Chlorophyll Fluorescence Emission Research Articles

Toxicogenetic, biochemical, and physiological effects of azoxystrobin and carbendazim fungicides over Lactuca sativa L. and Phaseolus vulgaris L.

Fungicides are pesticides that are frequently used in agriculture because of their action against fungal diseases. However, the widespread application of pesticides around the world raises environmental and public health concerns, since these compounds are toxic and can pose risks to ecosystems and human health. The aim of this study was to evaluate the phytotoxic, cytogenotoxic, and biochemical effects of azoxystrobin and carbendazim on Lactuca sativa L. and their physiological effects on Phaseolus vulgaris L. by analyzing the cell cycle and chromosomal and nuclear alterations in L. sativa; the biochemical effects of azoxystrobin and carbendazim on Phaseolus vulgaris L. and their physiological effects on Phaseolus vulgaris L. by analyzing the cell cycle and chromosomal and nuclear alterations in L. sativa; the biochemical effects by analyzing the activity of antioxidant enzymes in L. sativa; and the physiological effects by analyzing chlorophyll content and chlorophyll a fluorescence in P. vulgaris. It was observed that both fungicides were phytotoxic and cytotoxic, reducing root growth and the mitotic index, cytogenotoxic, increasing the occurrence of chromosomal alterations, as well as inducing oxidative stress and an increase in chlorophyll fluorescence emission and altered energy absorption in the plants used as a test system. In view of this, studies such as the one presented here indicate that the use of pesticides, even in small quantities, can lead to damage to the metabolism of plant organisms.

Deriving photosystem-level red chlorophyll fluorescence emission by combining leaf chlorophyll content and canopy far-red solar-induced fluorescence: Possibilities and challenges

Solar-induced chlorophyll fluorescence (SIF) emitted from photosystem I (PSI) and photosystem II (PSII) is characterized by two peaks centered in the red and far-red spectral regions. SIF provides a unique remotely sensible signal to track plant photosynthetic dynamics. Compared with far-red SIF, red SIF (RSIF) is more strongly linked to PSII and thus with plant photosynthetic activity, but is subject to stronger reabsorption within leaves and canopies. This hinders the understanding and use of canopy RSIF observations (RSIFobs), which is only a small fraction of the total RSIF emitted by the photosystems (RSIFtotal). Deriving RSIFtotal from RSIFobs is still challenging due to retrieval uncertainty, limited availability of RSIFobs and spectral overlap with chlorophyll absorption. To address the challenges associated with deriving RSIFtotal, we propose an exploratory method framework that combines canopy far-red SIF observations (FRSIFobs) and leaf chlorophyll content (LCC) to derive RSIFtotal. We first downscale FRSIFobs from canopy to leaf, and then leverage LCC information to estimate RSIF at the leaf level. Finally, we incorporate LCC information in the subsequent downscaling of RSIF from leaf to photosystem. To evaluate our approach, we use ground-based observation data in three crop types (rice, wheat, and maize) and SCOPE model simulations. Our results demonstrate that the seasonal patterns of RSIFtotal show a close agreement with the seasonal patterns of gross primary production (GPP) and absorbed photosynthetic active radiation (APAR). More importantly, RSIFtotal slightly outperforms FRSIFobs in estimating GPP for the three crop types. Our study has also revealed a strong linear relationship between the escape probability of RSIFtotal (fesc_R) and the RSIFobs/FRSIFobs ratio affected by LCC. The simplicity and robustness of our approach, along with its potential application in satellite remote sensing, will contribute to the improvement of large-scale GPP estimation and photosynthetic phenology detection. Moreover, our investigation of fesc_R will contribute to a better understanding the physiological and non-physiological dynamics of RSIFobs.

Hyperspectral and Photodiode Retrievals of Nighttime LED‐Induced Chlorophyll Fluorescence (LEDIF) for Tracking Photosynthetic Phenology in a Vineyard

AbstractThe magnitude of chlorophyll fluorescence emission represents both chlorophyll content and energy quenching processes enabling its application to serve as a proxy of photosynthetic activity. Thus, there is interest in advancing methods for canopy‐scale monitoring of chlorophyll fluorescence. Remotely sensed solar‐induced fluorescence (SIF) retrievals offer daytime monitoring of chlorophyll fluorescence, which can serve as an indicator of photosynthesis. However, it represents an instantaneous measurement during the day, which is strongly influenced by incoming radiation, solar angle, and sun/shade fraction—making it difficult to tease out baseline information on plant health and potential photosynthetic capacity—which could be tracked by changes in fluorescence yield (independent of sunlight). Recent advances have demonstrated the potential for inducing nighttime chlorophyll fluorescence via LED light sources at the canopy‐scale, which can be retrieved as LED‐induced chlorophyll fluorescence (LEDIF), potentially serving as a baseline indicator of plant health and photochemical capacity, independent of daytime conditions. In this study, we explored two methods of LEDIF retrievals: (a) hyperspectral sensor (1.33 nm full‐width half max) and (b) low‐cost Red‐Far‐Red photodiode sensor. LEDIF retrieved by the hyperspectral sensor demonstrated strong correlations with daytime SIF and gross primary productivity during mid to end of season phenology (R2 > 0.70). In contrast, phenological dynamics of LEDIF retrieved by the photodiode sensor was more subtle, likely due to weaker signal‐to‐noise ratio, but still demonstrated some potential. Overall, LEDIF offers a technique to monitor nighttime chlorophyll fluorescence emissions (and changes in its spectral shape with a hyperspectral sensor) to assess canopy‐scale phenology of photosynthetic potential.

Tracking canopy chlorophyll fluorescence with a low-cost light emitting diode platform.

Chlorophyll fluorescence measured at the leaf scale through pulse amplitude modulation (PAM) has provided valuable insight into photosynthesis. At the canopy- and satellite-scale, solar-induced fluorescence (SIF) provides a method to estimate the photosynthetic activity of plants across spatiotemporal scales. However, retrieving SIF signal remotely requires instruments with high spectral resolution, making it difficult and often expensive to measure canopy-level steady-state chlorophyll fluorescence under natural sunlight. Considering this, we built a novel low-cost photodiode system that retrieves far-red chlorophyll fluorescence emission induced by a blue light emitting diode (LED) light source, for 2 h at night, above the canopy. Our objective was to determine if an active remote sensing-based night-time photodiode method could track changes in canopy-scale LED-induced chlorophyll fluorescence (LEDIF) during an imposed drought on a broadleaf evergreen shrub, Polygala myrtifolia. Far-red LEDIF (720-740 nm) was retrieved using low-cost photodiodes (LEDIFphotodiode) and validated against measurements from a hyperspectral spectroradiometer (LEDIFhyperspectral). To link the LEDIF signal with physiological drought response, we tracked stomatal conductance (gsw) using a porometer, two leaf-level vegetation indices-photochemical reflectance index and normalized difference vegetation index-to represent xanthophyll and chlorophyll pigment dynamics, respectively, and a PAM fluorimeter to measure photochemical and non-photochemical dynamics. Our results demonstrate a similar performance between the photodiode and hyperspectral retrievals of LEDIF (R2 = 0.77). Furthermore, LEDIFphotodiode closely tracked drought responses associated with a decrease in photochemical quenching (R2 = 0.69), Fv/Fm (R2 = 0.59) and leaf-level photochemical reflectance index (R2 = 0.59). Therefore, the low-cost LEDIFphotodiode approach has the potential to be a meaningful indicator of photosynthetic activity at spatial scales greater than an individual leaf and over time.

Row orientation influences the diurnal cycle of solar-induced chlorophyll fluorescence emission from wheat canopy, as demonstrated by radiative transfer modeling

Although multiple studies have used solar-induced chlorophyll fluorescence (SIF) of the crop canopy to estimate photosynthetic activity, the effect of canopy geometry (row orientation) on diurnal SIF remains uncertain. Using a combination of 3D canopy radiative transfer modeling and spectroscopic observations collected in a winter wheat field, we investigated (i) the relationships between observed SIF and factors including temperature, humidity, absorbed photosynthetically active radiation (APAR), and row orientation; (ii) the temporal pattern and multi-angle distribution of diurnal canopy SIF; and (iii) the mechanisms influencing components of diurnal SIF under the effect of row orientation. The relationship between observed SIF and APAR was relatively strong at the early growth period, but SIF was influenced by various factors (temperature, humidity, and row–sun angle) at the middle of the growth period. To test an asymmetric crop row effect as a potential cause of this skewness, we evaluated the modeled SIF with a 3D northwestward (315°) crop row alignment and other orientations. Each row orientation resulted in a trough-like depression parallel to the row in simulated canopy SIF, which could cause a diurnal asymmetry of canopy SIF, a fraction of fluorescence escape, and a fraction of APAR. View angles, the dimension of canopy objects (1D/3D), inter-row width, and leaf structure parameters impacted the representativeness of these simulations. An increased shade SIF, synchronized with the time when row–sun angles become zero at the incident sunlight being parallel to the crop row, was linked to decreasing canopy SIF emissions and would cause a non-linear SIF–APAR relationship at certain row orientations especially facing diagonally to the south. The concept of row orientation provides a novel viewpoint of diurnal canopy SIF from which to test the effect of row geometry in cropland, and will promote a mechanistic understanding of photosynthesis and canopy light use.

Improved monitoring of cryptogam gas-exchange and volatile emissions during desiccation-rehydration cycles with a within-chamber hydration method

Desiccation-rehydration studies in cryptogams constitute an important tool to understand the relation of key physiological traits with species stress tolerance and environmental adaptability. Real-time monitoring of responses has been limited by the design of commercial or custom measuring cuvettes and difficulties in experimental manipulation. We developed a within-chamber rehydration method that allows to rewater the samples rapidly, without the need to open the chamber and take out the sample for manual rehydration by the investigator. Data is collected in real-time and simultaneously with an infrared gas-analyzer (LICOR-7000), a chlorophyll fluorometer (Maxi Imaging-PAM) and a proton transfer reaction time-of-flight mass-spectrometer (PTR-TOF-MS) for volatile organic compound emissions. The system was tested on four cryptogam species with contrasting ecological distributions. No major errors or kinetics disruptions were found during system testing and measurements. Our within-chamber rehydration method improved accuracy, as measurement periods were not lacking, and repeatability of the protocol by reducing error variance in sample manipulation. This method provides an improved technique to conduct desiccation-rehydration measurements, contributing to the standardization and accuracy of current existing methodologies. A close real-time and simultaneous monitoring of photosynthesis, chlorophyll fluorescence and volatile organic compound emission data, offers a novel perspective in the analysis of the cryptogam stress responses that is yet to be fully explored.

The photosynthetic response of spectral chlorophyll fluorescence differs across species and light environments in a boreal forest ecosystem

Chlorophyll fluorescence can serve as a proxy of photosynthesis in boreal forests. When sustained non-photochemical quenching (NPQS) relaxes towards summer, leaf chlorophyll fluorescence (ChlF) emission increases along with photosynthesis. Yet, other physical and physiological factors can also leave a measurable imprint on the fluorescence emission spectra, and disrupt this relationship.We measured spectral ChlF in leaves of three dominant evergreen species in the boreal ecosystem (Scots pine, Norway spruce and lingonberry) growing under contrasting light environments and throughout the spring recovery of photosynthesis. We also measured photosynthetic, biochemical and morphological traits. Correlations between traits and ChlF spectral components were analyzed to identify the mechanisms underlying both the spatial variation found between species and light environments, and the temporal variation along the spring recovery of photosynthesis.Spatially, we found evidence of baseline differences in leaf-level ChlF magnitude, which we attribute to species- and light environment-specific changes in leaf morphology. Temporally, ChlF magnitude followed the relaxation of NPQS towards summer, but only in upper canopy foliage and lingonberry, suggesting a seasonal compensation effect between sustained photochemical quenching (PQS) and NPQS, potentially decoupling the seasonal relationship between ChlF and photosynthesis in shaded foliage. Finally, we show subtle changes in the shape of the ChlF spectra that took place independently of chlorophyll concentration dynamics, pointing to the complexity of NPQS which could reflect structural rearrangements in the thylakoids and changes in the relative contribution of PSI to emitted ChlF.We conclude that the diversity of species and light environments found within an ecosystem generates a baseline level of variation in leaf spectral ChlF as well as contrasting seasonal photosynthetic acclimation patterns. These sources of variability should be taken into account when developing quantitative models for the interpretation of ChlF data, in particular for applications involving high resolution SIF imaging systems capable of resolving different plant individuals and their parts.

Downregulation of PSI regulates photosynthesis in early successional tree species. Evidence from a field survey across European forests

Photosynthesis rates and electron transport are key physiological traits that distinguish the successional status of plants (early successional – ES, and late successional – LS) in a community. ES plants can respond quickly to sudden changes in sunlight radiation exposure and display a greater photosynthetic plasticity, with respect to the LS ones, thanks to their ability to regulate the photosynthesis. Photosynthesis regulation is connected to the efficiency of the two photosystems (PSI and PSII). PSI plays an important role in the photosynthetic machinery functioning to respond against sudden changes of metabolism in response to heterogeneous environmental conditions. It can be downregulated under high pressure of reductants, to maintain the balance between the reductant pressure and the metabolic demand. The efficiency of PSII and of PSI can be estimated, respectively, by the maximum quantum yield of photosystem II (FV/FM) and the contribution of the thermal phase in the chlorophyll fluorescence emission (expressed by the parameter ΔVIP, that quantify the I-P phase in the fluorescence emission transient). Both parameters are evaluated in dark-adapted samples and can be measured by prompt fluorescence technique. Here, we analyzed a large dataset from a European-wide survey on mature forests, considering four representative forest tree species growing at six sampling sites distributed along a latitudinal gradient. Two early successional (Pinus sylvestris and Betula pendula) and two late successional (Picea abies and Fagus sylvatica) species were considered. The correlations between FV/FM and I-P phase were mostly positive for different plant species at each sampling site for FV/FM values < 0.82 (assumed as optimal threshold for this parameter), but not for FV/FM values > 0.82. In this last case, the correlations were mostly negative for early successional species, and not significant for late successional species. Foliar nitrogen concentration plays a relevant role for the PSI regulation in early successional species: trees with high nitrogen content have higher I-P phase values and negative correlations between FV/FM and I-P phase for FV/FM > 0.82. An opposite pattern was found in tree species with low foliar nitrogen level. We conclude that the late successional tree species show a substantial regulation and stability of the photosynthetic machinery and photosystem stoichiometry, whereas early successional species have more dynamic behavior of PSI. Early successional species can modulate the photosynthetic efficiency and are able both to up-regulate and down-regulate the PSI concentration and activity in relation to quantum yield capacity and leaf nitrogen content.

A novel hybrid machine learning phasor-based approach to retrieve a full set of solar-induced fluorescence metrics and biophysical parameters

The emission of solar-induced chlorophyll fluorescence (F) is a pivotal process to infer vegetation health and functioning that can be monitored by remote sensing. However, most of the current remote sensing methods retrieve only F at top-of-canopy level, therefore making the link with physiological processes occurring at photosystem level not trivial. In this study, we develop a novel machine learning Fourier (phasor)-based algorithm to retrieve F both at canopy level and after considering the reabsorption (i.e. photosystem level), consistently with relevant biophysical variables, exploiting the canopy apparent reflectance spectra (Rapp). In particular, Rapp is divided in consecutive spectral windows, where the Discrete Fourier Transform (DFT) is computed. Then, the DFT results in each window are exploited to estimate the fluorescence spectra and biophysical parameters, together with their uncertainties, by means of a supervised machine learning algorithm coupled to a statistical-based retrieval pipeline. The algorithm has been trained through synthetic Rapp spectra, obtained from simulations based on a Radiative Transfer (RT) model. As a proof of concept, the theoretical approach is then applied to experimental data, acquired both from crops and forests, at close and high soil-sensor distance respectively, to evaluate the retrieval accuracy of biophysical and F parameters. In particular, for the first time Rapp is used to extract the temporal evolution of F at canopy and photosystem levels and its quantum efficiency together with different biophysical variables, during the growing season of two agricultural crops. Furthermore, tower-based solar-induced fluorescence measurements in a deciduous forest are exploited to evaluate the performance of our algorithm when the atmospheric reabsorption and scattering are not negligible. The reliability of the proposed method is evaluated through a comparison with F spectra extracted from the state of the art SpecFit retrieval algorithm. This work promises a substantial advance toward a new accurate retrieval method for fluorescence signals and biophysical parameters at canopy and photosystem levels.

Iodine Enhances the Nutritional Value but Not the Tolerance of Lettuce to NaCl

Positive stress or essential and nonessential elements can improve nutritive values (biofortification) of edible plants. In the present study, we evaluate (i) the effect of moderate salinity on lettuce biofortification, evaluated as nutritional bioactive compound accumulation, and (ii) the role of iodine in enhancing salt tolerance by increasing photorespiration and the content of antioxidants in lettuce. Physiological (gas exchange and chlorophyll fluorescence emission) and biochemical (photosynthetic pigment and bioactive compound) analyses were performed on lettuce plants grown under moderate salinity (50 mM NaCl alone or 50 mM NaCl in combination with iodine, KIO3). Our results show that NaCl + iodine treatment improves the nutritional value of lettuce in terms of bioactive compounds acting as antioxidants. More specifically, iodine enhances the accumulation of photosynthetic pigments and polyphenols, such as anthocyanins, under salt but does not improve the salt tolerance. Our findings indicate that iodine application under moderate salinity could be a valid strategy in plant biofortification by improving nutritional bioactive compound accumulation, thus exercising functional effects on human health.

Canopy Solar-Induced Chlorophyll Fluorescence and Its Link to Transpiration in a Temperate Evergreen Needleleaf Forest during the Fall Transition

Northern hemisphere evergreen needleleaf forest (ENF) contributes a significant fraction of global water exchange but regional transpiration (T) observation in ENF ecosystems is still challenging. Traditional remote sensing techniques and terrestrial biosphere models reproduce the transpiration seasonality with difficulty, and with large uncertainties. Solar-induced chlorophyll fluorescence (SIF) emission from vegetation correlates to photosynthesis at multiple spatial and temporal scales. However, how SIF links to transpiration of evergreen forest during seasonal transition is unclear. Here, we explored the relationship between canopy SIF and T retrieved from ground observation towers in ENF. We also examined the role of meteorological and soil factors on the relationship between SIF and T. A slow decrease of SIF and T with a fast reduction in photosynthetically active radiation (PAR), air temperature, vapor pressure deficit (VPD), soil temperature and soil water content (SWC) were found in the ENF during the fall transition. The correlation between SIF and T at hourly and daily scales varied significantly among different months (Pearson correlation coefficient = 0.29–0.68, p &lt; 0.01). SIF and T were significantly linearly correlated at hourly (R2 = 0.53, p &lt; 0.001) and daily (R2 = 0.67, p &lt; 0.001) timescales in the October. Air temperature and PAR were the major moderating factors for the relationship between SIF and T in the fall transition. Soil water content (SWC) influenced the SIF-T relationship at an hourly scale. Soil temperature and VPD’s effect on the SIF-T relationship was evident at a daily scale. This study can help extend the possibility of constraining ecosystem T by SIF at an unprecedented spatiotemporal resolution during season transitions.

A new semi-analytical model for retrieving chlorophyll a concentration and magnitude of fluorescence in the South China Sea

The red band signal of the color spectrum is of significant interest because its remote sensing reflectance yields valuable information on phytoplankton. In this study, we combined a fluorescence model with a modified version of the Garver-Siegel-Maritorena (GSM) algorithm to retrieve the chlorophyll a concentration (Chla) and the magnitude of fluorescence in coastal waters. An analytical inverse optimization procedure was performed using the measured reflectance over the whole visible spectrum with a focus on the Ocean and Land Color Instrument (OLCI) sensor (from 413 to 709 nm). The coupled visible-infrared model (GSM&amp;Fluo) was validated by a comparison with Chla that was measured in situ in a wide variety of productive coastal water conditions in the South China Sea. The coefficients of determination (r2) were 0.79 and 0.88 for the GSM and our algorithm, respectively. The combined approach produced more accurate results in our study location. The coupled GSM&amp;Fluo model was able to avoid being trapped in a local optimum by adding an estimation of the fluorescence signal. The results also showed that the suspended particulate matter (SPM) played a major role in the magnitude of the reflectance peak in the chlorophyll fluorescence emission band, and the Fluorescence Line Height (FLH) calculated with a simple baseline approach tended to overestimate the true magnitude of the fluorescence emission as SPM increased.

Effects of NaCl and CaCl2 Salinization on Morpho-Anatomical and Physiological Traits of Potted Callistemon citrinus Plants

The aim of this work was to assess the possible coordination mechanisms between leaf functional anatomical traits and physiological acclimation of potted Callistemon citrinus plants when subjected to two saline iso-osmotic solutions using NaCl or CaCl2. Digital image analysis was applied to quantify anatomical parameters with a specific focus on the occurrence of signs of structural damage as well as on alterations, such as the occurrence of shrunk tissues and accumulation of phenolic compounds. Morpho-anatomical analyses were accompanied by measurements of leaf gas exchange and chlorophyll fluorescence emission. Results showed that C. citrinus plants, when irrigated with the CaCl2 solution, induced a leaf morpho-anatomical structure which did not allow the maintenance of high photosynthetic performance under such conditions, compared to both controls and plants treated with the iso-osmotic solution of NaCl. Data collected seem to suggest a close relationship between anatomical traits and eco-physiological strategies in maintaining the photosynthetic efficiency under saline stress conditions and highlight the fundamental role of leaf anatomy in imposing the limits of plant physiology.

Constraining water limitation of photosynthesis in a crop growth model with sun-induced chlorophyll fluorescence

Water fulfils key roles in maintaining a plant's biological activity. Water shortage induces stomatal closure, causing a reduction in photosynthesis and transpiration rates. Sun-induced chlorophyll fluorescence (SIF) emission is sensitive to subtle, stress-induced variations in non-photochemical quenching and in photosynthetic electron transport, caused by e.g., a fluctuation in the water availability. Based on this sensitivity, a framework for calibrating a water stress function in a crop growth model using ground-based SIF observations is proposed. SIF time series are simulated by coupling the AgroC crop growth model to the Soil Canopy Observations Photosynthesis Energy (SCOPE) model. This allowed parametrizing the water stress function in the AgroC crop growth model, resulting in improved estimates of actual evapotranspiration and net ecosystem exchange over a sugar beet stand during stressed periods. The improvement in the estimation of the water and carbon fluxes by AgroC during the summer months highlights the ability of canopyscale SIF observations to serve as a remote sensing metric to indicate the intensity of a stress condition. We argue that our framework, linking SIF emission to stress functions, can be used to extract information concerning drought stress from the Fluorescence Explorer (FLEX) satellite, scheduled for launch in 2024.

Photosynthesis dynamics and regulation sensed in the frequency domain.

Foundations of photosynthesis research have been established mainly by studying the response of plants to changing light, typically to sudden exposure to a constant light intensity after dark acclimation or light flashes. This approach remains valid and powerful, but can be limited by requiring dark acclimation before time-domain measurements and often assumes that rate constants determining the photosynthetic response do not change between dark and light acclimation. We show that these limits can be overcome by measuring plant responses to sinusoidally modulated light of varying frequency. By its nature, such frequency-domain characterization is performed in light-acclimated plants with no need for prior dark acclimation. Amplitudes, phase shifts, and upper harmonic modulation extracted from the data for a wide range of frequencies can target different kinetic domains and regulatory feedbacks. The occurrence of upper harmonic modulation reflects nonlinear phenomena, including photosynthetic regulation. To support these claims, we measured chlorophyll fluorescence emission of the green alga Chlorella sorokiniana in light that was sinusoidally modulated in the frequency range 1000-0.001 Hz. Based on these experimental data and numerical as well as analytical mathematical models, we propose that frequency-domain measurements can become a versatile tool in plant sensing.

Effects of Zinc Oxide Nanoparticles on Physiological and Anatomical Indices in Spring Barley Tissues.

The aim of the present work was to investigate the toxic effects of zinc oxide nanoparticles (ZnO NPs, particle size < 50 nm) on the physiological and anatomical indices of spring barley (Hordeum sativum L.). The results show that ZnO NPs inhibited H. sativum growth by affecting the chlorophyll fluorescence emissions and causing deformations of the stomatal and trichome morphology, alterations to the cellular organizations, including irregularities of the chloroplasts, and disruptions to the grana and thylakoid organizations. There was a lower number of chloroplasts per cell observed in the H. sativum leaf cells treated with ZnO NPs as compared to the non-treated plants. Cytomorphometric quantification revealed that ZnO NPs decreased the size of the chloroplast by 1.5 and 4 times in 300 and 2000 mg/L ZnO NP-treated plants, respectively. The elemental analysis showed higher Zn accumulation in the treated leaf tissues (3.8 and 10.18-fold with 300 and 2000 mg/L ZnO NPs, respectively) than the untreated. High contents of Zn were observed in several spots in ZnO NP-treated leaf tissues using X-ray fluorescence. Deviations in the anatomical indices were significantly correlated with physiological observations. The accumulation of Zn content in plant tissues that originated from ZnO NPs was shown to cause damage to the structural organization of the photosynthetic apparatus and reduced the photosynthetic activities.

Area-ratio Fraunhofer line depth (aFLD) method approach to estimate solar-induced chlorophyll fluorescence in low spectral resolution spectra in a cool-temperate deciduous broadleaf forest.

Solar-induced chlorophyll fluorescence (SIF) emissions were estimated by the "area-ratio Fraunhofer line depth (aFLD) method", a new retrieval methodology in spectra from a low spectral resolution (SR) spectroradiometer (MS-700: full width half maximum (FWHM) of 10nm and spectral sampling interval of 3.3nm), assisted with a scaling to reference SIF detected from high SR spectrum. The sparse pixels of a spectrum of low SR misses detecting the minimum of the O2A absorption band around at 760nm, which makes the SIF detection by conventional FLD methods lose accuracy considerably. To overcome this, the aFLD method uses the definite integral of spectra over a wide interval between 750 and 780nm. The integration of the spectrum is insusceptible to the change in shape of the depression curve, leading to higher accuracy of the aFLD method. Daily SIF, calculated by the aFLD method using the spectra obtained with MS-700, was scaled to reference daily SIF calculated by the spectral fitting method using the spectra obtained from August to December 2019 with an ultrafine SR spectroradiometer (QE Pro, FWHM = 0.24nm). As a result, SIF calculated from MS-700 spectra by aFLD method was strongly correlated with the reference SIF from QE Pro spectra (r2 = 0.81) and was successfully scaled. Then, the scaled 11-year SIF from MS-700 at a deciduous broadleaf forest showed the correlation with GPP at multiple time steps: daily, monthly, and yearly, consistently during 2008-2018. The comparison of aFLD-derived SIF with the global Orbiting Carbon Observatory-2 (OCO-2) SIF data set (GOSIF) showed high correlation on monthly values during 2008-2017 (r2 = 0.85). The combining approach of the aFLD method with a scaling to reference SIF successfully detected long-term canopy SIF emissions, which has great potential to provide essential information on ecosystem-level photosynthesis.

Low-Cost Chlorophyll Fluorescence Imaging for Stress Detection

Plants naturally contain high levels of the stress-responsive fluorophore chlorophyll. Chlorophyll fluorescence imaging (CFI) is a powerful tool to measure photosynthetic efficiency in plants and provides the ability to detect damage from a range of biotic and abiotic stresses before visible symptoms occur. However, most CFI systems are complex, expensive systems that use pulse amplitude modulation (PAM) fluorometry. Here, we test a simple CFI system, that does not require PAM fluorometry, but instead simply images fluorescence emitted by plants. We used this technique to visualize stress induced by the photosystem II-inhibitory herbicide atrazine. After applying atrazine as a soil drench, CFI and color images were taken at 15-minute intervals, alongside measurements from a PAM fluorometer and a leaf reflectometer. Pixel intensity of the CFI images was negatively correlated with the quantum yield of photosystem II (ΦPSII) (p < 0.0001) and positively correlated with the measured reflectance in the spectral region of chlorophyll fluorescence emissions (p < 0.0001). A fluorescence-based stress index was developed using the reflectometer measurements based on wavelengths with the highest (741.2 nm) and lowest variability (548.9 nm) in response to atrazine damage. This index was correlated with ΦPSII (p < 0.0001). Low-cost CFI imaging can detect herbicide-induced stress (and likely other stressors) before there is visual damage.

Mitigation of Pb toxicity by Mn in seedling of the cacao clonal CCN 51 genotype grown in soil: physiological, biochemical, nutritional and molecular responses.

Lead (Pb) is a highly toxic metal for humans, animals and plants even at low concentrations in the soil. The ingestion of chocolate produced from contaminated beans can contribute to consumer exposure to Pb. While, Mn is an element essential for plants and participates as enzymatic cofactors in several metabolic pathways. The objective of this study was to evaluate the influence of Mn on mitigation of Pb toxicity in seedling of the cacao clonal CCN 51 genotype grown in soils with different doses of Pb, Mn and Mn+Pb, through physiological, biochemical, molecular and nutritional responses. It was found that the seedling of the cacao clonal CCN 51 genotype grown in soils with high Pb, Mn and Mn+Pb contents accumulated these heavy metals in the roots and leaves. Mn doses reduced the Pb uptake by root system and prevented that the Pb accumulated at toxic levels in the roots and leaves of the plants. High doses of Pb applied in soil were highly toxic to the plants, leading, in some cases, them to death. However, no Mn toxicity was observed in cocoa plants, even at high doses in the soil. Uptake of Pb and Mn by the roots and its transport into the aerial part of the plant promoted changes in photosynthesis, leaf gas exchange, respiration, carboxylation and in the instantaneous efficiency of carboxylation, reducing in the treatments with the highest concentrations of Pb, and the emission of chlorophyll fluorescence, affecting the efficiency of photosystem 2 and the production of photoassimilates. Besides that, Pb, Mn and Mn+Pb toxicities activated defense mechanisms in plants that alter the gene expression of met, psbA and psbO, increasing in plants subjected to high concentrations of Pb and the activity of the enzymes involved in the cellular detoxification of excess ROS at the leaf level. In addition, high uptake of Mn by root system was found to reduced Pb uptake in plants grown with Mn+Pb in the soil. Therefore, application of Mn in the soil can be used to mitigate the Pb toxicity in seedling of the cacao clonal CCN 51 genotype grown in contaminated soils.

Challenges in the atmospheric characterization for the retrieval of spectrally resolved fluorescence and PRI region dynamics from space

In the coming years, Earth Observation missions like the FLuorescence EXplorer (FLEX) will acquire the radiance signal from the visible to the near-infrared at a very high spectral resolution, enabling exciting prospects for new insights in satellite-based photosynthetic studies. In this context, the process of de-coupling atmospheric and vegetation-related spectral signatures will become essential to guarantee a reliable estimation of the vegetation photosynthetic activity from space. Dynamic changes related to the vegetation photosynthetic status result in subtle contributions to the top of atmosphere radiance signal, e.g. due to the emission of the solar-induced chlorophyll fluorescence (~ 650–800 nm) or due to changes in surface reflectance spectra (500–600 nm) indicating variations in the vegetation photoprotection and light use efficiency. Conversely, atmospheric effects (molecular and aerosol absorption and scattering) dominate the spectral interval of interest for vegetation studies. This article presents a comprehensive overview of the atmospheric radiative effects caused by aerosols, ozone (O3), water vapor (H2O), oxygen (O2), and atmospheric pressure and temperature changes within the visible and near-infrared spectral interval, and paying special attention to the co-occurring vegetation-related spectral changes associated with the fluorescence emission and the activation of the photoprotection mechanisms. Since the largest uncertainties in the atmospheric correction process are associated with the characterization of the aerosol radiative effects, this work largely concentrates on the satellite retrieval-related implications under different aerosol absorbing and scattering scenarios on a global scale. Through a simulation exercise, it is evaluated to what extent aerosol climatology could influence the accuracy of satellite-derived surface apparent reflectance spectra impacting; therefore, any vegetation-related satellite product on a seasonal and global scale.