G. Schmuck

Estimating Leaf Biochemistry Using the PROSPECT Leaf Optical Properties Model

Abstract The biophysical, biochemical, and optical properties of 63 fresh leaves and 58 dry leaves were measured to investigate the potential of remote sensing to estimate the leaf biochemistry from space. Almost 2000 hemispherical reflectance and transmittance spectra were acquired from 400 nm to 2500 nm using a laboratory spectrophotometer. The amount of chlorophyll, water, protein, cellulose, hemicellulose, lignin, and starch was determined on these leaves using standard wet chemistry techniques. These experimental data were used to improve the PROSPECT model, a simple but effective radiative transfer model that calculates the leaf optical properties with a limited number of input parameters: a structure parameter and the leaf biochemistry. The new model construction mainly consisted in providing specific absorption coefficients for the biochemical constituents; the comparison with absorption spectra of pure materials derived from the literature showed good agreement. In the inversion, however, it was necessary to group some leaf components in order to estimate leaf biochemistry with reasonable accuracy. Predictive power varied with the chemistry variable, wavelengths used in analysis, and whether leaves were fresh or dry. r2 ranged from 0.39 to 0.88 for predictions on dry leaves; on fresh leaves, water and chlorophyll had high r2 values, 0.95 and 0.68 respectively, carbon based compounds reasonable r2, from 0.50 to 0.88, while the estimation of protein is still at issue.

Critique of stepwise multiple linear regression for the extraction of leaf biochemistry information from leaf reflectance data

Abstract This study examined the use of stepwise multiple linear regression to quantify leaf carbon, nitrogen, lignin, cellulose, dry weight, and water compositions from leaf level reflectance ( R ). Two fresh leaf and one dry leaf datasets containing a broad range of native and cultivated plant species were examined using unconstrained stepwise multiple linear regression and constrained regression with wavelengths reported from other leaf level studies and wavelengths derived from chemical spectroscopy. Although stepwise multiple linear regression explained large amounts of the variation in the chemical data, the bands selected were not related to known absorption bands, varied among datasets and expression bases for the chemical [concentration (g g −1 ) or content (g m −2 )], did not correspond to bands selected in other studies, and were sensitive to the samples entered into the regression. Stepwise multiple regression using artificially constructed datasets that randomized the association between nitrogen concentration and reflectance spectra produced coefficients of determination ( R 2 s) between 0.41 and 0.82 for first and second derivative log(1/ R ) spectra. The R 2 s for correctly-paired nitrogen data and first and second derivative log (1/ R ) only exceeded the average randomized R 2 s by 0.02–0.42. Replication of this randomization experiment on a larger dry ground leaf data set from the Harvard Forest showed the same trends but lower R 2 s. All of these results suggest caution in the use of stepwise multiple linear regression on fresh leaf reflectance spectra. Band selection does not appear to be based upon the absorption characteristics of the chemical being examined.

Investigation of leaf biochemistry by statistics

Abstract The biochemical concentration (total protein, cellulose, lignin, and starch) of 73 plant leaves has been related to their optical properties through statistical relationships. Both fresh and dry plant material, leaves and needles, were used in this study. Stepwise multiple regression analyses have been performed on reflectance, transmittance, and absorptance values (individual leaves) as well as on reflectance values of optically thick samples (stacked leaves + needles), on measured values and on transformations of them such as the first derivative or the logarithm of the reciprocal of the reflectance. They underscored good prediction performances for protein, cellulose, and lignin with high squared multiple correlation coefficients (r2) values. Starch, whose concentration in the leaf was smaller compared to the other components, was estimated with less accuracy. As expected, dry material and optically thick samples provided respectively stronger correlations than fresh material and individual leaves.

A new approach to quantifying abundances of materials in multispectral images

The authors tested a new technique for mapping abundances of materials using multispectral images which they call Foreground-Background Analysis (FBA). The method maximizes the contrast between sets of foreground and background spectra while simultaneously minimizing the variability within these sets. Spectral variability introduces errors in the fractions (abundance) of endmembers when simple mixture models (2-5 endmembers) are applied to complex natural surfaces. FBA was tested using CCD-camera measurements of known materials in the laboratory and included variations in illumination geometry. Lower abundance uncertainties were obtained using FBA than with simple mixture models.<<ETX>>

Time-resolved chlorophyll fluorescence spectra of intact leaves

Abstract Room temperature single photon counting measurements on intact leaves at low excitation energies have been analyzed using a four exponential kinetic model. The observed lifetimes and relative yields of the components have been compared to those obtained on isolated chloroplasts of the same species. At steady state conditions of fluorescence (Fs-level, reached after 5 min of preillumination with 625 nm pulsed laser light), the overall decay is characterized by lifetimes of approx. 20–30 ps ( τ 1 ), 80–100 ps ( τ 2 ), 400–450 ps ( τ 3 ), and 800–900 ps ( τ 4 ). By closing the reaction centers of PS II (application of the herbicide DCMU) the lifetimes of the two slowest components τ 3 and τ 4 increase by a factor of 4. The lifetimes of the two fastest components ( τ 1 , and τ 2 ) were found to be independent of PS II trap closure. A comparison of literature lifetime data of isolated pigment proteins (Hodges and Moya, 1986; Wittmershaus et al., 1987) with our results, obtained for the in vivo plant system suggsts that two fast decays can be attributed to PS I whereas three lifetime components are necessary to describe the fluorescence decay of PS II.

Interpretation Of Reflectance Spectra By Plant Physiological Parameters

Spectral signatures of the C3 plant wheat (Triticum aestivum) were confronted with those of the C4 plant maize (Zea mays). The reflectance spectra were taken with the IRIS spectroradiometer in the region between 400 nm and 2400 nm. Besides the reflectance, physiological parameters like water potential, photosynthetic activity (CO2 - gas exchange) and pigment concentration of the samples have been determined using the classical instrumentation of plant physiology. Changes in several bands of the reflectance spectra were compared with changes of these physiological parameters, An attempt has been made to transfer the results of the laboratory measurements, performed under controlled environmental conditions, to data acquired in the field over canopies of the same plant types. A critical analysis of the overall approach is reported.

Optical Properties of Leaves: Modelling and Experimental Studies

This paper deals with the interpretation of leaves spectra following an approach based on modelling and laboratory studies. First, the leaves structure and principal constituents are described together with the way they interact with light. The effects of growth, senescence and environmental factors on the leaf optical properties are summarised. A laboratory study conducted on drought stress of maize (Zea Mays) plants is reported as an example. A succinct review of the existing models is then made: ray tracing, Kubelka-Munk and developments, plate models, and the stochastic model. The use of these models to determine leaf constituents and structure by inversion on reflectance spectra is then discussed with an emphasis on the research of good specific absorption coefficients for the constituents. The validation of the PROSPECT model (generalised plate model) on the basis of leaves spectra is presented. The problems linked with the application of these procedures to remote sensing data is evoked, and an example of inversion on experimental spectra of sugar beet (Beta vulgaris L) fields is briefly reported.

The European optical goniometric facility: technical description and first experiments on spectral unmixing

Presents the new European goniometric facility installed at the Joint Research Center of the Commission of the European Union. Basically, this system allows independent positioning of a light source and a detector anywhere on a 2 m radius hemisphere centered on the target and thereby to perform bidirectional reflectance measurements. The control software allows full programming of an experiment, the graphical visualization of the measurement geometry and the interface to a dedicated data base for the experimental results. A first experiment is described in which multispectral CCD camera images were acquired on various targets (soils, canopies and synthetic leaves) under different illumination angles. The purpose was to provide a data set allowing the comparison of the projected area of spectral endmembers, as measured from the CCD images, with the result of linear spectral mixture inversion algorithms.<<ETX>>

Laser-induced time-resolved fluorescence of vegetation

Time-resolved measurements of the laser-induced chlorophyll fluorescence emission of vegetation detected by two different techniques are described. Fluorescence decay time measurements using single photon counting and picosecond laser pulse excitation have been used to analyze the fluorescence heterogeneity of plant leaves. The fluorescence is described by lifetimes of 10-40, 80-150, 400-500, and 700-1000 ps. By closing the reaction centers via application of the herbicide DCMU, the lifetimes of the two slowest components increase by a factor of about three. Another possible method to monitor the fluorescence after picosecond excitation could be a streak camera detection system. Measurements performed on the slow decay component of stressed and unstressed plants are presented. >

Estimation of the chlorophyll fluorescence lifetime of plant canopies: Validation of a deconvolution method based on the use of a 3-D canopy mockup

Abstract Fluorescence or backscattered signals measured over a plant canopy after a picosecond laser shot (nadir viewing) are composed of elementary contributions coming from different illuminated leaves, branches, twigs, and the soil background. They are affected by time delays, depending on their levels within the canopy. The global signals measured are then rather complex and depend on canopy architecture and fluorescence characteristics. Also, an amplitude decorrelation exists between fluorescence and backscattered signals, due to the difference existing between reflectance and fluorescence properties of plant canopy components. For this reason it was necessary to develop a specific method for retrieving the mean fluorescence lifetime of a complex plant canopy under field condition. In a first step, the position and the reflectance characteristics of each reflecting canopy element is determined from the deconvolution of the backscattered signal. In a second step, the parameters obtained are introduced into the fluorescence function in order to determine the mean fluorescence lifetime. The validity of the deconvolution method has been tested for numerous situations, using simulated laser shots on a 3-D canopy mockup. The results obtained show a very good agreement between estimated and input parameters.