Chlorophyll Fluorescence Signal Research Articles

A novel application of laser speckle imaging technique for prediction of hypoxic stress of apples

BackgroundFruit storage methods such as dynamic controlled atmosphere (DCA) technology enable adjusting the level of oxygen in the storage room, according to the physiological state of the product to slow down the ripening process. However, the successful application of DCA requires precise and reliable sensors of the oxidative stress of the fruit. In this study, respiration rate and chlorophyll fluorescence (CF) signals were evaluated after introducing a novel predictors of apples' hypoxic stress based on laser speckle imaging technique (LSI).ResultsBoth chlorophyll fluorescence and LSI signals were equally good for stress detection in principle. However, in an application with automatic detection based on machine learning models, the LSI signal proved to be superior, due to its stability and measurement repeatability. Moreover, the shortcomings of the CF signal appear to be its inability to indicate oxygen stress in tissues with low chlorophyll content but this does not apply to LSI. A comparison of different LSI signal processing methods showed that method based on the dynamics of changes in image content was better indicators of stress than methods based on measurements of changes in pixel brightness (inertia moment or laser speckle contrast analysis). Data obtained using the near-infrared laser provided better prediction capabilities, compared to the laser with red light.ConclusionsThe study showed that the signal from the scattered laser light phenomenon is a good predictor for the oxidative stress of apples. Results showed that effective prediction using LSI was possible and did not require additional signals. The proposed method has great potential as an alternative indicator of fruit oxidative stress, which can be applied in modern storage systems with a dynamically controlled atmosphere.

Chloroplast ATP synthase restricts photosynthesis under fluctuating light in tomato but not in maize

Photosynthesis in fluctuating light requires coordinated adjustments of diffusion conductance and biochemical capacity, but the role of chloroplast ATP synthase activity (gH+) in dynamic photosynthesis is not well understood. In this study, we measured gas exchange, chlorophyll fluorescence and electrochromic shift signals in fluctuating light for leaves of tomato (Solanum lycopersicum) and maize (Zea mays). During the transition from sun to shade, simultaneous increases in gH+, effective quantum yield of PSII, and net CO2 assimilation rate (AN) occurred in tomato but uncoupled in maize, indicating that gH + limited AN during the sun-to-shade transition in tomato but not in maize. During the shade-to-sun transition, gH + increased simultaneously with stomatal conductance, mesophyll conductance and Rubisco carboxylation capacity in tomato, suggesting that gH+ is an overlooked factor affecting light induction of AN in tomato. By comparison, gH + maintained at high levels in maize and its AN was mainly restricted by stomatal conductance. Our results reveal that the kinetics of gH+ in fluctuating light differs between species, and chloroplast ATP synthase may be a potential target for improving dynamic photosynthesis in crops such as tomato.

Novel technique for the ultra-sensitive detection of hazardous contaminants using an innovative sensor integrated with a bioreactor

This study introduces an evaluation methodology tailored for bioreactors, with the aim of assessing the stress experienced by algae due to harmful contaminants released from antifouling (AF) paints. We present an online monitoring system equipped with an ultra-sensitive sensor that conducts non-invasive measurements of algal culture's optical density and physiological stage through chlorophyll fluorescence signals. By coupling the ultra-sensitive sensor with flash-induced chlorophyll fluorescence, we examined the dynamic fluorescence changes in the green microalga Chlamydomonas reinhardtii when exposed to biocides. Over a 24-h observation period, increasing concentrations of biocides led to a decrease in photosynthetic activity. Notably, a substantial reduction in the maximum quantum yield of primary photochemistry (FV/FM) was observed within the first hour of exposure. Subsequently, we detected a partial recovery in FV/FM; however, this recovery remained 50% lower than that of the controls. Integrating the advanced submersible sensor with fluorescence decay kinetics offered a comprehensive perspective on the dynamic alterations in algal cells under the exposure to biocides released from antifouling coatings. The analysis of fluorescence relaxation kinetics revealed a significant shortening of the fast and middle phases, along with an increase in the duration of the slow phase, for the coating with the highest levels of biocides. Combining automated culturing and measuring methods, this approach has demonstrated its effectiveness as an ultrasensitive and non-invasive tool for monitoring the physiology of photosynthetic cultures. This is particularly valuable in the context of studying microalgae and their early responses to various environmental conditions, as well as the potential to develop an AF system with minimal harm to the environment.

SLC24A-mediated calcium exchange as an indispensable component of the diatom cell density-driven signaling pathway.

Diatom bloom is characterized by a rapid increase of population density. Perception of population density and physiological responses can significantly influence their survival strategies, subsequently impacting bloom fate. The population density itself can serve as a signal, which is perceived through chemical signals or chlorophyll fluorescence signals triggered by high cell density, and their intracellular signaling mechanisms remain to be elucidated. In this study, we focused on the model diatom, Phaeodactylum tricornutum, and designed an orthogonal experiment involving varying cell densities and light conditions, to stimulate the release of chemical signals and light-induced chlorophyll fluorescence signals. Utilizing RNA-Seq and Weighted Gene Co-expression Network Analysis, we identified four gene clusters displaying density-dependent expression patterns. Within these, a potential hub gene, PtSLC24A, encoding a Na+/Ca2+ exchanger, was identified. Based on molecular genetics, cellular physiology, computational structural biology, and in situ oceanic data, we propose a potential intracellular signaling mechanism related to cell density in marine diatoms using Ca2+: upon sensing population density signals mediated by chemical cues, the membrane-bound PtSLC24A facilitates the efflux of Ca2+ to maintain specific intracellular calcium levels, allowing the transduction of intracellular density signals, subsequently regulating physiological responses, including cell apoptosis, ultimately affecting algal blooms fate. These findings shed light on the calcium-mediated intracellular signaling mechanism of marine diatoms to changing population densities, and enhances our understanding of diatom bloom dynamics and their ecological implications.

Impact of Sodium Nitroprusside on the Photosynthetic Performance of Maize and Sorghum.

Nitric oxide (NO) is an important molecule in regulating plant growth, development and photosynthetic performance. This study investigates the impact of varying concentrations (0-300 µM) of sodium nitroprusside (SNP, a donor of NO) on the functions of the photosynthetic apparatus in sorghum (Sorghum bicolor L. Albanus) and maize (Zea mays L. Kerala) under physiological conditions. Analysis of the chlorophyll fluorescence signals (using PAM and the JIP-test) revealed an increased amount of open PSII reaction centers (qP increased), but it did not affect the number of active reaction centers per PSII antenna chlorophyll (RC/ABS). In addition, the smaller SNP concentrations (up to 150 μM) alleviated the interaction of QA with plastoquine in maize, while at 300 μM it predominates the electron recombination on QAQB-, with the oxidized S2 (or S3) states of oxygen evolving in complex ways in both studied plant species. At the same time, SNP application stimulated the electron flux-reducing end electron acceptors at the PSI acceptor side per reaction center (REo/RC increased up to 26%) and the probability of their reduction (φRo increased up to 20%). An increase in MDA (by about 30%) and H2O2 contents was registered only at the highest SNP concentration (300 µM). At this concentration, SNP differentially affected the amount of P700+ in studied plant species, i.e., it increased (by 10%) in maize but decreased (by 16%) in sorghum. The effects of SNP on the functions of the photosynthetic apparatus were accompanied by an increase in carotenoid content in both studied plants. Additionally, data revealed that SNP-induced changes in the photosynthetic apparatus differed between maize and sorghum, suggesting species specificity for SNP's impact on plants.

Water availability and atmospheric dryness controls on spaceborne sun-induced chlorophyll fluorescence yield

Climate change is amplifying the duration, frequency, and intensity of droughts, harming global ecosystems. During droughts, plants can close their stomata to save water, at the expense of a reduced carboxylation rate. When in a carboxylation-limited regime, plants benefit from an increase in water availability, as it increases their photosynthetic rate. The sun-induced chlorophyll fluorescence (SIF) signal, measurable from satellites, is mechanistically linked to this rate. Like canopy photosynthesis, SIF carries an imprint from the available irradiation (PAR) as well as the canopy structure and the efficiency of the photosynthesis at the photosystem level. Normalizing the global TROPOMI SIF observations with TROPOMI reflectance and MODIS Normalised Difference Vegetation Index (NDVI) data, we extracted the fluorescence quantum yield (ϕF), which lab-scale experiments have found to be linked to the photosynthetic electron transport. Plant physiologists have long proved the photosynthetic electron transport to be sensitive to plant water status. Here, the plant water status is controlled by the soil moisture (SM) and the vapour pressure deficit (VPD). Combining data from the TROPOMI, AIRS and SMAP satellite sensors, this study describes how SM and VPD control the ϕF at the global scale. We identify a VPD range (VPD<1.5 kPa) in which the ϕF is mainly controlled by VPD, and another (VPD>1.5 kPa) in which the ϕF is co-regulated by SM and VPD. The precise values of this range, as well as the magnitude of ϕF values, are modulated by the plant isohydricity. To gain a deeper understanding of the link between ϕF and photosynthetic efficiency at large scale, we used the link between ϕF and data on the canopy conductance (Gs), which were calculated using remote sensing data-driven models. A comparison found that the ϕF-Gs relationship at large scale is in line with the ϕF-Gs relationship described in plant-level studies.

Proton Gradient Regulation 5 is required to avoid photosynthetic oscillations during light transitions.

The production of ATP and NADPH by the light reactions of photosynthesis and their consumption by the Calvin-Benson-Bassham (CBB) cycle and other downstream metabolic reactions requires careful regulation. Environmental shifts perturb this balance, leading to photo-oxidative stress and losses in CO2 assimilation. Imbalances in the production and consumption of ATP and NADPH manifest themselves as transient instability in the chlorophyll fluorescence, P700, electrochromic shift, and CO2 uptake signals recorded on leaves. These oscillations can be induced in wild-type plants by sudden shifts in CO2 concentration or light intensity; however, mutants exhibiting increased oscillatory behaviour have yet to be reported. This has precluded an understanding of the regulatory mechanisms employed by plants to suppress oscillations. Here we show that the Arabidopsis pgr5 mutant, which is deficient in Proton Gradient Regulation 5 (PGR5)-dependent cyclic electron transfer (CET), exhibits increased oscillatory behaviour. In contrast, mutants lacking the NADH-dehydrogenase-like-dependent CET are largely unaffected. The absence of oscillations in the hope2 mutant which, like pgr5, lacks photosynthetic control and exhibits high ATP synthase conductivity, ruled out loss of these photoprotective mechanisms as causes. Instead, we observed slower formation of the proton motive force and, by inference, ATP synthesis in pgr5 following environmental perturbation, leading to the transient reduction of the electron transfer chain and photosynthetic oscillations. PGR5-dependent CET therefore plays a major role in damping the effect of environmental perturbations on photosynthesis to avoid losses in CO2 fixation.

First description of in situ chlorophyll fluorescence signal within East Antarctic coastal polynyas during fall and winter

Antarctic coastal polynyas are persistent and recurrent regions of open water located between the coast and the drifting pack-ice. In spring, they are the first polar areas to be exposed to light, leading to the development of phytoplankton blooms, making polynyas potential ecological hotspots in sea-ice regions. Knowledge on polynya oceanography and ecology during winter is limited due to their inaccessibility. This study describes i) the first in situ chlorophyll fluorescence signal (a proxy for chlorophyll-a concentration and thus presence of phytoplankton) in polynyas between the end of summer and winter, ii) assesses whether the signal persists through time and iii) identifies its main oceanographic drivers. The dataset comprises 698 profiles of fluorescence, temperature and salinity recorded by southern elephant seals in 2011, 2019-2021 in the Cape-Darnley (CDP;67˚S-69˚E) and Shackleton (SP;66˚S-95˚E) polynyas between February and September. A significant fluorescence signal was observed until April in both polynyas. An additional signal occurring at 130m depth in August within CDP may result from in situ growth of phytoplankton due to potential adaptation to low irradiance or remnant chlorophyll-a that was advected into the polynya. The decrease and deepening of the fluorescence signal from February to August was accompanied by the deepening of the mixed layer depth and a cooling and salinification of the water column in both polynyas. Using Principal Component Analysis as an exploratory tool, we highlighted previously unsuspected drivers of the fluorescence signal within polynyas. CDP shows clear differences in biological and environmental conditions depending on topographic features with higher fluorescence in warmer and saltier waters on the shelf compared with the continental slope. In SP, near the ice-shelf, a significant fluorescence signal in April below the mixed layer (around 130m depth), was associated with fresher and warmer waters. We hypothesize that this signal could result from potential ice-shelf melting from warm water intrusions onto the shelf leading to iron supply necessary to fuel phytoplankton growth. This study supports that Antarctic coastal polynyas may have a key role for polar ecosystems as biologically active areas throughout the season within the sea-ice region despite inter and intra-polynya differences in environmental conditions.

Early detection of phosphorus deficiency stress in cucumber at the cellular level using chlorophyll fluorescence signals

Abiotic stressors contribute to growth restriction and developmental disorders in plants. Early detection of the first signs of changes in plant functioning is very important. The objective of this study was to identify chlorophyll fluorescence parameters that change under phosphorus deficiency stress in cucumber. In this work, a trail to study the early changes caused by phosphorus deficiency in cucumber plants by analysing their photosynthetic performance is presented. Chlorophyll- a fluorescence (ChF) parameters were measured every 7 days for a period of 28 days. Measurements were made separately on young and old leaves and on cucumber fruit. Parameters that decreased during the stress were: p2G, PI abs, PI total, REo/CS o, and TRo/CSo. P deficiency decreased total electron carriers per RC ( ECo/RC), yields ( TRo/ABS ( Fv/Fm), ETo/TRo, REo/ETo, ETo/ABS and REo/ABS), fluxes ( REo/RC and REo/CSo) and fractional reduction of PSI end electron acceptors, and damaged all photochemical and non-photochemical redox reactions. Principal component analysis revealed a group of ChF parameters that may indicate early phosphorus deficiency in cucumber plants. Our results are used in the discovery of sensitive bioindicators of phosphorus deficiency in cucumber plants. Most JIP test parameters are linked to mathematical equations, so we recommend using of advanced statistical tools, such as principal component analysis, which should be considered very useful for stress identification. It has also been shown to be more effective in multivariate methods compared to univariate statistical methods was demonstrated.

The divergence of micrometeorology sensitivity leads to changes in GPP/SIF between cork oak and poplar

The observation of solar-induced chlorophyll fluorescence (SIF) signals on a spaceborne platform provides a new idea for estimating and monitoring the gross primary production (GPP) of terrestrial ecosystems at the regional scale. To improve the accuracy of remote sensing SIF signals to estimate GPP, the combination of ground tower-based continuous SIF signals with GPP to explore the mechanical connection in different ecosystems is needed. Based on continuous ground observations, the discrepancy of the links between SIF and GPP and the response to micrometeorology factors of two temperate deciduous broadleaf plantations was explored. The nonlinear relationship of SIF and GPP on the 30 min timescale transformed into a linear correlation with aggregation. With the increase in SIF, GPP gradually saturates and even declines in cork oak. We found SIF “promotion” by using intense incident radiation in cork oak and GPP “depression” or saturation in both species. GPP is more sensitive to instantaneous stresses such as intense radiation imposed high air temperature (Ta) in the midday/afternoon, and the stress threshold of cork oak is lower. We used GPP/SIF as a metric for the relationship, which is significantly higher in poplar, and found GPP/SIF of both species decrease with increasing photosynthetically active radiation (PAR). Since the difference in the amplitude of SIF between tree species was not prominent, the diversity of GPP/SIF performance was mainly caused by the divergence of GPP. The GPP/SIF of cork oak continues to decrease with increasing vapor pressure deficit (VPD), indicating that the relationship between SIF and GPP has a strong daily pattern in the constraint of micrometeorology factors. Our results show that characterizing the impact of micrometeorological conditions on the SIF-GPP relationship in a specific ecosystem is of great significance for the reliable estimation of GPP from remote sensing.

A study on cotton yield prediction based on the chlorophyll fluorescence parameters of upper leaves

The early and accurate monitoring of crop yield is important for field management, storage needs, and cash flow budgeting. Traditional cotton yield measurement methods are time-consuming, labor-intensive, and subjective. Chlorophyll fluorescence signals originate from within the plant and have the advantages of being fast and non-destructive, and the relevant parameters can reflect the intrinsic physiological characteristics of the plant. Therefore, in this study, the top four functional leaves of cotton plants at the beginning of the flocculation stage were used to investigate the pattern of the response of chlorophyll fluorescence parameters (e.g., F0, Fm, Fv/F0, and Fv/Fm) to nitrogen, and the cumulative fluorescence parameters were constructed by combining them with the leaf area index to clarify the correlation between chlorophyll fluorescence parameters and cotton yield. Support vector machine regression (SVM), an artificial neural network (BP), and an XGBoost regression tree were used to establish a cotton yield prediction model. Chlorophyll fluorescence parameters showed the same performance as photosynthetic parameters, which decreased as leaf position decreased. It showed a trend of increasing and then decreasing with increasing N application level, reaching the maximum value at 240 kg·hm-2 of N application. The correlation between fluorescence parameters and yield in the first, second, and third leaves was significantly higher than that in the fourth leaf, and the correlation between fluorescence accumulation and yield in each leaf was significantly higher than that of the fluorescence parameters, with the best performance of Fv/Fm accumulation found in the second leaf. The correlation between Fv/Fm accumulation and yield in the top three leaves combined was significantly higher than that in the top four leaves. The correlation coefficient between Fv/Fm accumulation and yield was the highest, indicating the feasibility of applying chlorophyll fluorescence to estimate yield. Based on the machine learning algorithm used to construct a cotton yield prediction model, the estimation models of Fv/F0 accumulation and yield of the top two leaves combined as well as top three leaves combined were superior. The estimation model coefficient of determination of the top two leaves combined in the BP algorithm was the highest. In general, the Fv/F0 accumulation of the top two leaves combined could more reliably predict cotton yield, which could provide technical support for cotton growth monitoring and precision management.

Species-specific responses of Antarctic terrestrial microalgae to salinity stress. Comparative study in Klebsormidium sp. and Stigeoclonium sp.

We studied the changes in PSII photochemical processes in the cells of Antarctic algae Klebsormidium sp. and Stigeoclonium sp. exposed to salinity stress (0 – 3M NaCl) for 3 h. Salinity stress induced a decrease in the potential (FV/FM) and effective quantum yield of PSII electron transport (FPSII). Salinity stress induced a decrease in vitality index (Rfd, relative decrease of chlorophyll fluorescence). Analyses of the polyphasic fast chlorophyll fluorescence transients (OJIP) showed that with the increase in salt concentration, the chlorophyll fluorescence signals recorded at the phases J, I, and P declined, and the transient flattened with increaseing NaCl concentration reaching close to zero ChlF values at salt concentration of 3 M NaCl after 180 min. exposition. Klebsormidium sp. was found more salinity stress resistant than Stigeoclonium sp.

The Effect of LED and HPS Assimilation Lighting on Leaf Anatomy, Chlorophyll and Carotenoid Autofluorescence Signals, and Some Physiological and Chemical Leaf Traits Related to the Productivity of Cucumber (Cucumis sativus L.) in High-Wire Cultivation

Supplemental lighting with light-emitting diode (LED) lamps and/or high-pressure sodium (HPS) lamps was applied to increase the activity of the photosynthetic apparatus and thus productivity of greenhouse cucumber (Cucumis sativus L.) in a high-wire growing system. The colocalisation of the chlorophyll of PSII (located mainly in grana) and carotenoid fluorescence signals in chloroplasts of cucumber leaves was studied under confocal microscopy. Leaf anatomy and some chemical quality traits (dry matter, chlorophyll, carotenoids, total soluble solids, total sugars and nitrate reductase activity) as well as selected chlorophyll fluorescence parameters were also investigated and subjected to the multidimensional principal component analysis together with the data on fruit yield. Under LED lighting, a lower correlation between the occurrence of chlorophyll and carotenoid fluorescence signals was observed, especially in older (lower-located) leaves, which may have resulted from changes in the distribution of carotenoids within chloroplasts and/or relative concentrations of chlorophyll and carotenoids. Compared to toplighting with HPS lamps, most commonly used in commercial greenhouse cucumber production, the application of LED interlighting, especially in combination with LED toplighting, led to the increase in chlorophyll and carotenoid content and photosynthetic performance index in older leaves, which was related to the increased cucumber productivity.

Water quality assessment in mosquito breeding habitats based on dissolved organic matter and chlorophyll measurements by laser-induced fluorescence spectroscopy.

Rapid urbanization and its associated pollution can affect water quality in mosquito breeding habitats and, as a result, the ecology and control of mosquito vectors. To understand the effects of pollution on mosquito vectors, an accurate assessment of water quality in breeding habitats is needed. Presently, water quality assessment of mosquito breeding habitats is usually based on the measurement of individual physicochemical parameters. However, several parameters are sometimes difficult to interpret or may not give a clear picture of the overall water quality of the breeding habitats, especially when the pollutants are in complex mixtures. This study employed the use of Laser-Induced Fluorescence (LIF) spectroscopy to assess water quality in breeding habitats of Anopheles, Aedes, and Culex mosquitoes in urban areas in Cape Coast, Ghana. The LIF spectra, using a 445-nm diode laser, were measured from field-collected water samples in the laboratory. The LIF spectra showed the presence of dissolved organic matter (DOM) and chlorophyll in the breeding habitats. The DOM and chlorophyll fluorescence signals were normalised by the Raman vibrational signals to determine water quality in each habitat. The overall water quality was better in Aedes breeding habitats than in Anopheles and Culex breeding habitats. The poor water quality in Anopheles and Culex breeding habitats was due to the presence of high fulvic acid and chlorophyll content, which often reflect pollutants from anthropogenic sources. Anopheles and Aedes habitats were made up of mainly An. coluzzii and Ae. aegypti respectively while Culex species were identified to genus level. The results add up to the growing concern about the breeding of Anopheles in polluted habitats. The study demonstrated for the first time the ability of LIF spectroscopy to assess water quality in mosquito breeding habitats.

Inferring Agronomical Insights for Wheat Canopy Using Image-Based Curve Fit K-Means Segmentation Algorithm and Statistical Analysis.

Phenomics and chlorophyll fluorescence can help us to understand the various stresses a plant may undergo. In this research work, we observe the image-based morphological changes in the wheat canopy. These changes are monitored by capturing the maximum area of wheat canopy image that has maximum photosynthetic activity (chlorophyll fluorescence signals). The proposed algorithm presented here has three stages: (i) first, derivation of dynamic threshold value by curve fitting of data to eliminate the pixels of low-intensity value, (ii) second, extraction and segmentation of thresholded region by application of histogram-based K-means algorithm iteratively (this scheme of the algorithm is referred to as the curve fit K-means (CfitK-means) algorithm); and (iii) third, computation of 23 grey level cooccurrence matrix (GLCM) texture features (traits) from the wheat images has been done. These features help to do statistical analysis and infer agronomical insights. The analysis consists of correlation, factor, and agglomerative clustering to identify water stress indicators. A public repository of wheat canopy images was used that had normal and water stress response chlorophyll fluorescence images. The analysis of the feature dataset shows that all 23 features are proved fruitful in studying the changes in the shape and structure of wheat canopy due to water stress. The best segmentation algorithm was confirmed by doing exhaustive comparisons of seven segmentation algorithms. The comparisons showed that the best algorithm is CfitK-means as it has a maximum IoU score value of 95.75.

Quantification of chlorophyll fluorescence in soybean seeds by multispectral images and their relationship with physiological potential

Abstract: The multispectral image analysis technique to detect chlorophyll fluorescence (CF) in soybean seeds was studied to assess the relationship between CF signals and seed physiological potential. Eight treatments, corresponding to 0%, 2%, 4%, 6%, 8%, 10%, 12%, and 14% green seeds, were used on two cultivars, BMX Desafio RR 8473 RSF and 96R10 IPRO, which passed through different seed quality tests. Initially, the CF of the seeds was determined using 660 nm and 730 nm spectra, and then the germination, electrical conductivity, accelerated aging with saturated NaCl solution, tetrazolium, and computerized seedling image analysis (Vigor-S) tests were performed on the same seeds. A completely randomized design was used, as well as replications of each treatment. Analysis of variance (ANOVA) was performed on the data from germination, vigor, and CF tests using the R® software, and the means were grouped by the Scott-Knott test (p ≤ 0.05). Pearson’s linear correlation coefficients (r) were calculated for all combinations among the evaluations with significance of the r values determined by the t-test (p ≤ 0.05), and multivariate analysis of the principal components was performed. Proportional increases in green seeds contribute to an increase in chlorophyll fluorescence signals and have a negative correlation with seed physiological quality; levels above 4% green seeds in the samples result in marked losses in physiological potential. Therefore, the chlorophyll fluorescence detected through multispectral images is inversely related to the physiological potential of soybean seeds.

Performance of Singular Spectrum Analysis in Separating Seasonal and Fast Physiological Dynamics of Solar‐Induced Chlorophyll Fluorescence and PRI Optical Signals

AbstractHigh temporal resolution measurements of solar‐induced chlorophyll fluorescence (F) and the Photochemical Reflectance Index (PRI) encode vegetation functioning. However, these signals are modulated by time‐dependent processes. We tested the applicability of the Singular Spectrum Analysis (SSA) for disentangling fast components (physiology‐driven) and slow components (controlled by structural and biochemical properties) from PRI, far‐red F (F760), and far‐red apparent fluorescence yield (Fy∗760). The proof of concept was developed on spectral and flux time series simulated with the Soil Canopy Observation of Photochemistry and Energy fluxes (SCOPE) model. This allowed the evaluation of SSA decomposition against variables that are independent of physiology or are modified by it. Slow SSA‐decomposed components of PRI and Fy∗760 showed high correlations with the reference variables (R2 = 0.97 and 0.96, respectively). Fast SSA‐decomposed components of PRI and Fy∗760 were better related to the physiological reference variables than the original signals during periods when leaf area index (LAI) was above 1 m2 m−2. The method was also successfully applied to predict light‐use efficiency (LUE) from the fast SSA‐decomposed components of PRI (R2 = 0.70) and Fy∗760 (R2 = 0.68) when discarding data modeled with LAI &lt; 1 m2 m−2 and short‐wave radiation Rin &lt; 250 W m−2. The method was then tested on data acquired in a Mediterranean grassland. In this case, the fast SSA‐decomposed component of apparent LUE∗ showed a stronger correlation with the fast SSA‐decomposed component of Fy∗760 (R2 = 0.42) than with original Fy∗760 (R2 = 0.01). SSA‐based approach is a promising tool for decoupling physiological information from measurements acquired with automated proximal sensing systems.

Reflectance and chlorophyll fluorescence-based retrieval of photosynthetic parameters improves the estimation of subtropical forest productivity

Forest ecosystems play a significant role in climate change mitigation and uptake a larger amount of atmospheric CO2 than other terrestrial ecosystems via photosynthesis process in form of gross primary production (GPP). The photosynthesis or GPP is largely determined by the photosynthetic capacity of vegetation (i.e., maximum rate of carboxylation, Vcmax) in ecosystem models. However, considerable uncertainties of Vcmax estimates may limit our potential to address scientific issues of GPP related to the increasing emission of atmospheric CO2. Recently, solar-induced chlorophyll fluorescence (SIF) signals have been used as a proxy for resolving photosynthesis. In this study, the biochemical and structural parameters were retrieved from hyperspectral reflectance and fluorescence quantum efficiency (FQEs) was retrieved from ground-based SIF. Then, retrieved parameters were incorporated into the Soil Canopy Observation Photosynthesis and Energy (SCOPE) model to explore the potential of ground-based SIF to track Vcmax variability for a subtropical evergreen mixed forest. Then, SIF-derived Vcmax was used to parameterize Boreal Ecosystem Production Simulator (BEPS) model to simulate the GPP. With retrieved vegetative parameters and FQEs, the ground-based SIF was strongly correlated with the model-based SIF simulation at O2-B and O2-A bands, demonstrating that the coefficient of determination (R2) improved from 0.15 (constant values) to 0.60 (retrieved values) for SIFB and from 0.79 to 0.94 for SIFA simulation. Using SIF-derived Vcmax, the R2 value of simulated GPP against eddy covariance-based measurements substantially increased from 0.18 (constant Vcmax) to 0.38 (SIF-derived Vcmax) for dry season and from 0.56 to 0.67 for wet season respectively. The utilization of SIF-derived Vcmax with its corrected temperature response function reduced the relative error in annual GPP simulations by 24.9%. Our results support the significant references toward reducing unbiased SIF simulation and highlighting the potential of ground SIF in deriving Vcmax at the site scale for defining forest management options.

Roles of alternative electron flows in response to excess light in Ginkgo biloba

Ginkgo biloba L., the only surviving species of Ginkgoopsida, is a famous relict gymnosperm, it may provide new insight into the evolution of photosynthetic mechanisms. Flavodiiron proteins (FDPs) are conserved in nonflowering plants, but the role of FDPs in gymnosperms has not yet been clarified. In particular, how gymnosperms integrate FDPs and cyclic electron transport (CET) to better adapt to excess light is poorly understood. To elucidate these questions, we measured the P700 signal, chlorophyll fluorescence and electrochromic shift signal under fluctuating and constant light in G. biloba. Within the first seconds after light increased, G. biloba could not build up a sufficient proton gradient (ΔpH). Concomitantly, photo-reduction of O2 mediated by FDPs contributed to the rapid oxidation of P700 and protected PSI under fluctuating light. Therefore, in G. biloba, FDPs mainly protect PSI under fluctuating light at acceptor side. Under constant high light, the oxidation of PSI and the induction of non-photochemical quenching were attributed to the increase in ΔpH formation, which was mainly caused by the increase in CET rather than linear electron transport. Therefore, under constant light, CET finely regulates the PSI redox state and non-photochemical quenching through ΔpH formation, protecting PSI and PSII against excess light. We conclude that, in G. biloba, FDPs are particularly important under fluctuating light while CET is essential under constant high light. The coordination of FDPs and CET fine-tune photosynthetic apparatus under excess light.

Detailed reconstruction of trees from terrestrial laser scans for remote sensing and radiative transfer modelling applications

AbstractThis study presents a method for three-dimensional (3D) reconstruction of forest tree species that are, for instance, required for simulations of 3D canopies in radiative transfer modelling. We selected three forest species of different architecture: Norway spruce (Picea abies) and European beech (Fagus sylvatica), representatives of European production forests, and white peppermint (Eucalyptus pulchella), a common forest species of Tasmania. Each species has a specific crown structure and foliage distribution. Our algorithm for 3D model construction of a single tree is based on terrestrial laser scanning (TLS) and ancillary field measurements of leaf angle distribution, percentage of current-year and older leaves, and other parameters that could not be derived from TLS data. The algorithm comprises four main steps: (i) segmentation of a TLS tree point cloud separating wooden parts from foliage, (ii) reconstruction of wooden parts (trunks and branches) from TLS data, (iii) biologically genuine distribution of foliage within the tree crown and (iv) separation of foliage into two age categories (for spruce trees only). The reconstructed 3D models of the tree species were used to build virtual forest scenes in the Discrete Anisotropic Radiative Transfer model and to simulate canopy optical signals, specifically: angularly anisotropic top-of-canopy reflectance (for retrieval of leaf biochemical compounds from nadir canopy reflectance signatures captured in airborne imaging spectroscopy data) and solar-induced chlorophyll fluorescence signal (for experimentally unfeasible sensitivity analyses).