Fraunhofer Line Depth Principle Research Articles

Sun-induced fluorescence - a new probe of photosynthesis: First maps from the imaging spectrometerHyPlant.

Variations in photosynthesis still cause substantial uncertainties in predicting photosynthetic CO2 uptake rates and monitoring plant stress. Changes in actual photosynthesis that are not related to greenness of vegetation are difficult to measure by reflectance based optical remote sensing techniques. Several activities are underway to evaluate the sun-induced fluorescence signal on the ground and on a coarse spatial scale using space-borne imaging spectrometers. Intermediate-scale observations using airborne-based imaging spectroscopy, which are critical to bridge the existing gap between small-scale field studies and global observations, are still insufficient. Here we present the first validated maps of sun-induced fluorescence in that critical, intermediate spatial resolution, employing the novel airborne imaging spectrometer HyPlant. HyPlant has an unprecedented spectral resolution, which allows for the first time quantifying sun-induced fluorescence fluxes in physical units according to the Fraunhofer Line Depth Principle that exploits solar and atmospheric absorption bands. Maps of sun-induced fluorescence show a large spatial variability between different vegetation types, which complement classical remote sensing approaches. Different crop types largely differ in emitting fluorescence that additionally changes within the seasonal cycle and thus may be related to the seasonal activation and deactivation of the photosynthetic machinery. We argue that sun-induced fluorescence emission is related to two processes: (i) the total absorbed radiation by photosynthetically active chlorophyll; and (ii) the functional status of actual photosynthesis and vegetation stress.

Spatio-temporal patterns of chlorophyll fluorescence and physiological and structural indices acquired from hyperspectral imagery as compared with carbon fluxes measured with eddy covariance

This study provides insight into the assessment of the spatio-temporal trends of chlorophyll fluorescence, narrow-band physiological indices, and structural indices acquired with a hyperspectral imager flown over a flux tower in a canopy characterized by small seasonal structural changes and a heterogeneous architecture. A total of seven flights between summer and autumn were conducted with a hyperspectral camera that captured 30cm resolution imagery and 260 spectral bands in the 400-900nm region. This enabled the identification of pure-vegetation tree crown pixels around an eddy covariance flux tower without shadow components or background effects. The hyperspectral imagery was used to study the temporal patterns of canopy fluorescence and reflectance indices related to physiology and canopy structure. The seasonal trends observed in the airborne indices and fluorescence and their relationship with gross primary production (GPP) demonstrated that vegetation indices mostly related to structure such as the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI) yielded non-significant relationships (r2=0.17; p>0.05) due to the small structural changes in the canopy through the season. By contrast, physiological indices related to chlorophyll content (TCARI/OSAVI), light use efficiency (PRI570), and canopy chlorophyll fluorescence calculated through the Fraunhofer Line Depth principle (FLD3) showed a similar seasonal trend to that of GPP measured at the same time of the flights (r2 in the range 0.75–0.84; p<0.01). These results are consistent with the physiological trend observed during summer and autumn, which showed that chlorophyll content increased by 17.9% while the NDVI and the estimated tree crown projected LAI (LAIp) remained almost constant during the experiment (3% variation). The time-series hyperspectral dataset demonstrated that the seasonal trajectories of the NDVI and EVI were weakly related (p>0.05) to the physiological indicators such as TCARI/OSAVI, PRI570 and fluorescence. The spatial variability of the hyperspectral indices investigated through the coefficient of variation (CV) showed that fluorescence around the tower varied up to 17% at the time of the maximum stress (summer), while LAIp showed little variation during that time (CV=1.8%). After the summer stress period, the CV for fluorescence and chlorophyll content decreased in autumn down to 9%. This study demonstrates that small physiological changes occurring in an evergreen canopy were still captured by remote sensing physiological indices and high-resolution airborne fluorescence. These indicators are required for GPP monitoring when the vegetation dynamics are not captured by remote sensing structural indices.

Using optical remote sensing techniques to track the development of ozone-induced stress

In this paper, a literature review about optical remote sensing (RS) of O 3 stress is presented. Studies on O 3-induced effects on vegetation reflectance have been conducted since late ‘70s based on the analysis of optical RS data. Literature review reveals that traditional RS techniques were able to detect changes in leaf and canopy reflectance related to O 3-induced stress when visible symptoms already occurred. Only recently, advanced RS techniques using hyperspectral sensors, demonstrated the feasibility of detecting the stress in its early phase by monitoring excess energy dissipation pathways such as chlorophyll fluorescence and non-photochemical quenching (NPQ). Steady-state fluorescence (Fs), measured by exploiting the Fraunhofer line depth principle and NPQ related xanthophyll-cycle, estimated through the photochemical reflectance index (PRI) responded to O 3 fumigation before visible symptoms occurred. This opens up new possibilities for the early detection of vegetation O 3 stress by means of hyperspectral RS.

Leaf level detection of solar induced chlorophyll fluorescence by means of a subnanometer resolution spectroradiometer

A leaf level investigation on the spectral signature of Phaseolus vulgaris was undertaken by using a very high spectral resolution spectroradiometer featuring full width at half maximum of 0.06 nm and spectral range of 635.5–802.5 nm. High spectral resolution allows detection of leaf reflected and emitted radiance fields in two narrow absorption bands at 687 and 760 nm, respectively, where solar irradiance is strongly reduced owing to molecular oxygen absorption of the terrestrial atmosphere. The flux emitted due to chlorophyll fluorescence was measured using the Fraunhofer line depth principle by spectrally modelling the signal, capitalizing on the high resolution of the spectroradiometer devices. An experiment was conducted on two potted bean plants. One was maintained in good health for use as a reference while the other was treated with a photosystem II inhibitor. Collected spectra show that the fluorescence emission produces a pair of characteristic peaks superimposed on the typical leaf-specific reflectance curve. The magnitude of the fluorescence signal of the herbicide-treated leaf was four times greater than that of the control plant, thus indicating damage to the photosynthetic apparatus of the plant.

Use of a prototype instrument to detect short-term changes in solar-excited leaf fluorescence

A prototype instrument was used under clear skies to determine the efficacy of the Fraunhofer Line–Depth Principle in detecting short-term (less than one day) changes in solar-excited fluorescence (F) as an indicator of plant physiological status. Corn (Zea mays L. var. Shoe Peg) and soybean (Glycine max L. var. Hutcheson) plants grown in pots outdoors were either assigned as controls or treated with bromacil, a photosystem II herbicide. The Plant Fluorescence System (PFS) measured the radiant flux of F and total upwelling radiant flux (M) from individual leaves in 10 nm bandwidths centred at 690 nm and 760 nm. The herbicide lowered (p=0.01) net photosynthetic CO2 assimilation rates to negative values in both species. In corn, this corresponded with increases in mean F to 5.9 mW m−2 nm−1 and 3.8 mW m−2 nm−1 at 690 nm and 760 nm, respectively, approximately twice control values. In soybean, mean F at 760 nm increased from 2.6 mW m−2 nm−1 to 3.8 mW m−2 nm−1, whereas F at 690 nm was unaffected. Leaf chlorophyll contents were unaffected by treatment. Although the stress induced was drastic, these changes in F increased M by only 1–2%, demonstrating the high radiometric sensitivity required for detection. It is expected that, for whole plant canopies, a still greater variability in F among leaves would further reduce its apparent contribution to M.

Detection of solar-excited chlorophyll a fluorescence and leaf photosynthetic capacity using a fraunhofer line radiometer

A Fraunhofer Line Radiometer (FLR) measured solar-excited chlorophyll a fluorescence ( F) as an indicator of photosynthetic capacity in leaves of Washingtonia robusta (palm) and Vitis vinifera (grape). Under clear skies, the FLR utilized the Fraunhofer line-depth principle (FLDP) to detect F within the O 2 absorption band centered at 687 nm wavelength. On 20 April, the morning after soils were treated with the photosystem II herbicide DCMU F increased significantly ( p = 0.05) in palm, while leaf reflectance at 687 nm ( R) did not change. By 21 April, leaf herbicide concentrations were lethal. Increased F and R in palm corresponded with net photosynthetic rates near zero. Photosynthesis in grape also decreased greatly, but F and R remained similar to the controls. Results for palm indicate that F could be measured while leaves remained exposed to full sunlight, and support earlier work to indicate a potential role for the FLDP in remotely sensing F and photosynthetic capacity.