenase‐like complex (NDH) accumulated in the thylakoid membrane in leaves of C4 plants, making it a candidate for the additional synthesis of ATP used in the CCM. In addition, C4 plants have higher levels of PROTON GRADIENT REGULATION 5 (PGR5) expression, but it has been unknown how PGR5 functions in C4 photosynthesis. In this study, PGR5 was overexpressed in a C4 dicot, Flaveria bidentis. In PGR5‐overproducing (OP) lines, PGR5 levels were 2.3‐ to 3.0‐fold greater compared with wild‐type plants. PGR5‐like PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1), which cooperates with PGR5, increased with PGR5. A spectroscopic analysis indicated that in the PGR5‐OP lines, the acceptor side limitation of PSI was reduced in response to a rapid increase in photon flux density. Although it did not affect CO2 assimilation, the overproduction of PGR5 contributed to an enhanced electron sink downstream of PSI. Significance Statement: We generated a transgenic C4 plant overexpressing PGR5 of Arabidopsis thaliana, in which PGR5 levels were 2.3‐ to 3.0‐fold greater compared with wild‐type plants. In the PGR5‐overproducing lines, acceptor side limitation of PSI was reduced in response to a rapid increase in photon flux density, which indicates that overproduction of PGR5 can lead to an enhanced electron sink downstream of PSI. [ABSTRACT FROM AUTHOR]

Plants are subjected to strong fluctuations in light intensity in their natural growth environment, caused both by unpredictable changes due to weather conditions and movement of clouds and upper canopy leaves and predictable changes during day-night cycle. The mechanisms of long-term acclimation to fluctuating light (FL) are still not well understood. Here, we used quantitative mass spectrometry to investigate long-term acclimation of low light-grown Arabidopsis thaliana to a FL condition that induces mild photooxidative stress. On the third day of exposure to FL, young and mature leaves were harvested in the morning and at the end of day for proteome analysis using a stable isotope labeling approach. We identified 2,313 proteins, out of which 559 proteins exhibited significant changes in abundance in at least one of the four experimental groups (morning-young, morning-mature, end-of-day-young, end-of-day-mature). A core set of 49 proteins showed significant responses to FL in three or four experimental groups, which included enhanced accumulation of proteins involved in photoprotection, cyclic electron flow around photosystem I, photorespiration, and glycolysis, while specific glutathione transferases and proteins involved in translation and chlorophyll biosynthesis were reduced in abundance. In addition, we observed pathway- and protein-specific changes predominantly at the end of day, whereas few changes were observed exclusively in the morning. Comparison of the proteome data with the matching transcript data revealed gene- and protein-specific responses, with several chloroplast-localized proteins decreasing in abundance despite increased gene expression under FL. Together, our data shows moderate but widespread alterations of protein abundance during acclimation to FL and suggests an important role of post-transcriptional regulation of protein abundance. [ABSTRACT FROM AUTHOR]

In nature, light intensity usually fluctuates and a sudden shade-sun transition can induce photodamage to photosystem I (PSI) in many angiosperms. Photosynthetic regulation in fluctuating light (FL) has been studied extensively in C 3 plants; however, little is known about how C 4 plants cope FL to prevent PSI photoinhibition. We here compared photosynthetic responses to FL between maize (Zea mays , C 4) and tomato (Solanum lycopersicum , C 3) grown under full sunlight. Maize leaves had significantly higher cyclic electron flow (CEF) activity and lower photorespiration activity than tomato. Upon a sudden shade-sun transition, maize showed a significant stronger transient PSI over-reduction than tomato, resulting in a significant greater PSI photoinhibition in maize after FL treatment. During the first seconds upon shade-sun transition, CEF was stimulated in maize at a much higher extent than tomato, favoring the rapid formation of trans-thylakoid proton gradient (ΔpH), which was helped by a transient down-regulation of chloroplast ATP synthase activity. Therefore, modulation of ΔpH by regulation of CEF and chloroplast ATP synthase adjusted PSI redox state at donor side, which partially compensated for the deficiency of photorespiration. We propose that C 4 plants use different photosynthetic strategies for coping with FL as compared with C 3 plants. • We compared photosynthetic regulation under fluctuating light in maize and tomato. • FL induced a significant greater PSI photoinhibition in maize than in tomato. • Maize displayed a faster ΔpH formation in FL to compensate for low photorespiration. • CEF and chloroplast ATP synthase jointly regulate ΔpH formation in maize. • Maize and tomato use different photosynthetic strategies to cope with FL. [ABSTRACT FROM AUTHOR]

Abstract The regulation of photosystem I (PSI) redox state under fluctuating light was investigated for four species using P700 measurement and electrochromic shift analysis. Species included the angiosperms Camellia japonica , Bletilla striata and Arabidopsis thaliana and the fern Cyrtomium fortunei. For the first seconds after transition from low to high light, all species showed relatively low levels of the proton gradient (ΔpH) across the thylakoid membranes. At this moment, PSI was highly oxidized in C. japonica and C. fortunei but was over-reduced in B. striata and A. thaliana. In B. striata and A. thaliana , the redox state of PSI was largely dependent on ΔpH. In contrast, the rapid oxidation of P700 in C. japonica was relatively independent of ΔpH, but was mainly dependent on the outflow of electrons to O 2 via the water-water cycle. In the fern C. fortunei , PSI redox state was rapidly regulated by the fast photo-reduction of O 2 rather than the ΔpH. These results indicate that mechanisms regulating PSI redox state under fluctuating light differ greatly between species. We propose that the water-water cycle is an important mechanism regulating the PSI redox state in angiosperms. Highlights • We examine the PSI redox state and ΔpH after transition from low to high light. • The water-water cycle alters P700 redox kinetics upon dark to light transition. • The water-water cycle can regulate PSI redox state under fluctuating light. • The water-water cycle can alleviate PSI photoinhibition under fluctuating light. [ABSTRACT FROM AUTHOR]

Highlights • We examine the PSI redox state and ΔpH after transition from low to high light. • PSI is over-reduced upon a sudden transition from low to high light in angiosperms. • The insufficient ΔpH induced over-reduction of PSI in fluctuating light. • PSI photoinhibition in fluctuating light is caused by the insufficient ΔpH. Abstract Despite photosystem I (PSI) is susceptible to photoinhibition under fluctuating light in wild-type angiosperms, the underlying mechanism is not well known. Because proton gradient (ΔpH) across the thylakoid membranes plays a crucial role in protecting PSI, we hypothesized that PSI photoinhibition under fluctuating light may be linked to the formation of ΔpH. To test this hypothesis, we measured chlorophyll fluorescence, PSI redox state, and the electrochromic shift signal during transition from low to high light in two angiosperms Bletilla striata (Orchidaceae) and Arabidopsis thaliana. The measurement of fast P700 redox kinetics indicated that the fast re-oxidation of P700 mediated by photo-reduction of O 2 was absent in B. striata. Furthermore, the redox state of PSI is highly determined by ΔpH in both species. For the first 20 s after transition from low to high light, both species could not build up a sufficient ΔpH, which was accompanied by the over-reduction of P700. During prolonged illumination at high light, the sufficient ΔpH made PSI to be highly oxidized. These results demonstrated that PSI photoinhibition under fluctuating light in wild-type angiosperms occurred mainly at the first 20 s after an increase in light intensity, which was caused by the insufficient ΔpH. [ABSTRACT FROM AUTHOR]

Higher plant Photosystem I (PSI) includes a peripheral antenna system (LHCI) composed of four light-harvesting proteins (Lhca1-Lhca4). The LHCI system is highly conserved, suggesting that it plays a specific role within the LHC family. In order to elucidate the specific function of LHCI, the phenotype of an Arabidopsis mutant devoid of the whole LHCI system was studied over a range of conditions, including rapid changes in irradiation. ΔLhca plants displayed stunted growth and reduced thylakoid lumen acidification respect to wild type, suggesting that the lack of LHCI affected electron transport rate. Under rapidly changing light intensity, growth rate of the mutant was further reduced and the redox balance of the photosynthetic electron chain impaired. Instead, under constant, excess light regime, the ΔLhca plants did not suffer enhanced photooxidation vs. wild type, implying LHCI optimizes the flow rate through the electron transport chain by maintaining high PSI activity at all irradiances. We conclude that a complete PSI supercomplex, including LHCI, is crucial for the dynamic regulation of PQ redox state and therefore for PSII photoprotection. [ABSTRACT FROM AUTHOR]

It has been reported that PSI photoinhibition is induced even in wild-type plants of Arabidopsis thaliana, rice and other species by exposure of leaves to fluctuating light (FL) for a few hours. Because plants are exposed to FL in nature, they must possess protective mechanisms against the FL induced photodamage. Here, using A. thaliana grown at various irradiances, we examined PSI photoprotection by far-red (FR) light at intensities comparable with those observed in nature. Dark-treated leaves were illuminated by red FL alternating high/low light at 1,200/30 μmol m-2 s-1 for 800 ms/10s. By this FL treatment without FR light for 120 min, the level of photo-oxidizable P700 was decreased by 30% even in the plants grown at high irradiances. The addition of continuous FR light during the FL suppressed this damage almost completely. With FR light, P700 was kept in a more oxidized state in both low- and high-light phases. The protective effect of FR light was diminished more in mutants of the NADH dehydrogenase-like complex (NDH)-mediated cyclic electron flow around PSI (CEF-PSI) than in the PGR5 (proton gradient regulation 5)-mediated CEF-PSI, indicating that the NDH-mediated CEF-PSI would be a major contributor to PSI photoprotection in the presence of FR light. We also confirmed that PSI photoinhibition decreased with the increase in growth irradiance in A. thaliana and field-grown plants, and that this PSI photodamage was largely suppressed by addition of FR light. These results clearly indicate that the most effective PSI protection is realized in the presence of FR light. [ABSTRACT FROM AUTHOR]

Shade-establishing plants growing in the forest understory are exposed to constant high light or fluctuating light when gaps are created by fallen trees. Our previous studies indicate that photosystem I (PSI) is sensitive to constant high light in shade-establishing tree species, however, the effects of fluctuating light on PSI and photosystem II (PSII) in shade-establishing species are little known. In the present study, we examined the responses of PSI and PSII to fluctuating light in comparison to constant high light in the shade-establishing species Psychotria henryi. Accompanying with significant activation of cyclic electron flow (CEF), the P700 oxidation ratio was maintained at high levels when exposed to strong light either under fluctuating light or constant high light. Under moderate fluctuating light, PSI and PSII activities were remained stable in P. henryi. Interestingly, PSI was insusceptible to fluctuating light but sensitive to constant high light in P. henryi. Furthermore, both PSI and PSII were more sensitive to constant high light than fluctuating light. These results suggest that CEF is essential for photoprotection of PSI under fluctuating light in P. henryi. Furthermore, photoinhibition of PSI under high light in P. henryi is more related to the accumulation of reactive oxygen species rather than to P700 redox state, which is different from the mechanisms of PSI photoinhibition in Arabidopsis thaliana and rice. Taking together, PSI is a key determiner of photosynthetic responses to fluctuating light and constant high light in the shade-establishing species P. henryi. [ABSTRACT FROM AUTHOR]

It has been claimed that the cyclic electron flow around PSI (CEF-PSI) plays an important role in protection of PSI against fluctuating light photoinhibition. However, the photoprotective mechanism of PSI is not fully elucidated. Here, we examined the mechanism, using two CEF-PSI mutants of Arabidopsis thaliana, and antimycin A, an inhibitor of the PGR5 (proton gradient regulation 5)-mediated CEF-PSI. Dark-adapted leaves in these plants were illuminated in fluctuating light alternating between high light at 1,200 μmol m-2 s-1 and low light at 30 μmol m-2 s-1 every 2 min, and PSI and PSII parameters were simultaneously measured for 160 min with 830 nm absorption and Chl fluorescence, respectively. When CEF-PSI, especially PGR5-mediated CEFPSI, did not operate, the acceptor-side limitation of PSI, Y(NA), increased stepwise, leading to marked PSI photoinhibition. The deficiency of CFE-PSI decreased not only the electron transport rate through PSI but also the donor-side limitation of PSI, Y(ND), in high light phases. These results showed that the large Y(ND), observed only when CEF-PSI operated, contributed to suppression of PSI photoinhibition. Taken together with our previous report that high Y(NA) was alleviated by the enhancement of CEF-PSI, a model for the protective mechanisms of PSI is proposed. In this model, both alleviation of Y(NA) and acceleration of Y(ND) are indispensable, and for realization of such a situation, regulation of the electron flows, especially the PGR5-mediated CEF-PSI, plays a key role. It is important for effective protection to regulate the balance of Y(ND) and Y(NA) through CEF-PSI. [ABSTRACT FROM AUTHOR]

The wheat lines affected by a decrease in the leaf chlorophyll content typically experience a biomass loss. A known major problem of the chlorophyll-deficient wheat mutants is their limited prevention of Photosystem I (PSI) over-reduction brought about by an insufficient cyclic electron flow, potentially exposing them to a higher sensitivity to light fluctuations. However, the resistance of some mutant lines against fluctuating light suggests the occurrence of regulatory processes compensating for the defect in cyclic electron flow. In this study, a phenotyping approach based on fast chlorophyll a fluorescence induction (OJIP transient), corroborated by P700 redox kinetics, was applied to a collection of chlorophyll-deficient wheat lines, grown under continuous or fluctuating light. Quantitative parameters calculated from the OJIP transient are considered informative about Photosystem II (PSII) functional antenna size and photochemistry, as well as the functioning of the entire photosynthetic electron transport chain. The mutants tended to recover a wild-type-like chlorophyll content, and mature plants could hardly be distinguished based on their effective PSII antenna size. Nevertheless, specific OJIP-derived parameters were strongly correlated with the phenotype severity, in particular the amplitude of the I-P phase and the I-P/J-P amplitude ratio, which are indicative of a more capacitive pool of PSI final electron acceptors (ferredoxin and ferredoxin-NADP+ oxidoreductase, FNR). We propose that the enlargement of such pool of electron carriers is a compensatory response operating at the acceptor side of PSI to alleviate potentially harmful over-reduced states of PSI. Our results also suggest that, in chlorophyll-deficient mutants, higher F V / F M cannot prove a superior PSII photochemistry and wider I-P phase is not indicative of a higher relative content of PSI. • Mature wheat mutants have WT-like PSII functional antenna size. • Higher F V /F M does not indicate a superior PSII activity or biomass productivity. • In mutants, electrons flow faster to the end PSI acceptors. • A more capacitive PSI end acceptors pool can help alleviate PSI acceptor-side limitation. • An enlarged PSI end acceptor pool can confer resistance to fluctuating light. [ABSTRACT FROM AUTHOR]

To assess the roles of the cyclic electron flow around PSI (CEF-PSI) and O2-dependent alternative pathways including the water–water cycle in fluctuating light, we grew the wild type and pgr5 mutant of Arabidopsis thaliana in constant light, and measured Chl fluorescence and P700 parameters in their leaves in the fluctuating light alternating between 240 (HL) and 30 µmol photons m−2 s−2 (LL) every 2 min. At 20% O2, the photochemical quantum yield of PSII decreased, in particular in the pgr5 plants, soon after the start of the fluctuating light treatment. PSI of the pgr5 plants was markedly photoinhibited by this treatment for 42 min. Slight PSI photoinhibition was also observed in the wild type. We measured energy sharing between PSII and PSI and estimated the PSI and PSII electron transport rates (ETRs). pgr5 showed larger energy allocation to PSI. In contrast to the wild type, the ratio of the PSI to the PSII ETR in pgr5 was higher in LL but lower in HL at 20% O2 due to PSI acceptor-side limitation. At 2.7% or 0% O2, the CEF-PSI of the pgr5 plants was enhanced, the acceptor-side limitation of PSI electron flow was released and PSI photoinhibition was not observed. The results suggest that the light fluctuation is a potent stress to PSI and that the CEF-PSI is essential to protect PSI from this stress. [ABSTRACT FROM AUTHOR]

Photosynthetic induction after a sudden increase in illumination affects carbon gain. Photosynthetic dynamics under fluctuating light (FL) have been widely investigated in C3 and C4 plants but are little known in CAM plants. In our present study, the chlorophyll fluorescence, P700 redox state and electrochromic shift signals were measured to examine photosynthetic characteristics under FL in the CAM orchid Vanilla planifolia. The light use efficiency was maximized in the morning but was restricted in the afternoon, indicating that the pool of malic acid dried down in the afternoon. During photosynthetic induction in the morning, electron flow through photosystem I rapidly reached the 95% of the maximum value in 4–6 min, indicating that V. planifolia showed a fast photosynthetic induction when compared with C3 and C4 plants reported previously. Upon a sudden transition from dark to actinic light, a rapid re-oxidation of P700 was observed in V. planifolia , indicating the fast outflow of electrons from PSI to alternative electron acceptors, which was attributed to the O 2 photo-reduction mediated by water-water cycle. The functioning of water-water cycle prevented photosystem I over-reduction after transitioning from low to high light and thus protected PSI under FL. In the afternoon, cyclic electron flow was stimulated under FL to fine-tune photosynthetic apparatus when photosynthetic CO 2 was restricted. Therefore, water-water cycle cooperates with cyclic electron flow to regulate the photosynthesis under FL in the CAM orchid V. planifolia. • We examine diurnal photosynthesis in the CAM plant Vanilla planifolia. • Vanilla planifolia shows rapid speed of photosynthetic induction in the morning. • Photosynthetic light use efficiency is largely restricted in the afternoon. • WWC is operational to protect PSI under fluctuating light. • WWC and CEF around PSI cooperate to regulate diurnal photosynthesis. [ABSTRACT FROM AUTHOR]

Fluctuating light is a typical light condition in nature and can cause selective photodamage to photosystem I (PSI). The sensitivity of PSI to fluctuating light is influenced by the amplitude of low/high light intensity. Tobacco mature leaves are tended to be horizontal to maximize the light absorption and photosynthesis, but young leaves are usually vertical to diminish the light absorption. Therefore, we tested the hypothesis that such regulation of the leaf angle in young leaves might protect PSI against photoinhibition under fluctuating light. We found that, upon a sudden increase in illumination, PSI was over-reduced in extreme young leaves but was oxidized in mature leaves. After fluctuating light treatment, such PSI over-reduction aggravated PSI photoinhibition in young leaves. Furthermore, the leaf angle was tightly correlated to the extent of PSI photoinhibition induced by fluctuating light. Therefore, vertical young leaves are more susceptible to PSI photoinhibition than horizontal mature leaves when exposed to the same fluctuating light. In young leaves, the vertical leaf angle decreased the light absorption and thus lowered the amplitude of low/high light intensity. Therefore, the regulation of the leaf angle was found for the first time as an important strategy used by young leaves to protect PSI against photoinhibition under fluctuating light. To our knowledge, we show here new insight into the photoprotection for PSI under fluctuating light in nature. [ABSTRACT FROM AUTHOR]

• We examine the roles of CEF and pseudo-CEF in Ginkgo biloba. • CEF helps ΔpH formation and protects PSI and PSII under constant high light. • Pseudo-CEF protects PSI under fluctuating light at the acceptor side. • CEF and pseudo-CEF cooperate to cope with excess light energy. Ginkgo biloba L., the only surviving species of Ginkgoopsida, is a famous relict gymnosperm, it may provide new insight into the evolution of photosynthetic mechanisms. Flavodiiron proteins (FDPs) are conserved in nonflowering plants, but the role of FDPs in gymnosperms has not yet been clarified. In particular, how gymnosperms integrate FDPs and cyclic electron transport (CET) to better adapt to excess light is poorly understood. To elucidate these questions, we measured the P700 signal, chlorophyll fluorescence and electrochromic shift signal under fluctuating and constant light in G. biloba. Within the first seconds after light increased, G. biloba could not build up a sufficient proton gradient (ΔpH). Concomitantly, photo-reduction of O 2 mediated by FDPs contributed to the rapid oxidation of P700 and protected PSI under fluctuating light. Therefore, in G. biloba , FDPs mainly protect PSI under fluctuating light at acceptor side. Under constant high light, the oxidation of PSI and the induction of non-photochemical quenching were attributed to the increase in ΔpH formation, which was mainly caused by the increase in CET rather than linear electron transport. Therefore, under constant light, CET finely regulates the PSI redox state and non-photochemical quenching through ΔpH formation, protecting PSI and PSII against excess light. We conclude that, in G. biloba , FDPs are particularly important under fluctuating light while CET is essential under constant high light. The coordination of FDPs and CET fine-tune photosynthetic apparatus under excess light. [ABSTRACT FROM AUTHOR]

Upon a sudden transition from low to high light, electrons transported from photosystem II (PSII) to PSI should be rapidly consumed by downstream sinks to avoid the over-reduction of PSI. However, the over-reduction of PSI under fluctuating light might be accelerated if primary metabolism is restricted by low stomatal conductance. To test this hypothesis, we measured the effect of diurnal changes in stomatal conductance on photosynthetic regulation under fluctuating light in tomato (Solanum lycopersicum) and common mulberry (Morus alba). Under conditions of high stomatal conductance, we observed PSI over-reduction within the first 10 s after transition from low to high light. Lower stomatal conductance limited the activity of the Calvin–Benson–Bassham cycle and aggravated PSI over-reduction within 10 s after the light transition. We also observed PSI over-reduction after transition from low to high light for 30 s at the low stomatal conductance typical of the late afternoon, indicating that low stomatal conductance extends the period of PSI over-reduction under fluctuating light. Therefore, diurnal changes in stomatal conductance significantly affect the PSI redox state under fluctuating light. Moreover, our analysis revealed an unexpected inhibition of cyclic electron flow by the severe over-reduction of PSI seen at low stomatal conductance. In conclusion, stomatal conductance can have a large effect on thylakoid reactions under fluctuating light. [ABSTRACT FROM AUTHOR]

Fluctuating light can cause selective photoinhibition of photosystem I (PSI) in angiosperms. Cyclic electron flow (CEF) around PSI and electron flux from water via the electron transport chain to oxygen (the water-water cycle) play important roles in coping with fluctuating light in angiosperms. However, it is unclear whether plant species in the same genus employ the same strategy to cope with fluctuating light. To answer this question, we measured P700 redox kinetics and chlorophyll fluorescence under fluctuating light in two Paphiopedilum (P.) Pftzer (Orchidaceae) species, P. dianthum and P. micranthum. After transition from dark to high light, P. dianthum displayed a rapid re-oxidation of P700, while P. micranthum displayed an over-reduction of P700. Furthermore, the rapid re-oxidation of P700 in P. dianthum was not observed when measured under anaerobic conditions. These results indicated that photo-reduction of O2 mediated by the water-water cycle was functional in P. dianthum but not in P. micranthum. Within the first few seconds after an abrupt transition from low to high light, PSI was highly oxidized in P. dianthum but was highly reduced in P. micranthum, indicating that the different responses of PSI to fluctuating light between P. micranthum and P. dianthum was attributed to the water-water cycle. In P. micranthum, the lack of the water-water cycle was partially compensated for by an enhancement of CEF. Taken together, P. dianthum and P. micranthum employed different strategies to cope with the abrupt change of light intensity, indicating the diversity of strategies for photosynthetic acclimation to fluctuating light in these two closely related orchid species. [ABSTRACT FROM AUTHOR]

• D. officinale shows a rapid oxidation of P700 upon dark-to-light transition. • D. officinale shows a rapid oxidation of PSI upon an increase in illumination. • Water-water cycle leads to the rapid oxidation of PSI in D. officinale. • WWC is more effective in protecting PSI under fluctuating light than CET. • Sensitivity of PSI to fluctuating light can be altered by WWC activity. Fluctuating light can cause selective photoinhibition of photosystem I (PSI) in angiosperms. However, the strategies for photosynthetic regulation under fluctuating light in wild epiphytic plants are little known. We here compared PSI and PSII performances under fluctuating light in the epiphytic orchid Dendrobium officinale and the terrestrial orchid Bletilla striata. Upon dark-to-light transition, D. officinale showed a rapid re-oxidation of P700, but this rapid re-oxidation of P700 was not observed in B. striata. Within the first 40 s after transition from dark to actinic light, B. striata showed much higher activation of cyclic electron transport (CET) than D. officinale. However, this strong activation of CET could not prevent the over-reduction of PSI. Therefore, the rapid re-oxidation of P700 in D. officinale was attributed to pseudo-cyclic electron transport (water-water cycle). Within the first 60 s after transition from low to high light, PSI was highly oxidized in D. officinale but was highly reduced in B. striata. Concomitantly, B. striata showed much higher stimulation of CET. These results indicate that water-water cycle significantly regulates PSI redox state under fluctuating light in the epiphytic orchid D. officinale. By comparison, the terrestrial orchid B. striata employs CET to cope with fluctuating light, similar to the phenotype of Arabidopsis thaliana. Taking together, these two orchids used different strategies for photosynthetic regulation to cope with fluctuating light. [ABSTRACT FROM AUTHOR]

• A disturbance in photosynthetic electron flow affects the chlorophyll-depleted wheat mutants. • Unexpectedly long-term acclimation to light fluctuations can be allowed in mutants. • Successful development of a fluctuating-light phenotype depends on the mutated locus. • Compensation of the mutation involves use of alternative electron sinks and thylakoid structural re-organization. The search for more productive crops is exploring accessions with reduced chlorophyll accumulation in leaves, which would promote plant growth mainly because of a higher light transmittance throughout the canopy. Under continuous light, chlorophyll-depleted chlorina mutants of wheat can reach yields similar to WT; however, their performance under fluctuating light could possibly be lowered by a disturbed photosynthetic electron transport. Six chlorina mutants of Triticum aestivum (ANBW4A, ANBW4B, ANK32A) or T. durum (ANDW7A, ANDW7B, ANDW8A) were compared to WT genotypes under continuous and fluctuating light regimes, the latter obtained through a randomized pattern of light intensity changes. After two weeks of plant acclimation under either regimes, light energy management was thoroughly analysed for four weeks. All chlorina mutants showed a defective ability to regulate the electron poise during a fast rise in irradiance. Different extent of deregulation depended on mutation and genomic background, durum wheat being more severely impaired than bread wheat. However, a great acclimative capacity to a fluctuating light regime unexpectedly still characterized the chlorina wheat plants, with the only exception of the mutants carrying the cn-B1b mutated locus (ANDW7B and ANBW4B). Under fluctuating light, all other chlorina mutants developed indeed an ability to improve their control of electron transport against the recurrent reducing bursts caused by lightflecks. Compensatory responses included the regulation of energy distribution between photosystems, the use of alternative electron sinks, and a structural reorganization of the thylakoid system. However, in no case a reduced chlorophyll content led to more productive plants. Among 23 measured photosynthetic parameters, especially the non-regulatory energy dissipation in photosystem II - Y(NO) –, when probed during a light rise, can be proposed in automated phenotyping experiments as a straightforward index to be used for chlorina wheat screening with respect to potential productivity. [ABSTRACT FROM AUTHOR]

Photosynthetic electron flux from water via photosystem II (PSII) and PSI to oxygen (water-water cycle) may act as an alternative electron sink under fluctuating light in angiosperms. We measured the P700 redox kinetics and electrochromic shift signal under fluctuating light in 11 Camellia species and tobacco leaves. Upon dark-to-light transition, these Camellia species showed rapid re-oxidation of P700. However, this rapid re-oxidation of P700 was not observed when measured under anaerobic conditions, as was in experiment with tobacco performed under aerobic conditions. Therefore, photo-reduction of O 2 mediated by water-water cycle was functional in these Camellia species but not in tobacco. Within the first 10 s after transition from low to high light, PSI was highly oxidized in these Camellia species but was over-reduced in tobacco leaves. Furthermore, such rapid oxidation of PSI in these Camellia species was independent of the formation of trans-thylakoid proton gradient (ΔpH). These results indicated that in addition to ΔpH-dependent photosynthetic control, the water-water cycle can protect PSI against photoinhibition under fluctuating light in these Camellia species. We here propose that the water-water cycle is an overlooked strategy for photosynthetic regulation under fluctuating light in angiosperms. • We compare photosynthetic regulation in eleven Camellia species and tobacco. • CET cannot prevent over-reduction of PSI under fluctuating light in tobacco. • WWC regulates PSI redox kinetics upon dark-to-light transition in Camellia species. • WWC regulates PSI redox state under fluctuating light in Camellia species. • WWC is more effective in protecting PSI under fluctuating light than CET. [ABSTRACT FROM AUTHOR]

The capacity of photoautotrophs to fix carbon depends on the efficiency of the conversion of light energy into chemical potential by photosynthesis. In nature, light input into photosynthesis can change very rapidly and dramatically. To analyze how genetic variation in Arabidopsis thaliana affects photosynthesis and growth under dynamic light conditions, 36 randomly chosen natural accessions were grown under uniform and fluctuating light intensities. After 14 days of growth under uniform or fluctuating light regimes, maximum photosystem II quantum efficiency (Fv/Fm) was determined, photosystem II operating efficiency (ΦPSII) and non-photochemical quenching (NPQ) were measured in low light, and projected leaf area (PLA) as well as the number of visible leaves were estimated. Our data show that ΦPSII and PLA were decreased and NPQ was increased, while Fv/Fm and number of visible leaves were unaffected, in most accessions grown under fluctuating compared to uniform light. There were large changes between accessions for most of these parameters, which, however, were not correlated with genomic variation. Fast growing accessions under uniform light showed the largest growth reductions under fluctuating light, which correlated strongly with a reduction in ΦPSII, suggesting that, under fluctuating light, photosynthesis controls growth and not vice versa. [ABSTRACT FROM AUTHOR]

In forests, understory plants are usually exposed to sunflecks on timescales of seconds or minutes. However, it is unclear how understory plants acclimate to fluctuating light. In this study, we compared chlorophyll fluorescence, PSI redox state and the electrochromic shift signal under fluctuating light between an understory plant Paris polyphylla (Liliaceae) and a light-demanding plant Bletilla striata (Orchidaceae). Within the first seconds after transition from low to high light, PSI was highly oxidized in P. polyphylla but was highly reduced in B. striata , although both species could not generate a sufficient trans-thylakoid proton gradient (ΔpH). Furthermore, the outflow of electrons from PSI to O 2 was not significant in P. polyphylla , as indicated by the P700 redox kinetics upon dark-to-light transition. Therefore, the different responses of PSI to fluctuating light between P. polyphylla and B. striata could not be explained by ΔpH formation or alternative electron transport. In contrast, upon a sudden transition from low to high light, electron flow from PSII was much lower in P. polyphylla than in B. striata , suggesting that the rapid oxidation of PSI in P. polyphylla was largely attributed to the lower PSII activity. We propose, for the first time, that down-regulation of PSII activity is an important strategy used by some understory angiosperms to cope with sunflecks. • PSI is highly oxidized in P. polyphylla upon a sudden transition from low to high light. • The rapid oxidation of PSI in P. polyphylla is not caused by an enough ΔpH. • Alternative electron flow from PSI to O 2 is not significant in P. polyphylla. • P. polyphylla shows low rate of electron flow from PSII under high light. • Down-regulation of PSII activity protects PSI under sunflecks in P. polyphylla. [ABSTRACT FROM AUTHOR]

• We examine the changes in PSI redox state and CEF under fluctuating light. • PSI is highly reduced within the first seconds after an increase in light intensity. • CEF is highly stimulated upon a sudden transition from low to high light. • Over-reduction of PSI stimulates the CEF activation under fluctuating light. • Stimulation of CEF alleviates PSI photoinhibiton at donor and acceptor sides. In angiosperms, cyclic electron flow (CEF) around photosystem I (PSI) is more important for photoprotection under fluctuating light than under constant light. However, the underlying mechanism is not well known. In the present study, we measured the CEF activity, P700 redox state and electrochromic shift signal upon a sudden transition from low to high light in wild-type plants of Arabidopsis thaliana and Bletilla striata (Orchidaceae). Within the first 20 s after transition from low to high light, P700 was highly reduced in both species, which was accompanied with a sufficient proton gradient (ΔpH) across the thylakoid membranes. Meanwhile, the level of CEF activation was elevated. After transition from low to high light for 60 s, the plants generated an optimal ΔpH. Under such condition, PSI was highly oxidized and the level of CEF activation decreased to the steady state. Furthermore, the CEF activation was positively correlated to the P700 reduction ratio. These results indicated that upon a sudden transition from low to high light, the insufficient ΔpH led to the over-reduction of PSI electron carriers, which in turn stimulated the CEF around PSI. This transient stimulation of CEF not only favored the rapid ΔpH formation but also accepted electrons from PSI, thus protecting PSI at donor and acceptor sides. These findings provide new insights into the important role of CEF in regulation of photosynthesis under fluctuating light. [ABSTRACT FROM AUTHOR]