Recent progress in the studies of structure and function of photosystem II

Lithium ore tailings harm the vegetative development, photosynthetic activity, and nutrition of tree species

Lithium (Li) exploitation promotes socioeconomic advances but may result in harmful environmental impacts. Thus, species selection for recovering environments degraded by Li mining is essential. We investigated the tolerance and early growth of four tree species to Li ore tailings (LOT), Enterolobium contortisiliquum and Handroanthus impetiginosus with wide geographic distribution and Hymenaea courbaril and H. stigonocarpa with restricted geographic distribution. The plants grew in LOT and soil for 255 days to evaluate photosynthesis, growth, and mineral nutrition. LOT negatively affected species growth, reducing the length of stems, roots, and biomass through structural and nutritional impoverishment. LOT favored the accumulation of Mg and decreased the absorption of K. The species presented a reduction in potential quantum efficiency and the chlorophyll index (b and total). E. contortisiliquum was the least tolerant species to LOT, and H. courbaril and H. stigonocarpa maintained their mass production in LOT, indicating greater tolerance to tailings. Furthermore, H. courbaril presented a translocation factor > 1 for Li and Mn, indicating the potential for phytoextraction of these metals. Our results offer first-time insights into the impacts of LOT on the early development of tree species with different geographic distribution ranges. This study may help in the tree species selection with a phytoremediation role, aiming at the recovery of areas affected by Li's mining activity.

Electron Transport in Chloroplasts: Regulation and Alternative Pathways of Electron Transfer

This work represents an overview of electron transport regulation in chloroplasts as considered in the context of structure-function organization of photosynthetic apparatus in plants. Main focus of the article is on bifurcated oxidation of plastoquinol by the cytochrome b6f complex, which represents the rate-limiting step of electron transfer between photosystems II and I. Electron transport along the chains of non-cyclic, cyclic, and pseudocyclic electron flow, their relationships to generation of the trans-thylakoid difference in electrochemical potentials of protons in chloroplasts, and pH-dependent mechanisms of regulation of the cytochrome b6f complex are considered. Redox reactions with participation of molecular oxygen and ascorbate, alternative mediators of electron transport in chloroplasts, have also been discussed.

Effect of Duration of LED Lighting on Growth, Photosynthesis and Respiration in Lettuce

Parameters of illumination including the spectra, intensity, and photoperiod play an important role in the cultivation of plants under greenhouse conditions, especially for vegetables such as lettuce. We previously showed that illumination by a combination of red, blue, and white LEDs with a high red light intensity, was optimal for lettuce cultivation; however, the effect of the photoperiod on lettuce cultivation was not investigated. In the current work, we investigated the influence of photoperiod on production (total biomass and dry weight) and parameters of photosynthesis, respiration rate, and relative chlorophyll content in lettuce plants. A 16 h (light):8 h (dark) illumination regime was used as the control. In this work, we investigated the effect of photoperiod on total biomass and dry weight production in lettuce plants as well as on photosynthesis, respiration rate and chlorophyll content. A lighting regime 16:8 h (light:dark) was used as control. A shorter photoperiod (8 h) decreased total biomass and dry weight in lettuce, and this effect was related to the suppression of the linear electron flow caused by the decreasing content of chlorophylls and, therefore, light absorption. A longer photoperiod (24 h) increased the total biomass and dry weight, nevertheless an increase in photosynthetic processes, light absorption by leaves and chlorophyll content was not recorded, nor were differences in respiration rate, thus indicating that changes in photosynthesis and respiration are not necessary conditions for stimulating plant production. A simple model to predict plant production was also developed to address the question of whether increasing the duration of illumination stimulates plant production without inducing changes in photosynthesis and respiration. Our results indicate that increasing the duration of illumination can stimulate dry weight accumulation and that this effect can also be induced using the equal total light integrals for day (i.e., this stimulation can be also caused by increasing the light period while decreasing light intensity). Increasing the duration of illumination is therefore an effective approach to stimulating lettuce production under artificial lighting.

Moderate photoinhibition of PSII and oxidation of P700 contribute to chilling tolerance of tropical tree species in subtropics of China

In the subtropics, a few tropical tree species are distributed and planted for ornamental and horticultural purposes; however, the photosynthesis of these species can be impaired by chilling. This study aimed to understand how these species respond to chilling. Light-dependent and CO2 assimilation reactions of six tropical tree species from geographically diverse areas, but grown at a lower subtropical site in China, were monitored during a chilling (≤ 10°C). Chilling induced stomatal and nonstomatal effects and moderate photoinhibition of PSII, with severe effect in Ixora chinensis. Woodfordia fruticosa was little affected by chilling, with negligible reduction of photosynthesis and PSII activity, higher cyclic electron flow (CEF), and oxidation state of P700 (P700+). Photoinhibition of PSII thus reduced electron flow to P700, while active CEF reduced oxidative damage of PSI and maintained photosynthesis during chilling. Studied parameters revealed that coupling between light-dependent and CO2 assimilation reactions was enhanced under chilling.

Upregulation of the mitochondrial alternative oxidase pathway improves PSII function and photosynthetic electron transport in tomato seedlings under chilling stress

The aim of this study was to explore how the mitochondrial alternative oxidase (AOX) pathway alleviates photoinhibition in chilled tomato (Solanum lycopersicum) seedlings. Chilling induced photoinhibition in tomato seedlings despite the increases in thermal energy dissipation and cyclic electron flow around PSI (CEF-PSI). Chilling inhibited the function of PSII and blocked electron transport at the PSII acceptor side, however, it did not affect the oxygen-evolving complex on the donor side of PSII. Upregulation of the AOX pathway protects against photoinhibition by improving PSII function and photosynthetic electron transport in tomato seedlings under chilling stress. The AOX pathway maintained the open state of PSII and the stability of the entire photosynthetic electron transport chain. Moreover, the protective role of the AOX pathway on PSII was more important than that on PSI. However, inhibition of the AOX pathway could be compensated by increasing CEF-PSI activity under chilling stress.

Ratio of Intensities of Blue and Red Light at Cultivation Influences Photosynthetic Light Reactions, Respiration, Growth, and Reflectance Indices in Lettuce

LED illumination can have a narrow spectral band; its intensity and time regime are regulated within a wide range. These characteristics are the potential basis for the use of a combination of LEDs for plant cultivation because light is the energy source that is used by plants as well as the regulator of photosynthesis, and the regulator of other physiological processes (e.g., plant development), and can cause plant damage under certain stress conditions. As a result, analyzing the influence of light spectra on physiological and growth characteristics during cultivation of different plant species is an important problem. In the present work, we investigated the influence of two variants of LED illumination (red light at an increased intensity, the "red" variant, and blue light at an increased intensity, the "blue" variant) on the parameters of photosynthetic dark and light reactions, respiration rate, leaf reflectance indices, and biomass, among other factors in lettuce (Lactuca sativa L.). The same light intensity (about 180 µmol m-2s-1) was used in both variants. It was shown that the blue illumination variant increased the dark respiration rate (35-130%) and cyclic electron flow around photosystem I (18-26% at the maximal intensity of the actinic light) in comparison to the red variant; the effects were dependent on the duration of cultivation. In contrast, the blue variant decreased the rate of the photosynthetic linear electron flow (13-26%) and various plant growth parameters, such as final biomass (about 40%). Some reflectance indices (e.g., the Zarco-Tejada and Miller Index, an index that is related to the core sizes and light-harvesting complex of photosystem I), were also strongly dependent on the illumination variant. Thus, our results show that the red illumination variant contributes a great deal to lettuce growth; in contrast, the blue variant contributes to stress changes, including the activation of cyclic electron flow around photosystem I.

Melatonin improves the photosynthesis in Dracocephalum kotschyi under salinity stress in a Ca2+/CaM-dependent manner

This study investigated: (1) the effects of various concentrations of melatonin (MT) and Ca2+; and (2) the impact of crosstalk between these signal molecules on photosynthesis and salt tolerance of Dracocephalum kotschyi Boiss. Results indicated that 5mM CaCl2, as well as 100μM MT were the best concentrations for increasing shoot dry weight, leaf area, SPAD index, maximum quantum efficiency of photosystem II (Fv/Fm), and decreasing malondialdehyde content under salinity stress. The impact of MT on growth and photosynthesis was closely linked to its effect on enhancing antioxidant enzyme activities in leaves. Application of p-chlorophenylalanine, as an inhibitor of MT biosynthesis, negated the impacts of MT on the aforementioned attributes. Salinity and MT boosted cytosolic Ca2+ concentration. Exogenous MT, as well as Ca2+, enhanced tolerance index, membrane stability, leaf area, the content of chlorophyll (Chl) a, Chl b, and carotenoids (Car), Fv/Fm, and stomatal conductance under salinity stress. These impacts of MT were eliminated by applying a calmodulin antagonist, a Ca2+ chelator and a Ca2+ channel blocker. These novel findings indicate that the MT-induced effects on photosynthetic parameters and salt-evoked oxidative stress were mediated through calcium/calmodulin (Ca2+/CaM) signalling.

Structural and Functional Aspects of Electron Transport Thermoregulation and ATP Synthesis in Chloroplasts

The review is focused on analysis of the mechanisms of temperature-dependent regulation of electron transport and ATP synthesis in chloroplasts of higher plants. Importance of photosynthesis thermoregulation is determined by the fact that plants are ectothermic organisms, whose own temperature depends on the ambient temperature. The review discusses the effects of temperature on the following processes in thylakoid membranes: (i) photosystem 2 activity and plastoquinone reduction; (ii) electron transfer from plastoquinol (via the cytochrome b₆f complex and plastocyanin) to photosystem 1; (iii) transmembrane proton transfer; and (iv) ATP synthesis. The data on the relationship between the functional properties of chloroplasts (photosynthetic transfer of electrons and protons, functioning of ATP synthase) and structural characteristics of membrane lipids (fluidity) obtained by electron paramagnetic resonance studies are presented.

Foliar application of nanoparticles mitigates the chilling effect on photosynthesis and photoprotection in sugarcane

Chilling is one of the main abiotic stresses that adversely affect the productivity of sugarcane, in marginal tropical regions where chilling incidence occurs with seasonal changes. However, nanoparticles (NPs) have been tested as a mitigation strategy against diverse abiotic stresses. In this study, NPs such as silicon dioxide (nSiO₂; 5-15 nm), zinc oxide (nZnO; <100 nm), selenium (nSe; 100 mesh), graphene (graphene nanoribbons [GNRs] alkyl functionalized; 2-15 μm × 40-250 nm) were applied as foliar sprays on sugarcane leaves to understand the amelioration effect of NPs against negative impact of chilling stress on photosynthesis and photoprotection. To this end, seedlings of moderately chilling tolerant sugarcane variety Guitang 49 was used for current study and spilt plot was used as statistical design. The changes in the level chilling tolerance after the application of NPs on Guitang 49 were compared with tolerance level of chilling tolerant variety Guitang 28. NPs treatments reduced the adverse effects of chilling by maintaining the maximum photochemical efficiency of PSII (F_v/F_m), maximum photo-oxidizable PSI (P_m), and photosynthetic gas exchange. Furthermore, application of NPs increased the content of light harvesting pigments (chlorophylls and cartinoids) in NPs treated seedlings. Higher carotenoid accumulation in leaves of NPs treated seedlings enhanced the nonphotochemical quenching (NPQ) of PSII. Among the NPs, nSiO₂ showed higher amelioration effects and it can be used alone or in combination with other NPs to mitigate chilling stress in sugarcane.

Simulation of a nonphotochemical quenching in plant leaf under different light intensities

An analysis of photosynthetic response on action of stressors is an important problem, which can be solved by experimental and theoretical methods, including mathematical modeling of photosynthetic processes. The aim of our work was elaboration of a mathematical model, which simulated development of a nonphotochemical quenching under different light conditions. We analyzed two variants of the model: the first variant included a light-induced activation of the electron transport chain; in contrast, the second variant did not describe this activation. Both variants of the model described interactions between transitions from open reaction centers to closed ones (and vice versa) and development of the nonphotochemical quenching. Investigation of both variants of the model showed well qualitative and quantitative accordance between simulated and experimental changes in coefficient of the nophotochemical quenching which were analyzed under different light regimes: (i) the stepped increase of the light intensity without dark intervals between steps, (ii) periodical illuminations by different light intensities with constant durations which were separated by constant dark intervals, and (iii) periodical illuminations by the constant light intensity with different durations which were separated by different dark intervals. Thus, the model can be used for theoretical prediction of stress changes in photosynthesis under fluctuations in light intensity and search of optimal regimes of plant illumination.

Identification and Expression of NAC Transcription Factors of Vaccinium corymbosum L. in Response to Drought Stress

Research Highlights: Phenotypic changes and expression profiles, phylogeny, conserved motifs, and expression correlations of NAC (NAM, ATAF1, ATAF2 and CUC2) transcription factors (TFs) in blueberry genome were detected under drought stress, and the expression patterns and functions of 12 NACs were analyzed. Background and Objectives: Blueberry is an important shrub species with a high level of flavonoids in fruit, which are implicated in a broad range of health benefits. However, the molecular mechanism of this shrub species in response to drought stress still remains elusive. NAC TFs widely participate in stress tolerance in many plant species. The characterization and expression profiles of NAC TFs were analyzed on the basis of genome data in blueberry when subjected to drought stress. Materials and Methods: Combined with the analysis of chlorophyll a fluorescence and endogenous phytohormones, the phenotypic changes of blueberry under drought stress were observed. The phylogenetic tree, conserved motifs, differently expressed genes, and expression correlation were determined by means of multiple bioinformatics analysis. The expression profiles of NACs in different organs were examined and compared through RNA-seq and qRT-PCR assay. Results: The chlorophyll a fluorescence parameters φPo, φEo, φRo, and PIabs of leaves were significantly inhibited under drought stress. ABA (abscisic acid) content noticeably increased over the duration of drought, whereas GA3 (gibberellic acid) and IAA (indole acetic acid) content decreased continuously. A total of 158 NACs were identified in blueberry genome and 62 NACs were differently expressed in leaf and root of blueberry under drought stress. Among them, 14 NACs were significantly correlated with the expression of other NAC genes. Conclusions: Our results revealed the phenotypic changes of this shrub under drought stress and linked them with NAC TFs, which are potentially involved in the process of response to drought stress.

Light acclimation of shade-tolerant and sun-resistant Tradescantia species: photochemical activity of PSII and its sensitivity to heat treatment

In this work, we have compared photosynthetic characteristics of photosystem II (PSII) in Tradescantia leaves of two contrasting ecotypes grown under the low light (LL) and high light (HL) regimes during their entire growth period. Plants of the same genus, T. fluminensis (shade-tolerant) and T. sillamontana (sun-resistant), were cultivated at 50-125 µmol photons m^-2 s^-1 (LL) or at 875-1000 µmol photons m^-2 s^-1 (HL). Analyses of intrinsic PSII efficiency was based on measurements of fast chlorophyll (Chl) a fluorescence kinetics (the OJIP test). The fluorescence parameters F_v/F_m (variable fluorescence) and F₀ (the initial level of fluorescence) in dark-adapted leaves were used to quantify the photochemical properties of PSII. Plants of different ecotypes showed different sustainability with respect to changes in the environmental light intensity and temperature treatment. The sun-resistant species T. sillamontana revealed the tolerance to variations in irradiation intensity, demonstrating constancy of maximum quantum efficiency of PSII upon variations of the growth light. In contrast to T. sillamontana, facultative shade species T. fluminensis demonstrated variability of PSII photochemical activity, depending on the growth light intensity. The susceptibility of T. fluminensis to solar stress was documented by a decrease in F_v/F_m and a rise of F₀ during the long-term exposition of T. fluminensis to HL, indicating the loss of photochemical activity of PSII. The short-term (10 min) heat treatment of leaf cuttings caused inactivation of PSII. The temperature-dependent heating effects were different in T. fluminensis and T. sillamontana. Sun-resistant plants T. sillamontana acclimated to LL and HL displayed the same plots of F_v/F_m versus the treatment temperature (t), demonstrating a decrease in F_v/F_m at t ≥ 45 °C. The leaves of shadow-tolerant species T. fluminensis grown under the LL and HL conditions revealed different sensitivities to heat treatment. Plants grown under the solar stress conditions (HL) demonstrated a gradual decline of F_v/F_m at lower heating temperatures (t ≥ 25 °C), indicating the "fragility" of their PSII as compared to T. fluminensis grown at LL. Different responses of sun and shadow species of Tradescantia to growth light and heat treatment are discussed in the context of their biochemical and ecophysiological properties.

Mitigation of drought-induced oxidative damage by enhanced carbon assimilation and an efficient antioxidative metabolism under high CO2 environment in pigeonpea (Cajanus cajan L.)

In the current study, pigeonpea (Cajanus cajan L.), a promising legume food crop was assessed for its photosynthetic physiology, antioxidative system as well as C and N metabolism under elevated CO₂ and combined drought stress (DS). Pigeonpea was grown in open top chambers under elevated CO₂ (600 µmol mol^-1) and ambient CO₂ (390 ± 20 µmol mol^-1) concentrations, later subjected to DS by complete water withholding. The DS plants were re-watered and recovered (R) to gain normal physiological growth and assessed the recoverable capacity in both elevated and ambient CO₂ concentrations. The elevated CO₂ grown pigeonpea showed greater gas exchange physiology, nodule mass and total dry biomass over ambient CO₂ grown plants under well-watered (WW) and DS conditions albeit a decrease in leaf relative water content (LRWC). Glucose, fructose and sucrose levels were measured to understand the role of hexose to sucrose ratios (H:S) in mediating the drought responses. Free amino acid levels as indicative of N assimilation provided insights into C and N balance under DS and CO₂ interactions. The enzymatic and non-enzymatic antioxidants showed significant upregulation in elevated CO₂ grown plants under DS thereby protecting the plant from oxidative damage caused by the reactive oxygen species. Our results clearly demonstrated the protective role of elevated CO₂ under DS at lower LRWC and gained comparative advantage of mitigating the DS-induced damage over ambient CO₂ grown pigeonpea.

Significance of PGR5-dependent cyclic electron flow for optimizing the rate of ATP synthesis and consumption in Arabidopsis chloroplasts

The proton motive force (PMF) across the chloroplast thylakoid membrane that is generated by electron transport during photosynthesis is the driving force for ATP synthesis in plants. The PMF mainly arises from the oxidation of water in photosystem II and from electron transfer within the cytochrome b₆f complex. There are two electron transfer pathways related to PMF formation: linear electron flow and cyclic electron flow. Proton gradient regulation 5 (PGR5) is a major component of the cyclic electron flow pathway, and the Arabidopsis pgr5 mutant shows a substantial reduction in the PMF. How the PGR5-dependent cyclic electron flow contributes to ATP synthesis has not, however, been fully delineated. In this study, we monitored in vivo ATP levels in Arabidopsis chloroplasts in real time using a genetically encoded bioluminescence-based ATP indicator, Nano-lantern(ATP1). The increase in ATP in the chloroplast stroma of pgr5 leaves upon illumination with actinic light was significantly slower than in wild type, and the decrease in ATP levels when this illumination stopped was significantly faster in pgr5 leaves than in wild type. These results indicated that PGR5-dependent cyclic electron flow around photosystem I helps to sustain the rate of ATP synthesis, which is important for growth under fluctuating light conditions.

Impact of weak water deficit on growth, photosynthetic primary processes and storage processes in pine and spruce seedlings

We investigated the influence of 40 days of drought on growth, storage processes and primary photosynthetic processes in 3-month-old Scots pine and Norway spruce seedlings growing in perlite culture. Water stress significantly affected seedling water status, whereas absolute dry biomass growth was not substantially influenced. Water stress induced an increase in non-structural carbohydrate content (sugars, sugar alcohols, starch) in the aboveground part of pine seedlings in contrast to spruce seedlings. Due to the relatively low content of sugars and sugar alcohols in seedling organs, their expected contribution to osmotic potential changes was quite low. In contrast to biomass accumulation and storage, photosynthetic primary processes were substantially influenced by water shortage. In spruce seedlings, PSII was more sensitive to water stress than PSI. In particular, electron transport in PSI was stable under water stress despite the substantial decrease of electron transport in PSII. The increase in thermal energy dissipation due to enhancement of non-photochemical quenching (NPQ) was evident in both species under water stress. Simultaneously, the yields of non-regulated energy dissipation in PSII were decreased in pine seedlings under drought. A relationship between growth, photosynthetic activities and storage processes is analysed under weak water deficit.

Three phases of energy-dependent induction of [Formula: see text] and Chl a fluorescence in Tradescantia fluminensis leaves

In plants, the short-term regulation (STR, seconds to minute time scale) of photosynthetic apparatus is associated with the energy-dependent control in the chloroplast electron transport, the distribution of light energy between photosystems (PS) II and I, activation/deactivation of the Calvin-Benson cycle (CBC) enzymes, and relocation of chloroplasts within the plant cell. In this work, using a dual-PAM technique for measuring the time-courses of P₇₀₀ photooxidation and Chl a fluorescence, we have investigated the STR events in Tradescantia fluminensis leaves. The comparison of Chl a fluorescence and [Formula: see text] induction allowed us to investigate the contribution of the trans-thylakoid pH difference (ΔpH) to the STR events. Two parameters were used as the indicators of ΔpH generation: pH-dependent component of non-photochemical quenching of Chl a fluorescence, and pH_in-dependent rate of electron transfer from plastoquinol (PQH₂) to [Formula: see text] (via the Cyt b₆f complex and plastocyanin). In dark-adapted leaves, kinetics of [Formula: see text] induction revealed three phases. Initial phase is characterized by rapid electron flow to [Formula: see text] (τ_1/2 ~ 5-10 ms), which is likely related to cyclic electron flow around PSI, while the outflow of electrons from PSI is restricted by slow consumption of NADPH in the CBC. The light-induced generation of ΔpH and activation of the CBC promote photooxidation of P₇₀₀ and concomitant retardation of [Formula: see text] reduction (τ_1/2 ~ 20 ms). Prolonged illumination induces additional slowing down of electron transfer to [Formula: see text] (τ_1/2 ≥ 30-35 ms). The latter effect is not accompanied by changes in the Chl a fluorescence parameters which are sensitive to ΔpH generation. We suggest the tentative explanation of the latter results by the reversal of Q-cycle, which causes the deceleration of PQH₂ oxidation due to the back pressure of stromal reductants.

In vivo regulation of thylakoid proton motive force in immature leaves

In chloroplast, proton motive force (pmf) is critical for ATP synthesis and photoprotection. To prevent photoinhibition of photosynthetic apparatus, proton gradient (ΔpH) across the thylakoid membranes needs to be built up to minimize the production of reactive oxygen species (ROS) in thylakoid membranes. However, the regulation of thylakoid pmf in immature leaves is little known. In this study, we compared photosynthetic electron sinks, P700 redox state, non-photochemical quenching (NPQ), and electrochromic shift (ECS) signal in immature and mature leaves of a cultivar of Camellia. The immature leaves displayed lower linear electron flow and cyclic electron flow, but higher levels of NPQ and P700 oxidation ratio under high light. Meanwhile, we found that pmf and ΔpH were higher in the immature leaves. Furthermore, the immature leaves showed significantly lower thylakoid proton conductivity than mature leaves. These results strongly indicated that immature leaves can build up enough ΔpH by modulating proton efflux from the lumenal side to the stromal side of thylakoid membranes, which is essential to prevent photoinhibition via thermal energy dissipation and photosynthetic control of electron transfer. This study highlights that the activity of chloroplast ATP synthase is a key safety valve for photoprotection in immature leaves.

Expression characterisation of cyclophilin BrROC1 during light treatment and abiotic stresses response in Brassica rapa subsp. rapa 'Tsuda'

ROC1 is a prototypic peptidyl prolyl cis/trans isomerase (PPIase) of the plant cytosol belonging to the large subfamily of cyclophilins that are associated with diverse functions through foldase, scaffolding, chaperoning or other unknown activities. Although many functions of plant cyclophilins have been reported, the molecular basis of stress-responsive expression of plant cyclophilins is still largely unknown. To characterise the roles of BrROC1 during light treatment and their responses in various abiotic stresses, we identified BrROC1 genes and characterised their expression patterns in Brassica rapa subsp. rapa 'Tsuda'. Our results showed that BrROC1 genes are multi-family genes. Transcript level analysis showed BrROC1-2 expressed higher than BrROC1-1 in 0 to 6-day-old seedlings under natural light. Moreover, BrROC1-2 genes were also induced to highly express in the cotyledon, upper hypocotyls and lower hypocotyls of seedlings under UV-A and blue-light treatment. In addition, the transcript level of BrROC1-1 was higher in pigment tissues than that in unpigment tissues (cotyledon and lower hypocotyl) under UV-A and blue-light treatment. Furthermore, when the unpigment epidermis (shaded light) of 2-month-old 'Tsuda' turnip roots was exposed to UV-A light, transcript levels of the BrROC1-1 and BrROC1-2 were significantly increased with time prolongation. These two BrROC1 genes might be involved in UV-A-induced anthocyanin synthesis in the root epidermis of 'Tsuda' turnip, which accumulates high levels of anthocyanin. These two BrROC1 genes were also induced to be regulated by abiotic stresses such as high or low temperature, dehydration, osmotic and salt stresses. Then, the results indicate that BrROC1 genes are involved in light induction response and may play important roles in adaptation of plants to various environmental stresses.

Photosynthetic physiological performance and proteomic profiling of the oleaginous algae Scenedesmus acuminatus reveal the mechanism of lipid accumulation under low and high nitrogen supplies

In this study, we presented cellular morphological changes, time-resolved biochemical composition, photosynthetic performance and proteomic profiling to capture the photosynthetic physiological response of Scenedesmus acuminatus under low nitrogen (3.6 mM NaNO₃, N^-) and high nitrogen supplies (18.0 mM NaNO₃, N⁺). S. acuminatus cells showed extensive lipid accumulation (53.7% of dry weight) and were enriched in long-chain fatty acids (C16 & C18) under low nitrogen supply. The activity of PSII and photosynthetic rate decreases, whereas non-photochemical quenching and dark respiration rates were increased in the N^- group. In addition, the results indicated a redistribution of light excitation energy between PSII and PSI in S. acuminatus exists before lipid accumulation. The iTRAQ results showed that, under high nitrogen supply, protein abundance of the chlorophyll biosynthesis, the Calvin cycle and ribosomal proteins decreased in S. acuminatus. In contrast, proteins associated with the photosynthetic machinery, except for F-type ATPase, were increased in the N⁺ group (N⁺, 3 vs. 9 days and 3 days, N⁺ vs. N^-). Under low nitrogen supply, proteins involved in central carbon metabolism, fatty acid synthesis and branched-chain amino acid metabolism were increased, whereas the abundance of proteins of the photosynthetic machinery had decreased, with exception of PSI (N^-, 3 vs. 9 days and 9 days, N⁺ vs. N^-). Collectively, the current study has provided a basis for the metabolic engineering of S. acuminatus for biofuel production.

Stimulation of cyclic electron flow around PSI as a response to the combined stress of high light and high temperature in grape leaves

Changes in cyclic electron flow (CEF) around PSI activity after exposing grape (Vitis vinifera L.) seedling leaves to the combined stress of high temperature (HT) and high light (HL) were investigated. The PSII potential quantum efficiency (Fv/Fm) decreased significantly under exposure to HT, and this decrease was greater when HT was combined with HL, whereas the PSI activity maintained stable. HT enhanced CEF mediated by NAD(P)H dehydrogenase remarkably. Compared with the control leaves, the half-time of P700+ re-reduction decreased during the HT treatment; this decrease was even more pronounced under the combined stress, implying significantly enhanced CEF as a result of the treatment. However, the heat-induced increase in nonphotochemical quenching (NPQ) was greater under HL, accompanied by a greater enhancement in high-energy state quenching. These results suggest that the combined stress of HT and HL resulted in severe PSII photoinhibition, whereas CEF showed plasticity in its response to environmental stress and played an important role in PSII and PSI photoprotection through accelerating generation of the thylakoid proton gradient and the induction of NPQ.

Leaf acclimation strategies to contrasting light conditions in saplings of different shade tolerance in a tropical cloud forest

To study the acclimation responses of the leaves of saplings of six tree species when changed to low or high levels of irradiance, we carried out a light exposure experiment. Species representative of contrasting shade tolerance groups were identified across a light gradient in the understorey of a Venezuelan Andean cloud forest. Measured traits included gas exchange, chlorophyll fluorescence, and morphoanatomical, biochemical and optical properties. Saplings were grown for 6 months in a shade-house receiving 20% photosynthetic photon flux (PPF) of full sunlight. Plant samples were then moved to shade-houses receiving low PPF (4%) or high PPF (65%). A factorial model (species×PPF), with repeated measurements (0, 15 and 120 days) was designed. Our results showed that morphological and anatomical traits were more plastic to PPF changes than photosynthetic traits. All species were susceptible to photoinhibition (15 days): shade-intolerant species showed dynamic photoinhibition (120 days), whereas shade-tolerant species presented chronic photoinhibition and the consequent inability to increase C assimilation rates under high PPF. The partially shade-tolerant species showed mixed responses; nonetheless, they exhibited larger adjustments in morphoanatomical and optical properties. Thus the acclimation responses of these species when subject to contrasting light conditions could help to explain their distribution along the light gradient in the understorey.

Photoinhibition of photosystem I in Nephrolepis falciformis depends on reactive oxygen species generated in the chloroplast stroma

We studied how high light causes photoinhibition of photosystem I (PSI) in the shade-demanding fern Nephrolepis falciformis, in an attempt to understand the mechanism of PSI photoinhibition under natural field conditions. Intact leaves were treated with constant high light and fluctuating light. Detached leaves were treated with constant high light in the presence and absence of methyl viologen (MV). Chlorophyll fluorescence and P700 signal were determined to estimate photoinhibition. PSI was highly oxidized under high light before treatments. N. falciformis showed significantly stronger photoinhibition of PSI and PSII under constant high light than fluctuating light. These results suggest that high levels of P700 oxidation ratio cannot prevent PSI photoinhibition under high light in N. falciformis. Furthermore, photoinhibition of PSI in N. falciformis was largely accelerated in the presence of MV that promotes the production of superoxide anion radicals in the chloroplast stroma by accepting electrons from PSI. From these results, we propose that photoinhibition of PSI in N. falciformis is mainly caused by superoxide radicals generated in the chloroplast stroma, which is different from the mechanism of PSI photoinhibition in Arabidopsis thaliana and spinach. Here, we provide some new insights into the PSI photoinhibition under natural field conditions.

Wheat plant selection for high yields entailed improvement of leaf anatomical and biochemical traits including tolerance to non-optimal temperature conditions

Assessment of photosynthetic traits and temperature tolerance was performed on field-grown modern genotype (MG), and the local landrace (LR) of wheat (Triticum aestivum L.) as well as the wild relative species (Aegilops cylindrica Host.). The comparison was based on measurements of the gas exchange (A/c_i, light and temperature response curves), slow and fast chlorophyll fluorescence kinetics, and some growth and leaf parameters. In MG, we observed the highest CO₂ assimilation rate [Formula: see text] electron transport rate (J_max) and maximum carboxylation rate [Formula: see text]. The Aegilops leaves had substantially lower values of all photosynthetic parameters; this fact correlated with its lower biomass production. The mesophyll conductance was almost the same in Aegilops and MG, despite the significant differences in leaf phenotype. In contrary, in LR with a higher dry mass per leaf area, the half mesophyll conductance (g_m) values indicated more limited CO₂ diffusion. In Aegilops, we found much lower carboxylation capacity; this can be attributed mainly to thin leaves and lower Rubisco activity. The difference in CO₂ assimilation rate between MG and others was diminished because of its higher mitochondrial respiration activity indicating more intense metabolism. Assessment of temperature response showed lower temperature optimum and a narrow ecological valence (i.e., the range determining the tolerance limits of a species to an environmental factor) in Aegilops. In addition, analysis of photosynthetic thermostability identified the LR as the most sensitive. Our results support the idea that the selection for high yields was accompanied by the increase of photosynthetic productivity through unintentional improvement of leaf anatomical and biochemical traits including tolerance to non-optimal temperature conditions.

Toward Escherichia coli bacteria machine for water oxidation

Nature uses a Mn oxide-based catalyst for water oxidation in plants, algae, and cyanobacteria. Mn oxides are among major candidates to be used as water-oxidizing catalysts. Herein, we used two straightforward and promising methods to form Escherichia coli bacteria/Mn oxide compounds. In one of the methods, the bacteria template was intact after the reaction. The catalysts were characterized by X-ray photoelectron spectroscopy, visible spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, Raman spectroscopy, and X-ray diffraction spectrometry. Electrochemical properties of the catalysts were studied, and attributed redox potentials were assigned. The water oxidation of the compounds was examined under electrochemical condition. Linear sweep voltammetry showed that the onsets of water oxidation in our experimental condition for bacteria and Escherichia coli bacteria/Mn oxide were 1.68 and 1.56 V versus the normal hydrogen electrode (NHE), respectively. Thus, the presence of Mn oxide in the catalyst significantly decreased (~ 120 mV) the overpotential needed for water oxidation.

Influence of the variation potential on photosynthetic flows of light energy and electrons in pea

Local damage (mainly burning, heating, and mechanical wounding) induces propagation of electrical signals, namely, variation potentials, which are important signals during the life of plants that regulate different physiological processes, including photosynthesis. It is known that the variation potential decreases the rate of CO2 assimilation by the Calvin-Benson cycle; however, its influence on light reactions has been poorly investigated. The aim of our work was to investigate the influence of the variation potential on the light energy flow that is absorbed, trapped and dissipated per active reaction centre in photosystem II and on the flow of electrons through the chloroplast electron transport chain. We analysed chlorophyll fluorescence in pea leaves using JIP-test and PAM-fluorometry; we also investigated delayed fluorescence. The electrical signals were registered using extracellular electrodes. We showed that the burning-induced variation potential stimulated a nonphotochemical loss of energy in photosystem II under dark conditions. It was also shown that the variation potential gradually increased the flow of light energy absorbed, trapped and dissipated by photosystem II. These changes were likely caused by an increase in the fraction of absorbed light distributed to photosystem II. In addition, the variation potential induced a transient increase in electron flow through the photosynthetic electron transport chain. Some probable mechanisms for the influence of the variation potential on the light reactions of photosynthesis (including the potential role of intracellular pH decrease) are discussed in the work.

Structural and functional disorder in the photosynthetic apparatus of radish plants under magnesium deficiency

Magnesium (Mg) is one of the significant macronutrients which is involved in the structural stabilisation of plant tissues and many enzymes such as PSII. The latter efficiency and performance were analysed, using chlorophyll (Chl) a fluorescence induction kinetics and microscopic images, to detect the changes in structure and function of photosynthetic apparatus of radish plants grown under Mg deficiency (Mgdef). Plants grown under Mgdef showed less PSII connectivity and fewer active primary electron acceptors (QA) oxidizing reaction centres than control plants. Confocal and electron microscopy analyses showed an increased amount of starch in chloroplasts, and 3,3'-diaminobenzidine (DAB)-uptake method revealed higher H2O2 accumulation under Mgdef. Prominent changes in the Chl a fluorescence parameters such as dissipated energy flux per reaction centre (DIo/RC), relative variable fluorescence at 150μs (Vl), and the sum of the partial driving forces for the events involved in OJIP fluorescence rise (DFabs) were observed under Mg deficiency. The latter also significantly affected some other parameters such as dissipated energy fluxes per cross-section (DIo/CSo), performance index for energy conservation from photons absorbed by PSII antenna until the reduction of PSI acceptors (PItotal), and relative variable fluorescence at 300μs (Vk). This work emphasises the use of chlorophyll fluorescence in combination with microscopic and statistical analyses to diagnose the effects of nutrients deficiency stress on plants at an early stage of its development as demonstrated for the example of Mgdef. Due to the short growth period and simple cultivation conditions of radish plant we recommend it as a new standard (model) plant to study nutrients deficiency and changes in plant photosynthetic efficiency under stress conditions.

The effect of light quality on the pro-/antioxidant balance, activity of photosystem II, and expression of light-dependent genes in Eutrema salsugineum callus cells

The antioxidant balance, photochemical activity of photosystem II (PSII), and photosynthetic pigment content, as well as the expression of genes involved in the light signalling of callus lines of Eutrema salsugineum plants (earlier Thellungiella salsuginea) under different spectral light compositions were studied. Growth of callus in red light (RL, maximum 660 nm), in contrast to blue light (BL, maximum 450 nm), resulted in a lower H₂O₂ content and thiobarbituric acid reactive substances (TBARS). The BL increased the activities of key antioxidant enzymes in comparison with the white light (WL) and RL and demonstrated the minimum level of PSII photochemical activity. The activities of catalase (CAT) and peroxidase (POD) had the highest values in BL, which, along with the increased H₂O₂ and TBARS content, indicate a higher level of oxidative stress in the cells. The expression levels of the main chloroplast protein genes of PSII (PSBA and PSBD), the NADPH-dependent oxidase gene of the plasma membrane (RbohD), the protochlorophyllide oxidoreductase genes (POR B, C) involved in the biosynthesis of chlorophyll, and the key photoreceptor signalling genes (CIB1, CRY2, PhyB, PhyA, and PIF3) were determined. Possible mechanisms of light quality effects on the physiological parameters of callus cells are discussed.

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

Nanolayered manganese oxides: insights from inorganic electrochemistry

The electrochemistry of nanolayered Mn oxides in the presence of LiClO₄ at pH = 6.3 under different conditions was studied.

The effect of lanthanum(III) and cerium(III) ions between layers of manganese oxide on water oxidation

Manganese oxide structure with lanthanum(III) or cerium(III) ions between the layers was synthesized by a simple method. The ratio of Mn to Ce or La in samples was 0.00, 0.04, 0.08, 0.16, 0.32, 0.5, 0.82, or 1.62. The compounds were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction studies, and atomic absorption spectroscopy. The compounds show efficient catalytic activity of water oxidation in the presence of cerium(IV) ammonium nitrate with a turnover frequency of 1.6 mmol O2/mol Mn.s. In contrast to the water-oxidizing complex in Photosystem II, calcium(II) has no specific role to enhance the water-oxidizing activity of the layered manganese oxides and other cations can be replaced without any significant decrease in water-oxidizing activities of these layered Mn oxides. Based on this and previously reported results from oxygen evolution in the presence of H 2 (18) O, we discuss the mechanism and the important factors influencing the water-oxidizing activities of the manganese oxides.

Carbon for engineering of a water-oxidizing catalyst

Herein we report that the reaction of KMnO4 with cobalt nanoparticles coated with multiple graphene layers forms a promising catalyst toward water oxidation. The compound was characterized by scanning electron microscopy, energy-dispersive spectroscopy, high resolution transmission electron microscopy, X-ray diffraction, electronic spectroscopy, Fourier transform infrared spectroscopy, and atomic absorption spectroscopy. In addition to the Mn oxide-based characteristics of the catalyst, it is a conductive, self-healing, recycling, highly dispersible, magnetically separable, environmentally friendly, and nano-sized catalyst for water oxidation. The turnover frequency for the catalyst toward water oxidation is 0.1 and 0.05 (mmol O2 per mol Mn s) in the presence of cerium(iv) ammonium nitrate and photo-produced Ru(bpy)3(3+).

Efficient Photoelectrochemical Energy Conversion using Spinach Photosystem II (PSII) in Lipid Multilayer Films

The need for clean, renewable energy has fostered research into photovoltaic alternatives to silicon solar cells. Pigment–protein complexes in green plants convert light energy into chemical potential using redox processes that produce molecular oxygen. Here, we report the first use of spinach protein photosystem II (PSII) core complex in lipid films in photoelectrochemical devices. Photocurrents were generated from PSII in a ∼2 μm biomimetic dimyristoylphosphatidylcholine (DMPC) film on a pyrolytic graphite (PG) anode with PSII embedded in multiple lipid bilayers. The photocurrent was ∼20 μA cm⁻² under light intensity 40 mW cm⁻². The PSII–DMPC anode was used in a photobiofuel cell with a platinum black mesh cathode in perchloric acid solution to give an output voltage of 0.6 V and a maximum output power of 14 μW cm⁻². Part of this large output is related to a five-unit anode–cathode pH gradient. With catholytes at higher pH or no perchlorate, or using an MnO₂ oxygen-reduction cathode, the power output was smaller. The results described raise the possibility of using PSII–DMPC films in small portable power conversion devices.

Nanolayered manganese oxide/C(60) composite: a good water-oxidizing catalyst for artificial photosynthetic systems

For the first time, we considered Mn oxide/C60 composites as water-oxidizing catalysts. The composites were synthesized by easy and simple procedures, and characterized by some methods. The water-oxidizing activities of these composites were also measured in the presence of cerium(iv) ammonium nitrate. We found that the nanolayered Mn oxide/C60 composites show promising activity toward water oxidation.

Gold nanorods or nanoparticles deposited on layered manganese oxide: new findings

Our results show that nano-sized gold has no significant effect on the water-oxidation activity of the Mn oxide phase in the presence of Ce(iv).

Nano-sized Mn oxides on halloysite or high surface area montmorillonite as efficient catalysts for water oxidation with cerium(iv) ammonium nitrate: support from natural sources

We used halloysite, a nano-sized natural mineral and high surface area montmorillonite as supports for nano-sized Mn oxides to synthesize efficient water-oxidising catalysts.

Photocatalytic H2 evolution from NADH with carbon quantum dots/Pt and 2-phenyl-4-(1-naphthyl)quinolinium ion

Carbon quantum dots (CQDs) were simply blended with platinum salts (K2PtCl4 and K2PtCl6) and converted into a hydrogen-evolution co-catalyst in situ, wherein Pt salts were dispersed on the surface of CQDs under photoirradiation of an aqueous solution of NADH (an electron and proton source) and 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA) employed as an organic photocatalyst. The co-catalyst (CQDs/Pt) exhibits similar catalytic reactivity in H2 evolution as that of pure Pt nanoparticles (PtNPs) although the Pt amount of CQDs/Pt was only 1/200 that of PtNPs previously reported. CQDs were able to capture the Pt salt acting as Pt supports. Meanwhile, CQDs act as electron reservoir, playing an important role to enhance electron transfer from QuPh(+)-NA to the Pt salt, which was confirmed by kinetic studies, XPS and HRTEM.

Mn oxide/nanodiamond composite: a new water-oxidizing catalyst for water oxidation

Herein, we reported nanosized Mn oxide/nanodiamond composites as water-oxidizing compounds.

Nanolayered manganese oxides as water-oxidizing catalysts: the effects of Cu(ii) and Ni(ii) ions

We synthesized nanolayered manganese oxides in the presence of copper(ii) or nickel(ii) ions, and considered the water oxidizing activities of them.

Age-dependent changes in the functions and compositions of photosynthetic complexes in the thylakoid membranes of Arabidopsis thaliana

Photosynthetic complexes in the thylakoid membrane of plant leaves primarily function as energy-harvesting machinery during the growth period. However, leaves undergo developmental and functional transitions along aging and, at the senescence stage, these complexes become major sources for nutrients to be remobilized to other organs such as developing seeds. Here, we investigated age-dependent changes in the functions and compositions of photosynthetic complexes during natural leaf senescence in Arabidopsis thaliana. We found that Chl a/b ratios decreased during the natural leaf senescence along with decrease of the total chlorophyll content. The photosynthetic parameters measured by the chlorophyll fluorescence, photochemical efficiency (F v/F m) of photosystem II, non-photochemical quenching, and the electron transfer rate, showed a differential decline in the senescing part of the leaves. The CO2 assimilation rate and the activity of PSI activity measured from whole senescing leaves remained relatively intact until 28 days of leaf age but declined sharply thereafter. Examination of the behaviors of the individual components in the photosynthetic complex showed that the components on the whole are decreased, but again showed differential decline during leaf senescence. Notably, D1, a PSII reaction center protein, was almost not present but PsaA/B, a PSI reaction center protein is still remained at the senescence stage. Taken together, our results indicate that the compositions and structures of the photosynthetic complexes are differentially utilized at different stages of leaf, but the most dramatic change was observed at the senescence stage, possibly to comply with the physiological states of the senescence process.

Re-evaluation of the side effects of cytochrome b6f inhibitor dibromothymoquinone on photosystem II excitation and electron transfer

Dibromothymoquinone (DBMIB) has been used as a specific inhibitor of plastoquinol oxidation at the Q0 binding site of the cytochrome b6f complex for 40 years. It is thought to suppress electron transfer between photosystem (PS) II and I, as well as cyclic electron transfer around PSI. However, DBMIB has also been reported to act as a quencher of chlorophyll excited states. In this study, we have re-evaluated the effects of DBMIB on chlorophyll excited states and PSII photochemistry. The results show that DBMIB significantly quenches the chlorophyll excited states of PSII antenna even at low concentration (from 0.1 μM), lowering the effective excitation rate of the actinic light. It also acts as a potent PSII electron acceptor retarding the reduction of the plastoquinone pool with almost maximal potency at 2 μM. Altogether, these results suggest that experiments using DBMIB can easily be misinterpreted and stress on the importance of taking into account all these side effects that occur in the same range of DBMIB concentration used for inhibition of plastoquinol oxidation (1 μM).