Early detection of plant stress by digital imaging within narrow stress-sensitive wavebands

Digital images of soybean canopies [Glycine max (L.) Merrill] were obtained within selected narrow wavebands (6–10 nm bandwidths) to determine their capability for early detection of plant stress. Images and physiological measurements of stress were acquired 2 days, 4 days, and 7 days following application of control, drought, and herbicide [(3,4-dichlorophenyl)-1, 1-dimethylurea, or DCMU] treatments. As a result of frequent rainfall, drought stress never occurred. However, exposure to herbicide rapidly induced plant stress. By day 4, the ratio of variable to maximum leaf fluorescence (F_v/F_m) decreased and leaf water potentials (ψ_w) increased in the herbicide treated soybean, indicating damage to the photosynthetic apparatus and stomatal closure. Also, Munsell leaf color had increased from approximately 5GY 4.6/5.7 to a lighter green-yellow value. Canopy reflectances at 670 nm, 694 nm, and in the 410–740 nm band (R_vis), as well as reflectance at 694 nm divided by reflectance at 760 nm (R₆₉₄/R₇₆₀), detected stress simultaneously with the physiological measurements and increased consistently with stress through day 7. Reflectances at 420 nm and 600 nm, together with R₆₀₀/R₇₆₀ and R_vis/R₇₆₀, did not increase until leaves were yellow or brown and wilted and canopies had begun to collapse on day 7. None of the reflectance or reflectance ratio images detected stress prior to visible color changes. This was attributed primarily to the rapid inducement of chlorosis by the herbicide. Reflectance in narrow wavebands within the 690–700 nm region and its ratio with near-infrared reflectance should provide earlier detection of stress-induced chlorosis compared with broad band systems or narrow bands located at lesser wavelengths.     

Distinguishing mangrove species with laboratory measurements of hyperspectral leaf reflectance

As a first step in developing classification procedures for remotely acquired hyperspectral mapping of mangrove canopies, we conducted a laboratory study of mangrove leaf spectral reflectance at a study site on the Caribbean coast of Panama, where the mangrove forest canopy is dominated by Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle. Using a high‐resolution spectrometer, we measured the reflectance of leaves collected from replicate trees of three mangrove species growing in productive and physiologically stressful habitats. The reflectance data were analysed in the following ways. First, a one‐way ANOVA was performed to identify bands that exhibited significant differences (P value<0.01) in the mean reflectance across tree species. The selected bands then formed the basis for a linear discriminant analysis (LDA) that classified the three types of mangrove leaves. The contribution of each narrow band to the classification was assessed by the absolute value of standardised coefficients associated with each discriminant function. Finally, to investigate the capability of hyperspectral data to diagnose the stress condition across the three mangrove species, four narrow band ratios (R ₆₉₅/R ₄₂₀, R ₆₀₅/R ₇₆₀, R ₆₉₅/R ₇₆₀, and R ₇₁₀/R ₇₆₀ where R ₆₉₅ represents reflectance at wavelength of 695nm, and so on) were calculated and compared between stressed and non‐stressed tree leaves using ANOVA.

Results indicate a good discrimination was achieved with an average kappa value of 0.9. Wavebands at 780, 790, 800, 1480, 1530, and 1550 nm were identified as the most useful bands for mangrove species classification. At least one of the four reflectance ratio indices proved useful in detecting stress associated with any of the three mangrove species. Overall, hyperspectral data appear to have great potential for discriminating mangrove canopies of differing species composition and for detecting stress in mangrove vegetation.     

Technical Note: Response of ratio vegetation indices to foot rot-infected citrus trees

Ratio vegetation indices were evaluated for differentiating foot rot-infected (Phytophthora parasitica Dast) from noninfected citrus trees (Citrus spp.). At two sites, field spectral measurements of infected and noninfected trees were obtained with a hand-held spectroradiometer. The following bands were assessed for developing the ratios: blue (B, 450–520?nm), green (G, 520–600?nm), red (R, 630–690?nm), near-infrared (NIR, 760–900?nm) and mid-infrared (MIR1, 1550–1750?nm; MIR2, 2080–2350?nm). For both sites, means of infected and noninfected trees were significantly different (t-test; p<0.05) for NIR/B, NIR/R and NIR/MIR1 ratios. Percentage difference values indicated the NIR/R ratio was most responsive to the disease. A relative index value was established for separating infected from noninfected trees. Findings demonstrated that ratio vegetation indices have high potential for distinguishing foot rot-infected from noninfected citrus trees.     

Hyperspectral indices to diagnose leaf biotic stress of apple plants,considering leaf phenology

Novel and existing hyperspectral vegetation indices were evaluated in this study, with the aim of assessing their utility for accurate tracking of leaf spectral changes due to differences in biophysical indicators caused by apple scab. Novel indices were extracted from spectral profiles by means of narrow‐waveband ratioing of all possible two‐band combinations between 350 nm and 2500 nm at nanometer intervals (2 311 250 combinations) and all possible two‐band derivative combinations. Narrow‐waveband ratios consisting of wavelengths of approximately 1500 nm and 2250 nm, associated with water content, have proven to be the most appropriate for detecting apple scab at early developmental stages. Logistic regression c‐values ranged from 0.80 to 0.88. At a more developed infection stage, vegetation indices such as R₄₄₀/R₆₉₀ and R₆₉₅/R₇₆₀ exhibited superior distinction between non‐infected and infected leaves. Identified derivative indices were located in similar regions. It therefore was concluded that the most appropriate indices at early stages of infection are ratios of wavelengths situated at the water band slopes. The choice of appropriate indices and their discriminatory performances, however, depended on the phenological stage of the leaves. Hence, an undisturbed 20‐day growth profile was examined to assess the effect of physiological changes on spectral variations at consecutive growth stages of leaves. Results suggested that an accurate distinction could be made between different leaf developmental stages using the 570 nm, 1460 nm, 1940 nm and 2400 nm wavelengths, and the red‐edge inflection point. These results are useful to crop managers interested in an early warning system to aid proactive system management and steering.     

Using multivariate analysis to detect the hyperspectral response of Chinese fir to acid stress

A useful method was developed to establish a diagnostic model using hyperspectral remote sensing to predict and monitor acid stress on plants. We analysed the hyperspectral response of Chinese fir to acid rain by measuring the spectral reflectance of the seedling leaves, sprayed by simulated acid rain (pH, 2.5, 4.0, and 5.6), for three periods. The sensitive bands were located and the rules for predicting classes of simulated acid stress on Chinese fir were established using a classification and regression tree (CART) approach. The acid-sensitive bands of Chinese fir were nearly all located between 380 and 410 nm, 460 and 560 nm, and 640 and 750 nm. CART predictor variables, which were selected from sensitive bands, reduce data dimensionality significantly. The misclassification errors of the CART training process in correctly attributing variables to respective target classes are 7.78%, 6.67%, and 11.67% respectively, at each measurement period, and the cross-validation misclassification errors are 16.6%, 11.1%, and 23.3%, respectively. Our results show that the spectral reference bands, which are related to chlorophyll-a and b around 670 and 450 nm, as well as the slight peak in the green around 550 nm, significantly affected the classification accuracy on acid stress. These provide useful optical response to acid stress.     

Relationships of leaf nitrogen concentration and canopy nitrogen density with spectral features parameters and narrow-band spectral indices calculated from field winter wheat (Triticum aestivum L.) spectra

Remote sensing is a promising tool that provides quantitative and timely information for crop stress detection over large areas. Nitrogen (N) is one of the important nutrient elements influencing grain yield and quality of winter wheat (Triticum aestivum L.). In this study, canopy spectral parameters were evaluated for N status assessment in winter wheat. A winter wheat field experiment with 25 different cultivars was conducted at the China National Experimental Station for Precision Agriculture, Beijing, China. Wheat canopy spectral reflectance over 350–2500 nm at different stages was measured with an ASD FieldSpec Pro 2500 spectrometer (Analytical Spectral Devices, Boulder, CO, USA) fitted with a 25° field of view (FOV) fibre optic adaptor. Thirteen narrow-band spectral indices, three spectral features parameters associated with the absorption bands centred at 670 and 980 nm and another three related to reflectance maximum values located at 560, 920, 1690 and 2230 nm were calculated and correlated with leaf N concentration (LNC) and canopy N density (CND). The results showed that CND was a more sensitive parameter than LNC in response to the variation of canopy-level spectral parameters. The correlation coefficient values between LNC and CND, on the one hand, and narrow-band spectral indices and spectral features parameters, on the other hand, varied with the growth stages of winter wheat, with no predominance of a single spectral parameter as the best variable. The differences in correlation results for the relationships of CND and LNC with narrow-band spectral indices and spectral features parameters decreased with wheat plant developing from Feekes 4.0 to Feekes 11.1. The red edge position (REP) was demonstrated to be a good indicator for winter wheat LNC estimation. The absorption band depth (ABD) normalized to the area of absorption feature (NBD) at 670 nm (NBD670) was the most reliable indicator for winter wheat canopy N status assessment.     

Plant spectral responses to gas leaks and other stresses

The study aimed to assess the ability of remote sensing to differentiate between plant stress caused by natural gas leakage and other stresses. In order to use satellite remote sensing to detect gas leaks it is necessary to determine whether the cause of the stress can be identified in the spectral response and distinguished from other stress factors. Field plots of oilseed rape (Brassica napus) were stressed using elevated levels of natural gas in the soil, dilute herbicide solution and extreme shade. Visible stress response, spectral stress response and chlorophyll content of plants from these three treatments were compared to control plants receiving no treatment. The reflectance from isolated leaves was measured in the laboratory. Spectral responses to stress included increased reflectance in the visible wavelengths and decreased reflectance in the near‐infrared. A shift of the red edge position towards shorter wavelengths was observed as a result of all three stresses, although the shift was greatest when stressed via extreme shade. Red edge position was strongly correlated with chlorophyll content across all the treatments. The ratio of reflectances centred on the wavelengths 670 and 560 nm was used to detect increases in red pigmentation in gassed and herbicide‐stressed leaves. Stress due to extreme shade could be distinguished from stress caused by natural gas or herbicide by changes in the reflectance spectra, however, stress caused by herbicide or natural gas could not be distinguished from one another in the spectra although symptoms of stress caused by elevated gas levels were identified earlier than symptoms caused by herbicide‐induced stress.     

Remote estimation of chlorophyll content in higher plant leaves

Indices for the non-destructive estimation of chlorophyll content were formulated using various instruments to measure reflectance and absorption spectra in visible and near-infrared ranges, as well as chlorophyll contents from several non-related species from different climatic regions. The proposed new algorithms are simple ratios between percentage reflectance at spectral regions that are highly sensitive (540 to 630nm and around 700nm) and insensitive (nearinfrared) to variations in chlorophyll content: R NIR / R 700 and R NIR / R 550. The developed algorithms predicting leaf chemistry from the leaf optics were validated for nine plant species in the range of chlorophyll content from 0.27 to 62.9mug cm -2. An error of less than 4.2 mugcm -2 in chlorophyll prediction was achieved. The use of green and red (near 700nm) channels increases the sensitivity of NDVI to chlorophyll content by about five-fold.     

Use of spatial structure analysis of hyperspectral data cubes for detection of insect‐induced stress in wheat plants

Wheat plants were experimentally infested with wheat stem sawflies, and hyperspectral images (reflectance range from 402.8–838.7 nm) were collected from leaves of infested and non‐infested plants. Mean and variance reflectance per leaf were calculated in five of 213 spectral bands (452, 553, 657, 725, and 760 nm) and compared with vegetation indices (NDVI, SI and PRI), and standard variogram parameters (nugget, sill and range values). Mean reflectance values and their variance values and vegetation indices showed significant effects of sawfly infestation in one dataset but not in another. Based on directional variogram analyses, we showed that: (1) better separation of leaf type and infested/non‐infested wheat plants was seen in variograms in longitudinal direction compared to transverse; (2) mainly spectral bands in the red edge and NIR showed consistent effect of sawfly infestation; (3) range values were not affected significantly by either sawfly infestation or leaf type; and (4) sawfly‐induced stress was most likely to be detected about three weeks after infestation. Variogram analysis is one of the key standards in quantitative spatial ecology, and this study supports further research into its use in remote sensing with particular emphasis on detection of biotic stress.     

水稻冠层光谱特征及其与LAI的关系研究

氮素营养是影响作物生长与产量的最主要限制因子之一。准确及时地监测或诊断出作物氮素营养状况,对提高氮素利用效率和作物管理水平、减少过度施氮造成的环境污染具有重要意义。本研究在不同施氮水平处理的水稻试验小区,对水稻整个生长期内冠层反射光谱进行了较系统、密集的测定,同时测定了几个重要生育期水稻的叶面积指数。研究结果表明:随着施氮量的增加,水稻冠层光谱在各生育期间呈现出一定的规律性,在近红外部分(710～1 220 nm),冠层光谱反射率随着施氮水平的提高而升高,而在可见光部分(460～680 nm),水稻冠层的光谱反射率反而逐渐降低。经冠层光谱差异显著性检验发现:水稻灌浆期以前,对施氮水平最为敏感的波段是绿光(560～610 nm)和近红外(710～760 nm)部分;转换为归一化植被指数(NDVI)以后,差异最显著的是(R760-R560)/(R760+R560)。不同氮肥处理的水稻LAI随时间变化曲线大致都呈抛物线型,中低水平施氮肥水稻LAI随时间的变化曲线比较平缓,而高水平施氮肥LAI曲线则变化比较剧烈。冠层光谱反射与叶面积的相关分析结果表明:在水稻抽穗前,叶面积与冠层光谱反射率相关性较差;而抽穗后,叶面积与冠层光谱有较高的相关性。     

OCEANcode: The complete set of algorithms and models for the level_2 processing of European CZCS historical data

A complete set of algorithms and models for the level_2 processing of the European CZCS historical data was integrated in the OCEANcode software package. The OCEANcode allows the calibration of the sensor-recorded signal taking into account the instrument sensitivity loss; the correction of the calibrated signal for atmospheric contamination and derive sub-surface reflectances; and then the estimation of the concentration of water constituents. The atmospheric correction is performed on the basis of a reflectance-model-based algorithm. The Rayleigh correction is applied consistently for all water pixels, using a multiple scattering approach, and introducing atmospheric pressure and Ozone concentration data in the computation. The marine aerosol correction uses a pixel-by-pixel iterative procedure, allowing successive estimates of both the marine reflectance in the red spectral region (670nm) and the Angstrom exponent, which links simple wavelengths ratios to reflectance ratios. For case 1 waters, the optical properties of which are essentially dominated by planktonic pigments, the interrelations between marine reflectances and reflectance ratios at various wavelengths are derived from modelled calculations. For identified case 2 waters, where water constituents other than planktonic pigments (i.e. dissolved organics and suspended sediments) dominate the water optical properties, the evaluation of marine reflectances is approximated by means of interpolated Angstrom exponent values computed over case 1 water pixels and of empirical relationships derived from in situ measurements. The computation of chlorophyll-like pigments is performed with algorithms based on blue/green (443-550nm) reflectance ratios, for lower pigment concentration, or on green/green (520-550nm) reflectance ratios, for higher pigment concentration. As for the case of atmospheric corrections, the inter-relations between pigment concentration and reflectance ratios are model-derived for case 1 waters, and empirically determined for case 2 waters.     

Usefulness of spectral reflectance indices as durum wheat yield predictors under contrasting Mediterranean conditions

Early prediction of crop yield can be an important tool for identifying promising genotypes in breeding programmes. To assess whether measurements of canopy reflectance at given stages of development could be used for yield forecasting and to identify the most appropriate indices, locations and growth stages for durum wheat yield assessment, nine field experiments, each including 20 or 25 durum wheat (Triticum turgidum L. var durum) genotypes, were carried out under a wide range of Mediterranean conditions. Canopy reflectance was recorded with a portable field spectroradiometer at several times from booting to physiological maturity, and nine indices were further derived. Grain yield was measured at harvesting. The results indicated that milk-grain stage was the most appropriate developmental stage for yield assessment. However, some indices were also sensitive to yield variations when determined at anthesis or even heading or booting. The capacity of spectral reflectance indices to forecast grain yield increased on locations that allowed genotypes to express their yield potentiality. Reflectance at 550?nm (R550), water index (WI), photochemical reflectance index (PRI), structural independent pigment index (SIPI), normalized difference vegetation index (NDVI) and simple ratio (SR) explained jointly a 95.7% of yield variability when all the experiments were analysed together, 92% being explained by R550. When regression analyses were carried out separately for each experiment, spectral reflectance indices explained from 17.3% to 65.2% of total variation in yield, and the indices that best explained differences in yield were experiment-dependent. Our data suggest that reflectance at 680?nm (R680), WI and SR may be suitable estimators of durum wheat grain yield under Mediterranean conditions, when determined at milk-grain stage.     

Comparison of measured and satellite-derived spectral diffuse attenuation coefficients for the Arabian Sea

We present here the results of our study comparing the spectral diffuse attenuation coefficients K _d(λ) measured in the Arabian Sea with those derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) using three algorithms, of which two are empirical-data-driven and one is semi-analytical. The measurements were carried out in all water types and the mean values of the measured spectral K _d(λ) are 0.105, 0.092, 0.077, 0.082, 0.110 and 0.490 m^?1 for wavelength λ at 412, 443, 490, 510, 555 and 670 nm, respectively. This profile corresponds to a chlorophyll value of about 1 mg m^?3. The maximum values of the measured K _d correspond to waters with chlorophyll of about 8 mg m^?3. Though the satellite-derived K _d(λ) are found to be overestimated in all bands, we have observed good correlations between the measured and satellite-derived values in all bands, and excluding the band at 670 nm, the mean absolute percent deviations are observed to be less than 50% in all bands. The performance of the data-driven empirical methods was found to be consistent in all the bands, except at the red band of 670 nm, which is uncorrelated with the measured values and has large errors. The performances of the empirical methods depend on the accuracy of the band ratios of the retrieved remote sensing reflectance. Though the performance of the semi-analytical algorithm is found to be spectrally varying, with large positive bias observed in the blue regions, this algorithm is recommended for hyperspectral applications. The performance of the semi-analytical algorithm could be improved by having a robust algorithm to accurately derive spectral inherent optical properties of absorption and backscattering coefficients from the satellite data.     

SeaWiFS data analysis and match-ups with in situ chlorophyll concentrations in Danish waters

For the year 1999 all Sea viewing Wide Field of view Sensor (SeaWiFS) scenes of the Danish waters from the North Sea to the Baltic Sea were browsed, and a total of 47 SeaWiFS scenes with reasonably low cloud cover and, therefore, potential in situ match-ups were found and processed. The in situ data used as match-ups were collected on routine monitoring cruises by Danish and Swedish environmental authorities. A few stations in the North Sea, Skagerak and the western Baltic Sea were sampled, while most stations were located in Kattegat and the inner Danish waters. A turbid water SeaWiFS atmospheric correction algorithm was applied, since the standard SeaWiFS algorithm for chlorophyll-a (CHL) has been shown to be fairly inaccurate in turbid coastal waters. This is due to both inaccurate atmospheric and to relatively high and variable abundance of yellow substance. The application of the turbid atmospheric correction substantially improved the SeaWiFS CHL estimates. Regressions between SeaWiFS estimates using the OC2 and OC4 algorithms used in the SeaDAS software (versions 3.3 and 4.0, respectively) and in situ CHL values were made as well, and regression with a number of other possible reflectance ratios with SeaWiFS channels. The best correlation was found to be R²=0.54 using a double-ratio algorithm using both R510/R555 and R443/R670, while the OC4v4 had the second best correlation of R²=0.39. Among other single ratios, the R510/R555 had the highest correlation with CHL, which was expected since this is also the ratio that OC4v4 most often switches to in the waters investigated here. The range of CHL concentrations in this study was rather limited (all but three points from 0.5–3?mg?m^?3) so there is a need for inclusion of more data to expand the concentration range. This should be possible using also data from 2000, 2001 and onwards and, hereafter, a more ‘stable’ empirical algorithm can be derived for the Danish waters.     

On the temporal stability of ground calibration targets: implications for the reproducibility of remote sensing methodologies

Ground calibration targets (GCT) fulfil an essential role in vicarious calibration and atmospheric correction methodologies. However, assumptions are often made about the temporal stability of GCT reflectance. This letter presents results from a multi‐year study aimed at testing the temporal stability of a typical weathered concrete GCT in southern England. Very accurate measurements of hemispherical‐directional reflectance factors in the 400–1000 nm range were collected using a mobile dual‐beam spectroradiometer. Results demonstrated that the calibration surface was subject to seasonal growth of a biological material, which caused the reflectance factor to vary by a factor of two during the year (range = 16.4% reflectance at 670 nm). The spectral effect of this was most noticeable in field spectra collected in April. As environmental conditions became drier throughout the summer, concrete reflectance factors increased. Over multiple seasons the same patterns in reflectance factors repeated, indicating the predictable nature of the biological signature. The research also suggested that the biological material was affected to a small but measurable extent on a daily basis by changes in relative humidity occurring after onset of a local sea breeze. The research highlights the dynamic nature of weathered GCTs, and has wider implications for those using similar sites for vicarious calibration or atmospheric correction purposes.     

High-spectral resolution data for monitoring Scots pine (Pinus sylvestris L.) regeneration

High-resolution field spectroradiometric measurements were used to investigate the spectral properties of naturally regenerating Scots pine ( Pinus sylvestris L.) in relation to sapling cover and season. Ratios of reflectance (R757/ R722) and first-derivative reflectance (D719/ D703 and D730/ D700) in the red edge region correlated most strongly with increasing sapling cover. The main control on these ratios was the contrast between the biomass of heather and pine saplings. At the site investigated, the strength of the correlation of reflectance ratios with sapling cover was not affected by seasonal changes in canopy colour.     

Spectral characterization of necrosis from reflectance of Eucalyptus globulus leaves with Mycosphaerella leaf disease or subjected to artificial lesions

Necrosis is one of a range of symptoms resulting from a number of different biotic and abiotic damage agents that may be detected and quantified with remote sensing as part of an operational forest health monitoring system. Mycosphaerella leaf disease (MLD) caused by Teratosphaeria spp. (formerly known as Mycosphaerella spp.) is the most common foliar disease in young Australian Eucalyptus globulus plantations. Necrosis often occurs in conjunction with other visible symptoms such as chlorosis and reddening, and we have tested whether these symptoms alter the ability of spectral approaches to detect necrosis. We completed two studies of necrosis with pot-grown E. globulus plants; one in which necrosis was induced by artificial infection of Teratosphaeria spp. and one in which necrosis was induced by injury, superimposed on plants with established reddening or chlorosis. Using spectral sensitivity analysis we found that across the two studies, wavelengths between 679 and 695 nm were most sensitive to the presence of necrosis and those between 706 and 726 nm were least sensitive. A new vegetation index (VI) was able to statistically group necrotic treatments together while grouping non-necrotic treatments together, regardless of reddening and chlorosis, which other relevant indices could not. Multivariate methods utilized many wavelengths throughout the spectrum and enabled much greater distinction of all treatment groups related to necrosis, compared with the VIs. Wavelengths in the 679–695 nm range were only occasionally selected as key wavelengths; therefore, results were not similar to the spectral sensitivity data.     

Reflectance characteristics of major land surfaces in slash‐and‐burn ecosystems in Laos

The in situ reflectance spectra in the 400–2500 nm wavelength region were obtained using a portable radiometer over a range of land surfaces including burnt fields, crop canopies, and fallow vegetation at different community ages in slash‐and‐burn ecosystems in Laos. Normalized difference spectral indices (NDSI[i,j] = [R_j ?R_i ]/[R_j +R_i ]) were derived using reflectance R_i and R_j at i and j nm wavelengths for a thorough combination (14 706 pairs) of 172 wavebands (10‐nm resolution). The separability of burnt fields from dry/senescent vegetation was highest at NDSI[1090, 2390], whereas it was highly discriminated from fallow and crop vegetation by NDSI[760, 1970]. NDSIs using 730–760 nm with 1970–1990 nm showed the largest differences between dry/senescent vegetation and fallow or crop vegetation. None of the NDSIs was useful in discriminating between fallow and crop vegetations or between slashed/senescent vegetation and crop residue/abandoned field. Community age and biomass of fallow vegetation could not be inferred directly from spectral information, since no NDSIs showed any significant differences among crop and fallow vegetation that had a large variability in the amount of green vegetation. Results would provide useful information for various applications of optical satellite sensor images especially in assessments of land use or post‐fire regeneration of vegetation.     

Estimation of plant water concentration by the reflectance Water Index WI (R900/R970)

Water Index WI (R900/R970) was used for the estimation of plant water concentration (PWC) by ground-based, reflectance measurements. Reflectance and PWC were measured for adult plants growing in the field throughout an annual cycle and in potted seedlings submitted to progressive desiccation. The species studied were characteristicly Mediterranean: Pinus halepensis, Quercus ilex, Quercus coccifera, Arbutus unedo, Cistus albidus, Cistus monspeliensis, Phillyrea angustifolia, Pistacia lentiscus and Brachypodium retusum . WI was significantly correlated with PWC when all the species were considered together, and with almost all the species considered individually, especially when a wider range of PWC was obtained by extreme dessication of experimental plants. The correlations increased when normalizing WI by NDVI. The wavelength of the trough corresponding to water absorption band tended to shift from 970-980 nm to lower wavelengths 930-950 nm with decreasing PWCs. Infrared measurement of plant temperature and T leaf - T air provided worse assessment of PWC. A simple radiometer measuring plant reflectance at 680, 900, and 970nm could speed up the measurement of PWC, and be useful in wildfire risk evaluation and drought assessment.     

Evaluation of nitrogen treatment effects on the reflectance of cotton at different spatial scales

ABSTRACT

The importance of assessing nitrogen (N) status in cotton is important from economic and environmental standpoints. In this study, visible and near-infrared reflectance (NIR) data were collected at cotton leaf-, canopy- and scene-scales at three levels of N treatments to determine the best spatial scale and growth stage that most effectively indicate N treatment effects. While N fertilization affected relative chlorophyll content, leaf area index (LAI), and ground cover (GC) simultaneously, these factors portrayed different effects on cotton reflectance measured at the three spatial scales. Leaf-scale measurement was mainly affected by chlorophyll content. Canopy-scale reflectance was controlled by chlorophyll content and LAI. Scene-scale reflectance was predominantly controlled by GC and to the least extent by chlorophyll content. In terms of visible reflectance, chlorophyll absorption decreased with decreasing N at all spatial scales. Nitrogen treatment effects were most apparent at 550 and 700 nm at the leaf-scale, 610 and 700 nm at the canopy-scale, and 685–690 nm at the scene-scale (after per cent GC exceeded 64%). Only measurements taken at the scene-scale demonstrated a consistent relationship between N fertilization and NIR (800–1000 nm). This information could be useful in the development of N-sensitive indices.