Lawrence A. Corp

Distinguishing nitrogen fertilization levels in field corn (Zea mays L.) with actively induced fluorescence and passive reflectance measurements

Abstract Laser-induced fluorescence (LIF) is an active sensing technique capable of capturing immediate and specific indications of changes in plant physiology and metabolism as they relate to the concentration and photosynthetic activity of the plant pigments. Reflectance is a passive sensing technique that can capture differences in the concentration of the primary plant pigments. Fluorescence and reflectance were compared for their ability to measure levels of plant stress that are of agronomic importance in corn ( Zea mays L.) crops. Laboratory LIF and reflectance spectra were made on excised leaves from field grown corn. Changes in the visible region of the spectrum were compared between groups of plants fertilized with seven different levels of nitrogen (N) fertilization. A pulsed nitrogen laser emitting photons at a wavelength of 337 nm was used as a fluorescence excitation source. Differences in maximum intensity of fluorescence occurred at 440 nm, 525 nm, 685 nm, and 740 nm. Significant separations were found between levels of N fertilization at several LIF wavelength ratios. Several reflectance algorithms also produced significant separations between certain levels of N fertilization.

Contribution of chlorophyll fluorescence to the apparent vegetation reflectance

Current strategies for monitoring the physiologic status of terrestrial vegetation rely on remote sensing reflectance data, which provide estimates of vigor based primarily on chlorophyll content. Chlorophyll fluorescence (ChlF) measurements offer a non-destructive alternative and a more direct approach for diagnosis of vegetation stress before a significant reduction in chlorophyll content has occurred. Thus, technology based on ChlF may allow more accurate carbon sequestration estimates and earlier stress detection than is possible when using reflectance data alone. However, the observed apparent vegetation reflectance (Ra) in reality includes contributions from both the reflected and fluoresced radiation. The aim of this study is to determine the relative contributions of reflectance and ChlF fractions to Ra in the red to near-infrared region (650-800 nm) of the spectrum. The practical objectives of the study are to: 1) evaluate the relationship between ChlF and reflectance at the foliar level for corn, soybean and maple; and 2) for corn, determine if the relationship established for healthy vegetation changes under nitrogen (N) deficiency. To obtain generally applicable results, experimental measurements were conducted on unrelated crop and tree species (corn, soybean and maple) under controlled conditions and a gradient of inorganic N fertilization levels. Optical reflectance spectra and actively induced ChlF emissions were collected on the same foliar samples, in conjunction with measurements of photosynthetic function, pigment levels, and carbon (C) and N content. The spectral trends were examined for similarities. On average, 10-20% of Ra at 685 nm was actually due to ChlF. The spectral trends in steady state and maximum fluorescence varied significantly, with steady state fluorescence (especially red, 685 nm) showing higher ability for species and treatment separation. The relative contribution of ChlF to Ra varied significantly among species, with maple emitting much higher fluorescence amounts, as compared to corn and soybean. Steady state fluorescence from individual red and far-red emission bands (F685 and F740, respectively) and their ratio consistently enabled species separation. For corn, the relative ChlF fraction increased in concert with the nutrient stress levels from <2% for non-stressed foliage to >7% for severely N deficient plants. Steady state ChlF at 685 nm provided optimal N treatment separation. This study confirms the trends in the steady state red/far-red ratio (F685s/F740s) associated with N deficiency and vegetation stress, previously established using active single narrow band excitation.

Red edge optical properties of corn leaves from different nitrogen regimes

High resolution (<2 nm) optical spectra and biophysical measurements were acquired from corn leaves from field plots having four nitrogen fertilizer application rates: 20%, 50%, 100% and 150% of optimal levels. Reflectance (R), transmittance (T), and absorptance (A) spectra were obtained for both adaxial and abaxial leaf surfaces. The strongest relationships between foliar chemistry and optical properties were demonstrated for C/N content and two optical parameters associated with the red edge inflection point (REIP): 1) anormalized first derivative maximum (Dmax) occurring between 695 and 730 nm (Dmax/D744); and 2) the wavelength associated with Dmax (WL of REIP). A nonlinear increase in the Dmax/D744 ratio as a function of C/N content was observed for all optical properties (r/sup 2/ = 0.90-0.95). Similarly, a nonlinear decrease in the WL of REIP as a function of C/N content was observed for all optical properties (RT, RB, TT, and AT) (r/sup 2/ = 0.85-0.96). The Dmax/D744 ratio increased as the WL of REIP declined from /spl sim/730 to 700 nm, with curves per optical property expressing different degrees of nonlinearity.

Fluorescence imaging system: application for the assessment of vegetation stresses

As a part of an ongoing laser induced fluorescence (LIF) project, out laboratories have developed a fluorescence imaging system (FIS) to acquire fluorescence images at wavelengths centered at 450 nm, 550 nm, 680 nm, and 740 nm. The system consists of ultraviolet (UV) fluorescent lamps as an exciting source, automated filter wheel, and charge coupled device (CCD) camera. The automated filter wheel and CD camera are controlled by a microcomputer via a computer interface,a nd digital images are captured. The FIS is capable of capturing steady state fluorescence and chlorophyll fluorescence induction images. Experimental studies were conducted to demonstrate the utility of the FIS. One such study included experiments to observe the effects of ethylenediurea (EDU) in soybean leaves with FIS. Five different concentrations of EDU were sued to establish a doe-response relationship. Although visual effects of EDU treatment were not apparent, the intensities of the fluorescence images of the plant leaves varied depending on the EDU concentration, the location on the leaf surface and the emission wavelength. EDU appeared mainly to affect the photosynthetic apparatus causing non-uniform increases in red and far-red fluorescence. Ratio images of red-green and blue/far-red were found to be sensitive indicators in detecting EDU effects. A ratio of fluorescence induction to steady state fluorescence had a curvilinear relationship with EDU-dosage. Such kinetic measurements can be used to assess photosynthetic activity in response to a range of chemical and environmental stresses. This study demonstrates that FIS is an excellent tool to detect stress symptoms before the onset of visible injury. It will enhance our understanding of the interactions among photosynthetic activity, vegetative stresses and fluorescence responses. Characterization of steady state fluorescence patterns in leaves is of significant value in our LIF research studies, and images taken with FIS greatly complement non-imaging fluorescence measurements by finding the spatial distribution of fluorescence in leaves.

The contribution of chlorophyll fluorescence to the reflectance spectra of green vegetation

Green vegetation has a relatively low reflectance in the red region (670 nm to 700 nm) due to strong absorption of chlorophyll a. Fluorescence of green vegetation has strong emission peaks at 685 nm and 740 nm. The red-NIR region (670 nm to 760 nm) has been widely used in remote sensing applications without considering the effects of chlorophyll fluorescence. It is vital to understand the contribution of chlorophyll fluorescence to the red-NIR region of the reflectance spectra. These investigation were conducted to determine the contribution of fluorescence to the reflectance of green vegetation. Measurements necessary for the determination the effects of fluorescence were made using soybeans grown at different nitrogen levels in order to obtain a range of reflectance and fluorescence spectra. The contribution of chlorophyll a fluorescence to reflectance in the red-NIR region is significant, and found to be as great as 23% of the reflectance at 685 nm and 4% at 740 nm. There is no significant contribution of fluorescence to the red edge shift.<<ETX>>

Optical reflectance and fluorescence for detecting nitrogen needs in Zea mays L

Nitrogen (N) status in field grown corn (Zea mays L.) was assessed using spectral techniques. Passive airborne hyperspectral reflectance remote sensing, passive leaf level reflectance, and both passive and active leaf level fluorescence sensing methods were tested. Reflectance of leaves could track total Cha levels in the red dip of the spectrum and auxiliary plant pigments of Chb and carotenoids in the yellow/orange/red edge reflectance. Based on leaf level reflectance behavior, a modified chlorophyll absorption reflectance index (MCARI) method was tested with narrow bands from the Airborne Imaging Spectroradiometer for Applications. MCARI indices could detect variations in N levels across field plots. At the leaf level, ratios of fluorescence emissions in the blue, green, red and far-red wavelengths sensed responses that were associated with the plant pigments, and were indicative of energy transfer in the photosynthetic process. Fluorescence emissions of leaves could distinguish N stressed corn from those with optimally applied N. Reflectance and fluorescence methods are sensitive in detecting corn N needs and may be especially powerful in monitoring crop conditions if both types of information can be combined.

Chlorophyll Fluorescence Emissions of Vegetation Canopies From High Resolution Field Reflectance Spectra

A two-year experiment was performed on corn (Zea mays L.) crops under nitrogen (N) fertilization regimes to examine the use of hyperspectral canopy reflectance information for estimating chlorophyll fluorescence (ChlF) and vegetation production. Fluorescence of foliage in the laboratory has proven more rigorous than reflectance for correlation to plant physiology. Especially useful are emissions produced from two stable red and far-red chlorophyll ChlF peaks centered at 685V10 nm and 735V5 nm. Methods have been developed elsewhere to extract steady state solar induced fluorescence (SF) from apparent reflectance of vegetation canopies/landscapes using the Fraunhofer Line Depth (FLD) principal. Our study utilized these methods in conjunction with field-acquired high spectral resolution canopy reflectance spectra obtained in 2004 and 2005 over corn crops, as part of an ongoing multi-year experiment at the USDA/Agriculture Research Service in Beltsville, MD. A spectroradiometer (ASD-FR Fieldspec Pro, Analytical Spectral Devices, Inc., Boulder, CO) was used to measure canopy radiances 1 m above plant canopies with a 22deg field of view and a 0deg nadir view zenith angle. Canopy and plant measurements were made at the R3 grain fill reproductive stage on 3-4 replicate N application plots provided seasonal inputs of 280, 140, 70, and 28 kg N/ha. Leaf level measurements were also made which included ChlF, photosynthesis, and leaf constituents (photosynthetic pigment, carbon (C), and N contents). Crop yields were determined at harvest. SIF intensities for ChlF were derived directly from canopy reflectance spectra in specific narrowband regions associated with atmospheric oxygen absorption features centered at 688 and 760 nm. The red/far-red S F ratio derived from these field reflectance spectra successfully discriminated foliar pigment levels (e.g., total chlorophyll, Chl) associated with N application rates in both corn crops. This canopy-level spectral ratio was also positively correlated to the foliar C/N ratio (r = 0.89, n = go), as was a leaf-level steady state fluorescence ratio (Fs/Chl, r = 0.92). The latter ratio was inversely correlated with crop grain yield (Kg 1 ha) (r = 0.9). This study has relevance to future passive satellite remote sensing approaches to monitoring C dynamics from space.

Contribution of chlorophyll fluorescence to the reflectance of corn foliage

To assess the contribution of chlorophyll fluorescence (ChlF) to apparent reflectance (Ra) in the red/far-red, spectra were collected on a C/sub 4/ agricultural species (corn, Zea mays L.) under conditions ranging from nitrogen deficiency to excess. A significant contribution of ChlF to Ra was observed, with on average 10-25% at 685nm and 2-6% at 740nm of Ra being due to ChlF. Higher ChlF was consistently measured from the abaxial leaf surface as compared to the adaxial. Using 350-665nm excitation, the study confirms the trends in three ChlF ratios established previously by active F technology, suggesting that the ChlF utility this technology has developed for monitoring vegetation physiological status is likely applicable also under natural solar illumination.

Fluorescence responses from nitrogen plant stress in 4 Fraunhofer band regions

The potential of solar Fraunhofer line features centered at 532, 607, 677 and 745nm for tracking changes in plant canopy chlorophyll content and photosynthetic capacity was studied. Excitation wavelengths similar to full sun light were considered. Canopy changes were tested experimentally by monitoring treatments of plant stress due to nitrogen application rate in corn. Corn leaves were obtained from field plots that were given different nitrogen application rates at 20, 50, 100, and 150% of optimal N in 2001. The data infers that leaves in plant canopies that have the greatest photosynthetic performance potential can be identified. Information collected in Fraunhofer regions compared favorably with data taken by laser induced fluorescence excitation and detection methods in peak emission areas. The technique may be useful in projecting what can be expected if a space-born interferometer type sensor can be developed for capturing plant canopy fluorescence.

Solar Induced Fluorescence and Reflectance Sensing Techniques for Monitoring Nitrogen Utilization in Corn

Remote sensing systems using either passive reflectance (R) or actively induced fluorescence (F) have long been explored as a means to monitor species composition and vegetative productivity. Passive F techniques using the Fraunhofer line depth (FLD) principle to isolate solar induced F (SIF) from the high resolution R continuum have also been suggested for the large-scale remote assessment of vegetation. The FLD principle was applied to both canopy R spectra and AISA multi-spectral imagery to discriminate the relatively weak in situ vegetation F in-fill of the telluric O2 bands located at 688 nm and 760 nm. The magnitudes of SIF retrieved from R ranged from 7 to 36 mW/m2/nm/sr and the ratio of the two spectral bands successfully discriminated the four N treatment levels. In addition, a number of R indices including but not limited to the physiological reflectance index (PRI), R550/R515 and R750/R800 were calculated from the AISA aircraft imagery and the high-resolution canopy R spectra. These indexes were then evaluated against georeferenced ground measurements of leaf area index (LAI), pigment contents, grain yields, and light use efficiency (LUE). A number of significant relationships were evident in both R and SIF indices to the biophysical changes in corn induced by N application rates. From this investigation we conclude that valuable SIF information can be extracted from high-resolution canopy R data and indices calculated from both data types can supply useful information for modeling N use for carbon sequestration by vegetation.