Action spectra of photosystems II and I and quantum yield of photosynthesis in leaves in State 1

Optimizing photosynthetic light-harvesting under stars: simple and general antenna models

In the next 10-20 years, several observatories will aim to detect the signatures of oxygenic photosynthesis on exoplanets, though targets must be carefully selected. Most known potentially habitable exo-planets orbit cool M-dwarf stars, which have limited emission in the photosynthetically active region of the spectrum (PAR, 400 < λ < 700 nm) used by Earth's oxygenic photoautotrophs. Still, recent experiments have shown that model cyanobacteria, algae, and non-vascular plants grow comfortably under simulated M-dwarf light, though vascular plants struggle. Here, we hypothesize that this is partly due to the different ways they harvest light, reflecting some general rule that determines how photosynthetic antenna structures may evolve under different stars. We construct a simple thermodynamic model of an oxygenic antenna-reaction centre supercomplex and determine the optimum structure, size and absorption spectrum under light from several star types. For the hotter G (e.g. the Sun) and K-stars, a small modular antenna is optimal and qualitatively resembles the PSII-LHCII supercomplex of higher plants. For the cooler M-dwarfs, a very large antenna with a steep 'energy funnel' is required, resembling the cyanobacterial phycobilisome. For the coolest M-dwarfs an upper limit is reached, where increasing antenna size further is subject to steep diminishing returns in photosynthetic output. We conclude that G- and K-stars could support a range of niches for oxygenic photo-autotrophs, including high-light adapted canopy vegetation that may generate detectable bio-signatures. M-dwarfs may only be able to support low light-adapted organisms that have to invest considerable resources in maintaining a large antenna. This may negatively impact global coverage and therefore detectability.

Far-red light enrichment affects gene expression and architecture as well as growth and photosynthesis in rice

Plants use light as a resource and signal. Photons within the 400-700 nm waveband are considered photosynthetically active. Far-red photons (FR, 700-800 nm) are used by plants to detect nearby vegetation and elicit the shade avoidance syndrome. In addition, FR photons have also been shown to contribute to photosynthesis, but knowledge about these dual effects remains scarce. Here, we study shoot-architectural and photosynthetic responses to supplemental FR light during the photoperiod in several rice varieties. We observed that FR enrichment only mildly affected the rice transcriptome and shoot architecture as compared to established model species, whereas leaf formation, tillering and biomass accumulation were clearly promoted. Consistent with this growth promotion, we found that CO2-fixation in supplemental FR was strongly enhanced, especially in plants acclimated to FR-enriched conditions as compared to control conditions. This growth promotion dominates the effects of FR photons on shoot development and architecture. When substituting FR enrichment with an end-of-day FR pulse, this prevented photosynthesis-promoting effects and elicited shade avoidance responses. We conclude that FR photons can have a dual role, where effects depend on the environmental context: in addition to being an environmental signal, they are also a potent source of harvestable energy.

Rational design of ROS scavenging and fluorescent gold nanoparticles to deliver siRNA to improve plant resistance to Pseudomonas syringae

Bacterial diseases are one of the most common issues that result in crop loss worldwide, and the increasing usage of chemical pesticides has caused the occurrence of resistance in pathogenic bacteria and environmental pollution problems. Nanomaterial mediated gene silencing is starting to display powerful efficiency and environmental friendliness for improving plant disease resistance. However, the internalization of nanomaterials and the physiological mechanisms behind nano-improved plant disease resistance are still rarely understood. We engineered the polyethyleneimine (PEI) functionalized gold nanoparticles (PEI-AuNPs) with fluorescent properties and ROS scavenging activity to act as siRNA delivery platforms. Besides the loading, protection, and delivery of nucleic acid molecules in plant mature leaf cells by PEI-AuNPs, its fluorescent property further enables the traceability of the distribution of the loaded nucleic acid molecules in cells. Additionally, the PEI-AuNPs-based RNAi delivery system successfully mediated the silencing of defense-regulated gene AtWRKY1. Compared to control plants, the silenced plants performed better resistance to Pseudomonas syringae, showing a reduced bacterial number, decreased ROS content, increased antioxidant enzyme activities, and improved chlorophyll fluorescence performance. Our results showed the advantages of AuNP-based RNAi technology in improving plant disease resistance, as well as the potential of plant nanobiotechnology to protect agricultural production.

Characterization of the far-red light absorbing light-harvesting chlorophyll a/b binding complex, a derivative of the distinctive Lhca gene family in green algae

Prasiola crispa, an aerial green alga, exhibits remarkable adaptability to the extreme conditions of Antarctica by forming layered colonies capable of utilizing far-red light for photosynthesis. Despite a recent report on the structure of P. crispa's unique light-harvesting chlorophyll (Chl)-binding protein complex (Pc-frLHC), which facilitates far-red light absorption and uphill excitation energy transfer to photosystem II, the specific genes encoding the subunits of Pc-frLHC have not yet been identified. Here, we report a draft genome sequence of P. crispa strain 4113, originally isolated from soil samples on Ongul Island, Antarctica. We obtained a 92 Mbp sequence distributed in 1,045 scaffolds comprising 10,244 genes, reflecting 87.1% of the core eukaryotic gene set. Notably, 26 genes associated with the light-harvesting Chl a/b binding complex (LHC) were identified, including four Pc-frLHC genes, with similarity to a noncanonical Lhca gene with four transmembrane helices, such as Ot_Lhca6 in Ostreococcus tauri and Cr_LHCA2 in Chlamydomonas reinhardtii. A comparative analysis revealed that Pc-frLHC shares homology with certain Lhca genes found in Coccomyxa and Trebouxia species. This similarity indicates that Pc-frLHC has evolved from an ancestral Lhca gene with four transmembrane helices and branched out within the Trebouxiaceae family. Furthermore, RNA-seq analysis conducted during the initiation of Pc-frLHC gene induction under red light illumination indicated that Pc-frLHC genes were induced independently from other genes associated with photosystems or LHCs. Instead, the genes of transcription factors, helicases, chaperones, heat shock proteins, and components of blue light receptors were identified to coexpress with Pc-frLHC. Those kinds of information could provide insights into the expression mechanisms of Pc-frLHC and its evolutional development.

Beyond the visible: Accounting for ultraviolet and far-red radiation in vegetation productivity and surface energy budgets

Photosynthetically active radiation (PAR) is typically defined as light with a wavelength within 400-700 nm. However, ultra-violet (UV) radiation within 280-400 nm and far-red (FR) radiation within 700-750 nm can also excite photosystems, though not as efficiently as PAR. Vegetation and land surface models (LSMs) typically do not explicitly account for UV's contribution to energy budgets or photosynthesis, nor FR's contribution to photosynthesis. However, whether neglecting UV and FR has significant impacts remains unknown. We explored how canopy radiative transfer (RT) and photosynthesis are impacted when explicitly implementing UV in the canopy RT model and accounting for UV and FR in the photosynthesis models within a next-generation LSM that can simulate hyperspectral canopy RT. We validated our improvements using photosynthesis measurements from plants under different light sources and intensities and surface reflection from an eddy-covariance tower. Our model simulations suggested that at the whole plant level, after accounting for UV and FR explicitly, chlorophyll content, leaf area index (LAI), clumping index, and solar radiation all impact the modeling of gross primary productivity (GPP). At the global scale, mean annual GPP within a grid would increase by up to 7.3% and the increase is proportional to LAI; globally integrated GPP increases by 4.6 PgC year-1 (3.8% of the GPP without accounting for UV + FR). Further, using PAR to proxy UV could overestimate surface albedo by more than 0.1, particularly in the boreal forests. Our results highlight the importance of improving UV and FR in canopy RT and photosynthesis modeling and the necessity to implement hyperspectral or multispectral canopy RT schemes in future vegetation and LSMs.

Excitation transfer and quenching in photosystem II, enlightened by carotenoid triplet state in leaves

Accumulation of carotenoid (Car) triplet states was investigated by singlet-triplet annihilation, measured as chlorophyll (Chl) fluorescence quenching in sunflower and lettuce leaves. The leaves were illuminated by Xe flashes of 4 μs length at half-height and 525-565 or 410-490 nm spectral band, maximum intensity 2 mol quanta m-2 s-1, flash photon dose up to 10 μmol m-2 or 4-10 PSII excitations. Superimposed upon the non-photochemically unquenched Fmd state, fluorescence was strongly quenched near the flash maximum (minimum yield Fe), but returned to the Fmd level after 30-50 μs. The fraction of PSII containing a 3Car in equilibrium with singlet excitation was calculated as Te = (Fmd-Fe)/Fmd. Light dependence of Te was a rectangular hyperbola, whose initial slope and plateau were determined by the quantum yields of triplet formation and annihilation and by the triplet lifetime. The intrinsic lifetime was 9 μs, but it was strongly shortened by the presence of O2. The triplet yield was 0.66 without nonphotochemical quenching (NPQ) but approached zero when NP-Quenched fluorescence approached 0.2 Fmd. The results show that in the Fmd state a light-adapted charge-separated PSIIL state is formed (Sipka et al., The Plant Cell 33:1286-1302, 2021) in which Pheo-P680+ radical pair formation is hindered, and excitation is terminated in the antenna by 3Car formation. The results confirm that there is no excitonic connectivity between PSII units. In the PSIIL state each PSII is individually turned into the NPQ state, where excess excitation is quenched in the antenna without 3Car formation.

Far-red light modulates grapevine growth by increasing leaf photosynthesis efficiency and triggering organ-specific transcriptome remodelling : Author

BACKGROUND

Growing evidence demonstrates that the synergistic interaction of far-red light with shorter wavelength lights could evidently improve the photosynthesis efficiency of multiple species. However, whether/how far-red light affects sink organs and consequently modulates the source‒sink relationships are largely unknown.

RESULTS

Here, equal intensities of white and far-red lights were added to natural light for grape plantlets to investigate the effects of far-red light supplementation on grapevine growth and carbon assimilate allocation, as well as to reveal the underlying mechanisms, through physiological and transcriptomic analysis. The results showed that additional far-red light increased stem length and carbohydrate contents in multiple organs and decreased leaf area, specific leaf weight and dry weight of leaves in comparison with their counterparts grown under white light. Compared to white light, the maximum net photosynthetic rate of the leaves was increased by 31.72% by far-red light supplementation, indicating that far-red light indeed elevated the photosynthesis efficiency of grapes. Transcriptome analysis revealed that leaves were most responsive to far-red light, followed by sink organs, including stems and roots. Genes related to light signaling and carbon metabolites were tightly correlated with variations in the aforementioned physiological traits. In particular, VvLHCB1 is involved in light harvesting and restoring the balance of photosystem I and photosystem II excitation, and VvCOP1 and VvPIF3, which regulate light signal transduction, were upregulated under far-red conditions. In addition, the transcript abundances of the sugar transporter-encoding genes VvSWEET1 and VvSWEET3 and the carbon metabolite-encoding genes VvG6PD, VvSUS7 and VvPGAM varied in line with the change in sugar content.

CONCLUSIONS

This study showed that far-red light synergistically functioning with white light has a beneficial effect on grape photosystem activity and is able to differentially affect the growth of sink organs, providing evidence for the possible addition of far-red light to the wavelength range of photosynthetically active radiation (PAR).

Significance of moss pretreatments in active biomonitoring surveys

The present study examines the impact of pretreatment procedures on the metal concentrations in bags that are to be exposed. We examine Mn, Fe, Cu, Zn, Cd, and Pb amounts in Sphagnum fallax and Dicranum polysetum mosses using atomic absorption spectrometry. The concentration of Hg was also determined using a mercury analyzer. Two sample preparation ways were tested (with and without rinsing) and their influence was evaluated by determining the coefficient of variation (CV). Chlorophyll content was also determined in mosses collected from three habitats (deep woodland, forest road, and wood lot). The results indicate, that the concentration of elements deposited in mosses depends on the species and the habitat where they were collected (ANOVA, p < 0.001). Rinsing of mosses reduces the CV for Mn, Fe, Cu, and Zn and uniform the material prior to exposure (CV for the majority of metals <10%). Selected correlations were found for element concentrations with chlorophyll content. Photosynthetic activity of mosses decreased by about 80% during their one-month storage in the laboratory. Due to the varying concentration of metals in the collected samples, proper, and standardized preparation of mosses before exposure, they can be effectively used in active biomonitoring.

Supplementary Low Far-Red Light Promotes Proliferation and Photosynthetic Capacity of Blueberry In Vitro Plantlets

Far-red light exerts an important regulatory influence on plant growth and development. However, the mechanisms underlying far-red light regulation of morphogenesis and photosynthetic characteristics in blueberry plantlets in vitro have remained elusive. Here, physiological and transcriptomic analyses were conducted on blueberry plantlets in vitro supplemented with far-red light. The results indicated that supplementation with low far-red light, such as 6 μmol m-2 s-1 and 14 μmol m-2 s-1 far-red (6FR and 14FR) light treatments, significantly increased proliferation-related indicators, including shoot length, shoot number, gibberellin A3, and trans-zeatin riboside content. It was found that 6FR and 14 FR significantly reduced chlorophyll content in blueberry plantlets but enhanced electron transport rates. Weighted correlation network analysis (WGCNA) showed the enrichment of iron ion-related genes in modules associated with photosynthesis. Genes such as NAC, ABCG11, GASA1, and Erf74 were significantly enriched within the proliferation-related module. Taken together, we conclude that low far-red light can promote the proliferative capacity of blueberry plantlets in vitro by affecting hormone pathways and the formation of secondary cell walls, concurrently regulating chlorophyll content and iron ion homeostasis to affect photosynthetic capacity.

Growth Light Quality Influences Leaf Surface Temperature by Regulating the Rate of Non-Photochemical Quenching Thermal Dissipation and Stomatal Conductance

Significant efforts have been made to optimise spectrum quality in indoor farming to maximise artificial light utilisation and reduce water loss. For such an improvement, green (G) light supplementation to a red-blue (RB) background was successfully employed in our previous studies to restrict both non-photochemical quenching (NPQ) and stomatal conductance (gs). At the same time, however, the downregulation of NPQ and gs had the opposite influence on leaf temperature (Tleaf). Thus, to determine which factor plays the most prominent role in Tleaf regulation and whether such a response is temporal or permanent, we investigated the correlation between NPQ and gs and, subsequently, Tleaf. To this end, we analysed tomato plants (Solanum lycopersicum L. cv. Malinowy Ozarowski) grown solely under monochromatic LED lamps (435, 520, or 662 nm; 80 µmol m-2 s-1) or a mixed RGB spectrum (1:1:1; 180 µmol m-2 s-1) and simultaneously measured gs and Tleaf with an infrared gas analyser and a thermocouple or an infrared thermal camera (FLIR) during thermal imaging analyses. The results showed that growth light quality significantly modifies Tleaf and that such a response is not temporal. Furthermore, we found that the actual adaxial leaf surface temperature of plants is more closely related to NPQ amplitude, while the temperature of the abaxial surface corresponds to gs.

From remotely sensed solar-induced chlorophyll fluorescence to ecosystem structure, function, and service: Part I-Harnessing theory

Solar-induced chlorophyll fluorescence (SIF) is a remotely sensed optical signal emitted during the light reactions of photosynthesis. The past two decades have witnessed an explosion in availability of SIF data at increasingly higher spatial and temporal resolutions, sparking applications in diverse research sectors (e.g., ecology, agriculture, hydrology, climate, and socioeconomics). These applications must deal with complexities caused by tremendous variations in scale and the impacts of interacting and superimposing plant physiology and three-dimensional vegetation structure on the emission and scattering of SIF. At present, these complexities have not been overcome. To advance future research, the two companion reviews aim to (1) develop an analytical framework for inferring terrestrial vegetation structures and function that are tied to SIF emission, (2) synthesize progress and identify challenges in SIF research via the lens of multi-sector applications, and (3) map out actionable solutions to tackle these challenges and offer our vision for research priorities over the next 5-10 years based on the proposed analytical framework. This paper is the first of the two companion reviews, and theory oriented. It introduces a theoretically rigorous yet practically applicable analytical framework. Guided by this framework, we offer theoretical perspectives on three overarching questions: (1) The forward (mechanism) question-How are the dynamics of SIF affected by terrestrial ecosystem structure and function? (2) The inference question: What aspects of terrestrial ecosystem structure, function, and service can be reliably inferred from remotely sensed SIF and how? (3) The innovation question: What innovations are needed to realize the full potential of SIF remote sensing for real-world applications under climate change? The analytical framework elucidates that process complexity must be appreciated in inferring ecosystem structure and function from the observed SIF; this framework can serve as a diagnosis and inference tool for versatile applications across diverse spatial and temporal scales.

Active and passive chlorophyll fluorescence measurements at canopy level on potato crops. Evidence of similitude of diurnal cycles of apparent fluorescence yields

We performed active and passive measurements of diurnal cycles of chlorophyll fluorescence on potato crops at canopy level in outdoors conditions for 26 days. Active measurements of the stationary fluorescence yield (Fs) were performed using Ledflex, a fluorescence micro-LIDAR described in Moya et al. (Photosynth Res 142:1-15, 2019), capable of remote measurements of chlorophyll fluorescence under full sun-light in the wavelength range from 650 to 800 nm. Passive measurements of solar-induced fluorescence (SIF) fluxes were performed with Spectroflex, an instrument based on the method of filling-in in the O₂A and O₂B absorption bands at 760 nm (F760) and 687 nm (F687), respectively.Diurnal cycles of Fs showed significant variations throughout the day, directly attributed to changes in photosystem II yield. Contrasting patterns were observed according to illumination conditions. Under cloudy sky, Fs varied in parallel with photosynthetically active radiation (PAR). By contrast, during clear sky days, the diurnal cycle of Fs showed a "M" shape pattern with a minimum around noon.F687 and F760 showed different patterns, according to illumination conditions. Under low irradiance associated with cloudy conditions, F687 and F760 followed similar diurnal patterns, in parallel with PAR. Under high irradiance associated with clear sky we observed an increase of the F760/F687 ratio, which we attributed to the contributions in the 760 nm emission of photosystem I fluorescence from deeper layers of the leaves, on one end, and by the decrease of 687 nm emission as a result of red fluorescence re-absorption, on the other end.We defined an approach to derive a proxy of fluorescence yield (FYSIF) from SIF measurements as a linear combination of F687 and F760 normalized by vegetation radiance, where the coefficients of the linear combination were derived from the spectral transmittance of Ledflex. We demonstrated a close relationship between diurnal cycles of FYSIF and Fs, which outperformed other approaches based on normalization by incident light.

Light-induced changes of far-red excited chlorophyll fluorescence: further evidence for variable fluorescence of photosystem I in vivo

Recently, the long-standing paradigm of variable chlorophyll (Chl) fluorescence (Fv) in vivo originating exclusively from PSII was challenged, based on measurements with green algae and cyanobacteria (Schreiber and Klughammer 2021, PRES 149, 213-231). Fv(I) was identified by comparing light-induced changes of Fv > 700 nm and Fv < 710 nm. The Fv(I) induced by strong light was about 1.5 × larger in Fv > 700 nm compared to Fv < 710 nm. In the present communication, concentrating on the model green alga Chlorella vulgaris, this work is extended by comparing the light-induced changes of long-wavelength fluorescence (> 765 nm) that is excited by either far-red light (720 nm, mostly absorbed in PSI) or visible light (540 nm, absorbed by PSI and PSII). Polyphasic rise curves of Fv induced by saturating 540 nm light are measured, which after normalization of the initial O-I1 rises, assumed to reflect Fv(II), display a 2 × higher I2-P transient with 720 nm excitation (720ex) compared with 540ex. Analysis of the Fo(I) contributions to Fo(720ex) and Fo(540ex) reveals that also Fo(I)720ex is 2 × higher than Fo(I)540ex, which supports the notion that the whole I2-P transient is due to Fv(I). The twofold increase of the excitation ratio of F(I)/F(II) from 680 to 720 nm is much smaller than the eight-tenfold increase of PSI/PSII known from action spectra. It is suggested that the measured F > 765 nm is not representative for the bulk chlorophyll of PSI, but rather reflects a small fraction of far-red absorbing chlorophyll forms ("red Chls") with particular properties. Based on the same approach (comparison of polyphasic rise curves measured with 720ex and 540ex), the existence of Fv(I) is confirmed in a variety of other photosynthetic organisms (cyanobacteria, moss, fern, higher plant leaves).

Uphill energy transfer mechanism for photosynthesis in an Antarctic alga

Prasiola crispa, an aerial green alga, forms layered colonies under the severe terrestrial conditions of Antarctica. Since only far-red light is available at a deep layer of the colony, P. crispa has evolved a molecular system for photosystem II (PSII) excitation using far-red light with uphill energy transfer. However, the molecular basis underlying this system remains elusive. Here, we purified a light-harvesting chlorophyll (Chl)-binding protein complex from P. crispa (Pc-frLHC) that excites PSII with far-red light and revealed its ring-shaped structure with undecameric 11-fold symmetry at 3.13 Å resolution. The primary structure suggests that Pc-frLHC evolved from LHCI rather than LHCII. The circular arrangement of the Pc-frLHC subunits is unique among eukaryote LHCs and forms unprecedented Chl pentamers at every subunit‒subunit interface near the excitation energy exit sites. The Chl pentamers probably contribute to far-red light absorption. Pc-frLHC's unique Chl arrangement likely promotes PSII excitation with entropy-driven uphill excitation energy transfer.

Single chloroplast in folio imaging sheds light on photosystem energy redistribution during state transitions

Oxygenic photosynthesis is driven by light absorption in photosystem I (PSI) and photosystem II (PSII). A balanced excitation pressure between PSI and PSII is required for optimal photosynthetic efficiency. State transitions serve to keep this balance. If PSII is overexcited in plants and green algae, a mobile pool of light-harvesting complex II (LHCII) associates with PSI, increasing its absorption cross-section and restoring the excitation balance. This is called state 2. Upon PSI overexcitation, this LHCII pool moves to PSII, leading to state 1. Whether the association/dissociation of LHCII with the photosystems occurs between thylakoid grana and thylakoid stroma lamellae during state transitions or within the same thylakoid region remains unclear. Furthermore, although state transitions are thought to be accompanied by changes in thylakoid macro-organization, this has never been observed directly in functional leaves. In this work, we used confocal fluorescence lifetime imaging to quantify state transitions in single Arabidopsis (Arabidopsis thaliana) chloroplasts in folio with sub-micrometer spatial resolution. The change in excitation-energy distribution between PSI and PSII was investigated at a range of excitation wavelengths between 475 and 665 nm. For all excitation wavelengths, the PSI/(PSI + PSII) excitation ratio was higher in state 2 than in state 1. We next imaged the local PSI/(PSI + PSII) excitation ratio for single chloroplasts in both states. The data indicated that LHCII indeed migrates between the grana and stroma lamellae during state transitions. Finally, fluorescence intensity images revealed that thylakoid macro-organization is largely unaffected by state transitions. This single chloroplast in folio imaging method will help in understanding how plants adjust their photosynthetic machinery to ever-changing light conditions.

Active in situ and passive airborne fluorescence measurements for water stress detection on a fescue field

Ledflex is a fluorometer adapted to measure chlorophyll fluorescence at the canopy level. It has been described in detail by Moya et al. (2019), Photosynthesis Research. https://doi.org/10.1007/s11120-019-00642-9 . We used this instrument to determine the effect of water stress on the fluorescence of a fescue field under extreme temperature and light conditions through a 12 days campaign during summer in a Mediterranean area. The fescue field formed part of a lysimeter station in "las Tiesas," near Albacete-Spain. In addition to the fluorescence data, the surface temperature was measured using infrared radiometers. Furthermore, "Airflex," a passive fluorometer measuring the filling-in of the atmospheric oxygen absorption band at 760 nm, was installed in an ultralight plane and flown during the most critical days of the campaign. We observed with the Ledflex fluorometer a considerable decrease of about 53% of the stationary chlorophyll fluorescence level at noon under water stress, which was well correlated with the surface temperature difference between the stressed and control plots. Airflex data also showed a decrease in far-red solar-induced fluorescence upon water stress in agreement with surface temperature data and active fluorescence measurements after correction for PS I contribution. Notwithstanding, the results from airborne remote sensing are not as precise as in situ active data.

Bioaccumulation of Trace Elements from Aqueous Solutions by Selected Terrestrial Moss Species

The interrelationship between metal concentrations in mosses and their surroundings prompts research toward examining their accumulation properties, as it is particularly important for their usage in biomonitoring studies that use mosses. In this study, the kinetics of elemental sorption in three moss species (Pleurozium schreberi, Dicranum polysetum, and Sphagnum fallax) were investigated under laboratory conditions. Sorption from metal salt solutions was carried out under static conditions with decreasing elemental concentration. Functional groups responsible for binding metal cations to the internal structures of the mosses were also identified. It was shown that the equilibrium state was reached after about 60 min. Under the conditions of the experiment, in the first 10 min of the process, about 70.4-95.3% of metal ions were sorbed from the solution into the moss gametophytes by P. schreberi (57.1-89.0% by D. polysetum and 54.1-84.5% by S. fallax) with respect to the concentration of this analyte accumulated in the mosses at equilibrium. It can be assumed that the exposure of mosses with little contamination by heavy metals in an urbanized area under active biomonitoring will cause an increase in the concentration of these analytes in proportion to their concentration in atmospheric aerosols. In the case of P. schreberi and D. polysetum, the O-H/N-H band was enormously affected by the adsorption process. On the other hand, FTIR (Fourier transform infrared spectroscopy) analysis of S. fallax after adsorption showed slight changes for most of the bands analyzed. Based on this study, it can be concluded that mosses can be used as, for example, a biomonitor in monitoring of urban ecosystems, but also in the phytoremediation of surface waters.

Prying into the green black-box

Life-long efforts of the Tartu photosynthesis research group have been summarized. The measurements were facilitated by self-designed instruments, distinct in multifunctionality and fastresponse time. The black-box type kinetical analysis on intact leaves has revealed several physiologically significant features of leaf photosynthesis. Rubisco studies reflected competition for the active site between the substrates and products, linearizing in vivo kinetics compared with the low-K_m in vitro responses. Rubisco Activase usually activates only a small part of the Rubisco, making the rest of it a storage protein. Precisely quantifying absorbed photons and the responding transmittance changes, electron flow rates through cytochrome b₆f, plastocyanin and photosystem I were measured, revealing competition between the proton-uncoupled cyclic electron flow from PSI to Cyt b₆f to P700⁺ and the proton-coupled linear flow from PSII to Cyt b₆f to P700⁺. Analyzing responses of O₂ evolution and Chl fluorescence to ms-length light pulses we concluded that explanation of the sigmoidal fluorescence induction by excitonic connectivity between PSII units is a misconception. Each PSII processes excitation from its own antenna, but the sigmoidicity is caused by rise of the fluorescence yield of the Q_A-reduced PSII units after their Q_B site becomes occupied by reduced plastoquinone (or diuron). Unlike respiration, photosynthetic electrons must prepare their acceptor by coupled synthesis of 3ATP/4e^-. Feedback regulation of this ratio leads to oscillations under saturating light and CO₂, when the rate is P_i-limited. The slow oscillations (period 60s) indicate that the magnitudes of the deflections in the 3ATP/4e^- ratio, corrected by regulating cyclic and alternative electron flow (including the Mehler type O₂ reduction), are only a fraction of a per cent. The P_i limitation causes slip in the ATP synthase, slightly increasing the basic 12H⁺/3ATP requirement.

Photosynthesis in sun and shade: the surprising importance of far-red photons

The current definition of photosynthetically active radiation includes only photons from 400 up to 700 nm, despite evidence of the synergistic interaction between far-red photons and shorter-wavelength photons. The synergy between far-red and shorter-wavelength photons has not been studied in sunlight under natural conditions. We used a filter to remove photons above 700 nm to quantify the effects on photosynthesis in diverse species under full sun, medium light intensity and vegetation shade. Far-red photons (701 to 750 nm) in sunlight are used efficiently for photosynthesis. This is especially important for leaves in vegetation shade, where far-red photons can be > 50% of the total incident photons between 400 and 750 nm. Far-red photons accounted for 24-25% of leaf gross photosynthesis (P_gross ) in a C₃ and a C₄ species when sunlight was filtered through a leaf, and 10-14% of leaf P_gross in a tree and an understory species in deep shade. Accounting for the photosynthetic activity of far-red photons is critical for accurate measurement and modeling of photosynthesis at single leaf, canopy and ecosystem scales. This, in turn, is crucial in understanding crop productivity, the global carbon cycle and climate change impacts on agriculture and ecosystems.

The noninvasive monitoring of the redox status of photosynthetic electron transport chains in Hibiscus rosa-sinensis and Tradescantia leaves

We present an approach to the noninvasive determination of the electron capacity of the intersystem pool of electron carriers in chloroplasts in situ. As apt experimental models, we used the leaves of Hibiscus rosa-sinensis and Tradescantia species. Electron paramagnetic resonance and optical response of P₇₀₀ (the primary electron donor in Photosystem I) were applied to measuring electron transport in chloroplasts. Electron capacities of the intersystem electron transport chain (ETC) were determined from redox transients of P₇₀₀ upon chromatic transitions (white light → far-red light). During the induction period, we observed the nonmonotonic changes in the number of electron equivalents in the intersystem ETC per P₇₀₀ (parameter Q). In Hibiscus rosa-sinensis, the light-induced rise of Q from ≈2.5 (in the dark) to Q ≈ 12 was followed by its decrease to Q ≈ 6. The data obtained are discussed in the context of pH-dependent regulation of electron transport in chloroplasts, which provides the well-balanced operation of the intersystem ETC. The decay of Q is explained by the attenuation of Photosystem II activity due to the lumen acidification and the acceleration of plastoquinol re-oxidation as a result of the Calvin-Benson cycle activation. Our computer model of electron and proton transport coupled to ATP synthesis in chloroplasts was used to analyze the up and down feedbacks responsible for pH-dependent regulation of electron transport in chloroplasts. The procedures introduced here may be important for subsequent works aimed at defining the plastoquinone participation in regulation of photosynthetic processes in chloroplasts in situ.

A Photosynthetic Light Acclimation Model Accounting for the Effects of Leaf Age, Chlorophyll Content, and Intra-Leaf Radiation Transfer

Mechanistic models of canopy photosynthesis usually upscale leaf photosynthesis to crop level. A detailed prediction of canopy microclimate with accurate leaf morphological and physiological model parameters is the pre-requisite for accurate predictions. It is well established that certain leaf model parameters (V cmax, J max) of the frequently adopted Farquhar and Caemmerer photosynthesis model change with leaf age and light interception history. Previous approaches to predict V cmax and J max focused primarily on light interception, either by cumulative intercepted photosynthetic photon flux density (PPFD) or by closely related proxy variables such as leaf nitrogen content per leaf area. However, for plants with monopodial growth, such as vertically grown tomatoes or cucumber crops, in greenhouse production, there is a strong relationship between leaf age and light interception, complicating the experimental and mathematical separation of both effects. We propose a modeling framework that separates age and light intensity-related acclimation effects in a crop stand: Improved approximation of intra-leaf light absorption profiles with cumulative chlorophyll content (Chl) is the basis, while parameters are estimated via Gaussian process regression from total Chl, carotenoid content (Car), and leaf mass per area (LMA). The model approximates light absorption profiles within a leaf and links them to leaf capacity profiles of photosynthetic electron transport. Published datasets for Spinacia oleracea and Eucalyptus pauciflora were used to parameterize the relationship between light and capacity profiles and to set the curvature parameter of electron transport rate described by a non-rectangular hyperbola on Cucumis sativus. Using the modified capacity and light absorption profile functions, the new model was then able to predict light acclimation in a 2-month period of a fully grown tomato crop. An age-dependent lower limit of the electron transport capacity per unit Chl was essential in order to capture the decline of V cmax and J max over time and space of the investigated tomato crop. We detected that current leaf photosynthetic capacity in tomato is highly affected by intercepted light-sum of 3-5 previous days.

Genetic architecture of photosynthesis energy partitioning as revealed by a genome-wide association approach

The photosynthesis process is determined by the intensity level and spectral quality of the light; therefore, leaves need to adapt to a changing environment. The incident energy absorbed can exceed the sink capability of the photosystems, and, in this context, photoinhibition may occur in both photosystem II (PSII) and photosystem I (PSI). Quantum yield parameters analyses reveal how the energy is managed. These parameters are genotype-dependent, and this genotypic variability is a good opportunity to apply mapping association strategies to identify genomic regions associated with photosynthesis energy partitioning. An experimental and mathematical approach is proposed for the determination of an index which estimates the energy per photon flux for each spectral bandwidth (Δ_λ) of the light incident (QI index). Based on the QI, the spectral quality of the plant growth, environmental lighting, and the actinic light of PAM were quantitatively very similar which allowed an accurate phenotyping strategy of a rice population. A total of 143 genomic single regions associated with at least one trait of chlorophyll fluorescence were identified. Moreover, chromosome 5 gathers most of these regions indicating the importance of this chromosome in the genetic regulation of the photochemistry process. Through a GWAS strategy, 32 genes of rice genome associated with the main parameters of the photochemistry process of photosynthesis in rice were identified. Association between light-harvesting complexes and the potential quantum yield of PSII, as well as the relationship between coding regions for PSI-linked proteins in energy distribution during the photochemical process of photosynthesis is analyzed.

Insight into Details of the Photosynthetic Light Reactions and Selected Metabolic Changes in Tomato Seedlings Growing under Various Light Spectra

The objective of our study was to characterise the growth of tomato seedlings under various light spectra, but special attention has been paid to gaining a deeper insight into the details of photosynthetic light reactions. The following light combinations (generated by LEDs, constant light intensity at 300 μmol m⁻² s⁻¹) were used: blue/red light; blue/red light + far red; blue/red light + UV; white light that was supplemented with green, and white light that was supplemented with blue. Moreover, two combinations of white light for which the light intensity was changed by imitating the sunrise, sunset, and moon were also tested. The reference point was also light generated by high pressure sodium lamps (HPS). Plant growth/morphological parameters under various light conditions were only partly correlated with the photosynthetic efficiency of PSI and PSII. Illumination with blue/red as the main components had a negative effect on the functioning of PSII compared to the white light and HPS-generated light. On the other hand, the functioning of PSI was especially negatively affected under the blue/red light that was supplemented with FR. The FT-Raman studies showed that the general metabolic profile of the leaves (especially proteins and β-carotene) was similar in the plants that were grown under the HPS and under the LED-generated white light for which the light intensity changed during a day. The effect of various light conditions on the leaf hormonal balance (auxins, brassinosteroids) is also discussed.

Photosynthetic activity and chlorophyll pigment concentration in Medicago x varia T. Martyn leaves treated with the Tytanit growth regulator

The purpose of the research was to determine the effect of the foliar use of a growth regulator with the trade name of Tytanit, containing titanium ascorbate, on photosynthetic activity and chlorophyll content in Medicago × varia T. Martyn leaves. There were two kinds of plots: C – control series; Ti – plants treated with Tytanit, containing 8.5 g of titanium in 1 dm³. The following parameters were determined: maximum photosystem II efficiency (F_v/F_m) in a dark-adapted state, actual photosystem II efficiency (ΔF/F_m’) in a light-adapted state, photochemical quenching factor (QP), non-photochemical quenching factor (QN), and chlorophyll a and b content. The Fisher-Snedecor test was used to determine whether the impact of experimental factors was significant, and the HSD 0.05 value was calculated using Tukey’s test. Compared to control, the photosynthetic apparatus performance of alfalfa was positively affected by the regulator compared to control. Tytanit applied to plant leaves increased their photosynthetic activity as a result of an increase in the content of chlorophyll pigments. It was also found that periods of rainfall deficiency did not affect the beneficial effects of the regulator.

Aqueous-soluble bipyridine cobalt(ii/iii) complexes act as direct redox mediators in photosystem I-based biophotovoltaic devices

Sustainable energy production is critical for meeting growing worldwide energy demands. Due to its stability and reduction potential, photosystem I (PSI) is attractive as the photosensitizer in biophotovoltaic devices. Herein, we characterize aqueous and organic solvent soluble synthetic bipyridine-based cobalt complexes as redox mediators for PSI-based biophotovoltaics applications. Cobalt-based complexes are not destructive to protein and have appropriate midpoint potentials for electron donation to PSI. We report on PSI stability in organic solvents commonly used in biophotovoltaics. We also show the effects of a mixed organic solvent phase on PSI reduction kinetics, slowing reduction rates approximately 8–38 fold as compared to fully aqueous systems, with implications for dye regeneration rates in PSI-based biophotovoltaics. Further, we show evidence of direct electron transfer from cobalt complexes to PSI. Finally, we report on photocurrent generation from Co mediator-PSI biophotovoltaic devices. Taken together, we discuss the development of novel Co complexes and our ability to fine-tune their characteristics via functional groups and counteranion choice to drive interaction with a biological electron acceptor on multiple levels from redox midpoints, spectral overlap, and solvent requirements, among others. This work suggests that fine-tuning of redox active species for interaction with a biological partner is possible for the creation and improvement of low cost, carbon-neutral energy production in the future.

Sustainable energy production is critical for meeting growing worldwide energy demands.

Fabrication of Electronic Junctions between Oriented Multilayers of Photosystem I and the Electrodes of Optoelectronic Solid-State Devices

The efficient optoelectronic properties of photosynthetic proteins were explored in the quest for the fabrication of novel solid biohybrid devices. As most optoelectronic devices function in a dry environment, an attempt was made to fabricate an efficient electronic junction by covalent binding of photosynthetic reaction center proteins to metals, transparent semiconductor polymers, and solid semiconductors that function in a dry environment. The primary stages of photosynthesis take place in nanometric-size protein-chlorophyll complexes. Such is photosystem I (PSI), which generates a photovoltage of 1 V. The isolated PSI generates an absorbed light-energy conversion efficiency of ∼47% (∼23% solar energy) and internal quantum efficiency of ∼100%. The robust cyanobacterial PSI was used in the fabrication of solid-state optoelectronic devices by forming oriented multilayers from genetically engineered cysteine mutants between metal and transparent conducting semiconductor electrodes. Current-voltage measurements of the cells generated diode- and photodiode-like responses in the dark and the light, respectively. The cells were stable for many months in a dry environment. The generation of photocurrent and V_oc indicated the formation of good electronic coupling between PSI and the electrodes, which can serve in the fabrication of solid-state biohybrid optoelectronic devices.

Electron transport in Tradescantia leaves acclimated to high and low light: thermoluminescence, PAM-fluorometry, and EPR studies

Using thermoluminescence, PAM-fluorometry, and electron paramagnetic resonance (EPR) for assaying electron transport processes in chloroplasts in situ, we have compared photosynthetic characteristics in Tradescantia fluminensis leaves grown under low light (LL, 50-125 µmol photons m^-2 s^-1) or high light (HL, 875-1000 µmol photons m^-2 s^-1) condition. We found differences in the thermoluminescence (TL) spectra of LL- and HL-acclimated leaves. The LL and HL leaves show different proportions of the Q (~ 0 °C) and B (~ 25-30 °C) bands in their TL spectra; the ratios of the "light sums" of the Q and B bands being S_Q/S_B ≈ 1/1 (LL) and S_Q/S_B ≈ 1/3 (HL). This suggests the existence of different redox states of electron carriers on the acceptor side of PSII in LL and HL leaves, which may be affected, in particular, by different capacities of their photo-reducible PQ pools. Enhanced content of PQ in chloroplasts of LL leaves may be the reason for an efficient performance of photosynthesis at low irradiance. Kinetic studies of slow induction of Chl a fluorescence and measurements of P₇₀₀ photooxidation by EPR demonstrate that HL leaves have faster (about 2 times) response to switching on actinic light as compared to LL leaves grown at moderate irradiation. HL leaves also show higher non-photochemical quenching (NPQ) of Chl a fluorescence. These properties of HL leaves (faster response to light and generation of enhanced NPQ) reflect the flexibility of their photosynthetic apparatus, providing sustainability and rapid response to fluctuations of environmental light intensity and solar stress resistance. Analysis of time-courses of the EPR signals of [Formula: see text] induced by far-red (λ_max = 707 nm), exciting predominantly PSI, and white light, exciting both PSI and PSII, suggests that there is a contribution of cyclic electron flow around PSI to electron flow through PSI in HL leaves. The data obtained are discussed in terms of photosynthetic apparatus sustainability of HL and LL leaves under variable irradiation conditions.

Steady-state stomatal responses of C3 and C4 species to blue light fraction: Interactions with CO2 concentration

Blue light induced stomatal opening has been studied by applying a short pulse (~5 to 60 s) of blue light to a background of saturating photosynthetic red photons, but little is known about steady-state stomatal responses. Here we report stomatal responses to blue light at high and low CO₂ concentrations. Steady-state stomatal conductance (g_s ) of C₃ plants increased asymptotically with increasing blue light to a maximum at 20% blue (120 μmol m^-2 s^-1 ). This response was consistent from 200 to 800 μmol mol^-1 atmospheric CO₂ (C_a ). In contrast, blue light induced only a transient stomatal opening (~5 min) in C₄ species above a C_a of 400 μmol mol^-1 . Steady-state g_s of C₄ plants generally decreased with increasing blue intensity. The net photosynthetic rate of all species decreased above 20% blue because blue photons have lower quantum yield (moles carbon fixed per mole photons absorbed) than red photons. Our findings indicate that photosynthesis, rather than a blue light signal, plays a dominant role in stomatal regulation in C₄ species. Additionally, we found that blue light affected only stomata on the illuminated side of the leaf. Contrary to widely held belief, the blue light-induced stomatal opening minimally enhanced photosynthesis and consistently decreased water use efficiency.

Steady-state stomatal responses of C3 and C4 species to blue light fraction: Interactions with CO2 concentration

Blue light induced stomatal opening has been studied by applying a short pulse (~5 to 60 s) of blue light to a background of saturating photosynthetic red photons, but little is known about steady-state stomatal responses. Here we report stomatal responses to blue light at high and low CO₂ concentrations. Steady-state stomatal conductance (g_s ) of C₃ plants increased asymptotically with increasing blue light to a maximum at 20% blue (120 μmol m^-2 s^-1 ). This response was consistent from 200 to 800 μmol mol^-1 atmospheric CO₂ (C_a ). In contrast, blue light induced only a transient stomatal opening (~5 min) in C₄ species above a C_a of 400 μmol mol^-1 . Steady-state g_s of C₄ plants generally decreased with increasing blue intensity. The net photosynthetic rate of all species decreased above 20% blue because blue photons have lower quantum yield (moles carbon fixed per mole photons absorbed) than red photons. Our findings indicate that photosynthesis, rather than a blue light signal, plays a dominant role in stomatal regulation in C₄ species. Additionally, we found that blue light affected only stomata on the illuminated side of the leaf. Contrary to widely held belief, the blue light-induced stomatal opening minimally enhanced photosynthesis and consistently decreased water use efficiency.

Action spectrum of the redox state of the plastoquinone pool defines its function in plant acclimation

The plastoquinone (PQ) pool mediates electron flow and regulates photoacclimation in plants. Here we report the action spectrum of the redox state of the PQ pool in Arabidopsis thaliana, showing that 470-500, 560 or 650-660 nm light favors Photosystem II (PSII) and reduces the PQ pool, whereas 420-440, 520 or 690 nm light favors Photosystem I (PSI) and oxidizes PQ. These data were used to construct a model predicting the redox state of PQ from the spectrum of any polychromatic light source. Moderate reduction of the PQ pool induced transition to light state 2, whereas state 1 required highly oxidized PQ. In low-intensity PSI light, PQ was more oxidized than in darkness and became gradually reduced with light intensity, while weak PSII light strongly reduced PQ. Natural sunlight was found to favor PSI, which enables plants to use the redox state of the PQ pool as a measure of light intensity.

Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO2 Fixation and Increased Photon Capture During Long-Term Studies: Implications for Re-Defining PAR

Far-red photons regulate shade avoidance responses and can have powerful effects on plant morphology and radiation capture. Recent studies have shown that far-red photons (700 to 750 nm) efficiently drive photosynthesis when added to traditionally defined photosynthetic photons (400-700 nm). But the long-term effects of far-red photons on canopy quantum yield have not yet been determined. We grew lettuce in a four-chamber, steady-state canopy gas-exchange system to separately quantify canopy photon capture, quantum yield for CO₂ fixation, and carbon use efficiency. These measurements facilitate a mechanistic understanding of the effect of far-red photons on the components of plant growth. Day-time photosynthesis and night-time respiration of lettuce canopies were continuously monitored from seedling to harvest in five replicate studies. Plants were grown under a background of either red/blue or white light, each background with or without 15% (50 μmol m^-2 s^-1) of far-red photons substituting for photons between 400 and 700 nm. All four treatments contained 31.5% blue photons, and an equal total photon flux from 400 to 750 nm of 350 μmol m^-2 s^-1. Both treatments with far-red photons had higher canopy photon capture, increased daily carbon gain (net photosynthesis minus respiration at night), and 29 to 31% more biomass than control treatments. Canopy quantum yield was similar among treatments (0.057 ± 0.002 mol of CO₂ fixed in gross photosynthesis per mole of absorbed photons integrated over 400 to 750 nm). Carbon use efficiency (daily carbon gain/gross photosynthesis) was also similar for mature plants (0.61 ± 0.02). Photosynthesis increased linearly with increasing photon capture and had a common slope among all four treatments, which demonstrates that the faster growth with far-red photon substitution was caused by enhanced photon capture through increased leaf expansion. The equivalent canopy quantum yield among treatments indicates that the absorbed far-red photons were equally efficient for photosynthesis when acting synergistically with the 400-700 nm photons.

Time- and reduction-dependent rise of photosystem II fluorescence during microseconds-long inductions in leaves

Lettuce (Lactuca sativa) and benth (Nicotiana benthamiana) leaves were illuminated with 720 nm background light to mix S-states and oxidize electron carriers. Green-filtered xenon flashes of different photon dose were applied and O2 evolution induced by a flash was measured. After light intensity gradient across the leaf was mathematically considered, the flash-induced PSII electron transport (= 4·O2 evolution) exponentially increased with the flash photon dose in any differential layer of the leaf optical density. This proved the absence of excitonic connectivity between PSII units. Time courses of flash light intensity and 680 nm chlorophyll fluorescence emission were recorded. While with connected PSII the sigmoidal fluorescence rise has been explained by quenching of excitation in closed PSII by its open neighbors, in the absence of connectivity the sigmoidicity indicates gradual rise of the fluorescence yield of an individual closed PSII during the induction. Two phases were discerned: the specific fluorescence yield immediately increased from Fo to 1.8Fo in a PSII, whose reaction center became closed; fluorescence yield of the closed PSII was keeping time-dependent rise from 1.8Fo to about 3Fo, approaching the flash fluorescence yield Ff = 0.6Fm during 40 μs. The time-dependent fluorescence rise was resolved from the quenching by 3Car triplets and related to protein conformational change. We suggest that QA reduction induces a conformational change, which by energetic or structural means closes the gate for excitation entrance into the central radical pair trap-efficiently when QB cannot accept the electron, but less efficiently when it can.

Quieting a noisy antenna reproduces photosynthetic light-harvesting spectra

Photosynthesis achieves near unity light-harvesting quantum efficiency yet it remains unknown whether there exists a fundamental organizing principle giving rise to robust light harvesting in the presence of dynamic light conditions and noisy physiological environments. Here, we present a noise-canceling network model that relates noisy physiological conditions, power conversion efficiency, and the resulting absorption spectra of photosynthetic organisms. Using light conditions in full solar exposure, light filtered by oxygenic phototrophs, and light filtered under seawater, we derived optimal absorption characteristics for efficient solar power conversion. We show how light-harvesting antennae can be tuned to maximize power conversion efficiency by minimizing excitation noise, thus providing a unified theoretical basis for the observed wavelength dependence of absorption in green plants, purple bacteria, and green sulfur bacteria.

Variable fluorescence of closed photochemical reaction centers

Chlorophyll fluorescence induction during 0.4 to 200 ms multiple-turnover pulses (MTP) was measured in parallel with O₂ evolution induced by the MTP light. Additionally, a saturating single-turnover flash (STF) was applied at the end of each MTP and the total MTP +STF O₂ evolution was measured. Quantum yield of O₂ evolution during the MTP transients was calculated and related to the number of open PSII centers, found from the STF O₂ evolution. Proportionality between the number of open PSII and their running photochemical activity showed the quantum yield of open PSII remained constant independent of the closure of adjacent centers. During the induction, total fluorescence was partitioned between F_o of all the open centers and F_c of all the closed centers. The fluorescence yield of a closed center was 0.55 of the final F_m while less than a half of the centers were closed, but later increased, approaching F_m to the end of the induction. In the framework of the antenna/radical pair equilibrium model, the collective rise of the fluorescence of centers closed earlier during the induction is explained by an electric field, facilitating return of excitation energy from the Pheo^- P680⁺ radical pair to the antenna.

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

Cyclic electron flow (CEF) around photosystem I (PSI) is essential for generating additional ATP and enhancing efficient photosynthesis. Accurate estimation of CEF requires knowledge of the fractions of absorbed light by PSI (fI) and PSII (fII), which are only known for a few model species such as spinach. No measures of fI are available for C4 grasses under different irradiances. We developed a new method to estimate (1) fII in vivo by concurrently measuring linear electron flux through both photosystems [Formula: see text] in leaf using membrane inlet mass spectrometry (MIMS) and total electron flux through PSII (ETR2) using chlorophyll fluorescence by a Dual-PAM at low light and (2) CEF as ETR1-[Formula: see text]. For a C3 grass, fI was 0.5 and 0.4 under control (high light) and shade conditions, respectively. C4 species belonging to NADP-ME and NAD-ME subtypes had fI of 0.6 and PCK subtype had 0.5 under control. All shade-grown C4 species had fI of 0.6 except for NADP-ME grass which had 0.7. It was also observed that fI ranged between 0.3 and 0.5 for gymnosperm, liverwort and fern species. CEF increased with irradiance and was induced at lower irradiances in C4 grasses and fern relative to other species. CEF was greater in shade-grown plants relative to control plants except for C4 NADP-ME species. Our study reveals a range of CEF and fI values in different plant functional groups. This variation must be taken into account for improved photosynthetic calculations and modelling.

Demonstration of a relationship between state transitions and photosynthetic efficiency in a higher plant

A consequence of the series configuration of PSI and PSII is that imbalanced excitation of the photosystems leads to a reduction in linear electron transport and a drop in photosynthetic efficiency. Achieving balanced excitation is complicated by the distinct nature of the photosystems, which differ in composition, absorption spectra, and intrinsic efficiency, and by a spectrally variable natural environment. The existence of long- and short-term mechanisms that tune the photosynthetic apparatus and redistribute excitation energy between the photosystems highlights the importance of maintaining balanced excitation. In the short term, state transitions help restore balance through adjustments which, though not fully characterised, are observable using fluorescence techniques. Upon initiation of a state transition in algae and cyanobacteria, increases in photosynthetic efficiency are observable. However, while higher plants show fluorescence signatures associated with state transitions, no correlation between a state transition and photosynthetic efficiency has been demonstrated. In the present study, state 1 and state 2 were alternately induced in tomato leaves by illuminating leaves produced under artificial sun and shade spectra with a sequence of irradiances extreme in terms of PSI or PSII overexcitation. Light-use efficiency increased in both leaf types during transition from one state to the other with remarkably similar kinetics to that of F'm/Fm, F'o/Fo, and, during the PSII-overexciting irradiance, ΦPSII and qP. We have provided compelling evidence for the first time of a correlation between photosynthetic efficiency and state transitions in a higher plant. The importance of this relationship in natural ecophysiological contexts remains to be elucidated.

Far-red light enhances photochemical efficiency in a wavelength-dependent manner

Linear electron transport depends on balanced excitation of photosystem I and II. Far-red light preferentially excites photosystem I (PSI) and can enhance the photosynthetic efficiency when combined with light that over-excites photosystem II (PSII). The efficiency of different wavelengths of far-red light exciting PSI was quantified by measuring the change in quantum yield of PSII (ΦPSII ) of lettuce (Lactuca sativa) under red/blue light with narrowband far-red light added (from 678 to 752 nm, obtained using laser diodes). The ΦPSII of lettuce increased with increasing wavelengths of added light from 678 to 703 nm, indicating longer wavelengths within this region are increasingly used more efficiently by PSI than by PSII. Adding 721 nm light resulted in similar ΦPSII as adding 703 nm light, but ΦPSII tended to decrease as wavelength increased from 721 to 731 nm, likely due to decreasing absorptance and low photon energy. Adding 752 nm light did not affect ΦPSII . Leaf chlorophyll fluorescence light response measurements showed lettuce had higher ΦPSII under halogen light (rich in far-red) than under red/blue light (which over-excites PSII). Far-red light is more photosynthetically active than commonly believed, because of its synergistic interaction with light of shorter wavelengths.

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

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

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.

A Mathematical Model of Photosynthetic Electron Transport in Response to the Light Spectrum Based on Excitation Energy Distributed to Photosystems

To enable us to analyze more systematically the effects of the spectral distribution of light (i.e. light quality) on photosynthetic electron transport, we propose a simple mathematical model which describes electron transport reactions under light-limited conditions based on the excitation energy distributed to the photosystems. The model assumes that the rate-limiting photosystem performs the photochemical reaction at its maximum yield, while the yield in the other photosystem is passively down-regulated to equalize the rates of linear electron transport through the photosystems. Using intact cucumber leaves, we tested the model by comparing actual and estimated photosynthetic parameters under several combinations of photon flux densities of red and far-red lights (R and FR, respectively). Simultaneously provided R and FR yielded greater gross photosynthetic rates than the sums of the rates under only R and only FR, which is known as the 'enhancement effect'. The present model reproduced these non-additive increases in the gross photosynthetic rates in response to supplemental FR to R and provided more accurate estimates than an existing method that did not take the enhancement effect into account (root mean square errors: 0.11 and 0.21 μmol m-2 s-1, respectively). Using the present model, the photon flux density of the supplemental FR which gives the changing point of rate-limiting photosystem and the photochemical yields of the non-rate-limiting photosystems were estimated reasonably well. The present study has therefore formulated a simplified quantitative electron transport model in response to the light spectrum based on generally accepted concepts and demonstrated its validity experimentally.

Partitioning of absorbed light energy within photosystem II in barley can be affected by chloroplast movement

Plants have developed many ways to protect reaction centres of photosystems against overexcitation. One of the mechanisms involves reduction of the leaf absorption cross-section by light-induced chloroplast avoidance reaction. Decrease in the probability of photon absorption by the pigments bound within photosystem II (PSII) complexes leads to the increase in quantum yield of PSII photochemistry (Φ_PSII). On the other hand, the decrease of PSII excitation probability causes reduction of chlorophyll a fluorescence intensity which is manifested as the apparent increase of determined quantum yield of regulated light-induced non-photochemical quenching (Φ_NPQ). Absorption of different light intensity by phototropins led to the different chloroplast distribution within barley leaves, estimated by measurement of the leaf transmittance. Due to a weak blue light used for transmittance measurements, leaves exposed to actinic light with wavelengths longer than 520 nm undergo chloroplast accumulation reaction, in contrast with leaves exposed to light with shorter wavelengths, that showed a different extent of chloroplast avoidance reaction. Based on the Φ_NPQ action spectra measured simultaneously with the transmittance, the influence of different chloroplast distribution on Φ_NPQ was assessed. The analysis of results showed that decrease in the leaf absorption cross-section due to increasing part of chloroplasts reaching profile position significantly affected the partitioning of excitation energy within PSII and such rearrangement also distorted measured Φ_NPQ and cannot be neglected in its interpretation. When the majority of chloroplasts reached profile position, the photoprotective effect appeared to be the most prominent for strong blue light that has the highest absorption in the upper leaf layers in comparison with green or red ones.

Auxin-to-Gibberellin Ratio as a Signal for Light Intensity and Quality in Regulating Soybean Growth and Matter Partitioning

The intensity and quality (red to far-red (R/Fr) ratio) of light directly affect growth of plant under shading. Gibberellins (GAs) and auxin [indole-3-acetic acid (IAA)] play important roles in mediating the shading adaptive responses of plants. Thus, the intensity and quality of the uncoupling light from shading were assessed to identify the influence of each component on the morphology and matter distribution of the leaf, stem, and petiole. This assessment was based on the changes in endogenous Gibberellin 1 (GA1) and IAA levels. Soybean plants were grown in a growth chamber with four treatments [normal (N), N+Fr, low (L), and L+Fr light]. Results revealed that the reductions in photosynthetically active radiation (PAR) and R/Fr ratio equally increased height and stem mass fractions (SMFs) of the soybean seedling. The light intensity significantly influenced the dry mass per unit area and mass fraction of soybean leaves, whereas the light quality regulated the petiole elongation and mass fraction. Low R/Fr ratio (high Fr light) increased the soybean biomass by improving the photosynthetic assimilation rate and quantum yield of photosystem II. In addition, the IAA and GA1 levels in the leaf, stem, and petiole did not reflect the growth response trends of each tissue toward light intensity and quality; however, trends of the IAA-to-GA1 content ratios were similar to those of the growth and matter allocation of each soybean tissue under different light environments. Therefore, the response of growth and matter allocation of soybean to light intensity and quality may be regulated by the IAA-to-GA1 content ratio in the tissues of the soybean plant.

Quantification of excitation energy distribution between photosystems based on a mechanistic model of photosynthetic electron transport

Absorbed light energy is converted into excitation energy. The excitation energy is distributed to photosystems depending on the wavelength and drives photochemical reactions. A non-destructive, mechanistic and quantitative method for estimating the fraction of the excitation energy distributed to photosystem II (f) was developed. For the f values for two simultaneously provided actinic lights (ALs) with different spectral distributions to be estimated, photochemical yields of the photosystems were measured under the ALs and were then fitted to an electron transport model assuming the balance between the electron transport rates through the photosystems. For the method to be tested using leaves with different properties in terms of the long-term and short-term acclimation (adjustment of photosystem stoichiometry and state transition, respectively), the f values for red and far-red light (R and FR) were estimated in leaves grown (~1 week) under white light without and with supplemental FR and adapted (~10 min) to R without and with supplemental FR. The f values for R were clearly greater than those for FR and those of leaves grown with and adapted to supplemental FR tended to be higher than the controls. These results are consistent with previous studies and therefore support the validity of the proposed method.

Light Quality Affects Chloroplast Electron Transport Rates Estimated from Chl Fluorescence Measurements

Chl fluorescence has been used widely to calculate photosynthetic electron transport rates. Portable photosynthesis instruments allow for combined measurements of gas exchange and Chl fluorescence. We analyzed the influence of spectral quality of actinic light on Chl fluorescence and the calculated electron transport rate, and compared this with photosynthetic rates measured by gas exchange in the absence of photorespiration. In blue actinic light, the electron transport rate calculated from Chl fluorescence overestimated the true rate by nearly a factor of two, whereas there was closer agreement under red light. This was consistent with the prediction made with a multilayer leaf model using profiles of light absorption and photosynthetic capacity. Caution is needed when interpreting combined measurements of Chl fluorescence and gas exchange, such as the calculation of CO2 partial pressure in leaf chloroplasts.

Connectivity between electron transport complexes and modulation of photosystem II activity in chloroplasts

In chloroplasts, photosynthetic electron transport complexes interact with each other via the mobile electron carriers (plastoquinone and plastocyanin) which are in surplus amounts with respect to photosystem I and photosystem II (PSI and PSII), and the cytochrome b ₆ f complex. In this work, we analyze experimental data on the light-induced redox transients of photoreaction center P₇₀₀ in chloroplasts within the framework of our mathematical model. This analysis suggests that during the action of a strong actinic light, even significant attenuation of PSII [for instance, in the result of inhibition of a part of PSII complexes by DCMU or due to non-photochemical quenching (NPQ)] will not cause drastic shortage of electron flow through PSI. This can be explained by "electronic" and/or "excitonic" connectivity between different PSII units. At strong AL, the overall flux of electrons between PSII and PSI will maintain at a high level even with the attenuation of PSII activity, provided the rate-limiting step of electron transfer is beyond the stage of PQH₂ formation. Results of our study are briefly discussed in the context of NPQ-dependent mechanism of chloroplast protection against light stress.