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At least 127 records · Page 7

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NASA NTRS1973

Light ray tracing through a leaf cross section

A light ray, incident at about 5 deg to the normal, is geometrically plotted through the drawing of the cross section of a soybean leaf using Fresnel's equations and Snell's law. The optical mediums of the leaf considered for ray tracing are: air, cell sap, chloroplast, and cell wall. The ray is also drawn through the same leaf cross section with cell wall and air as the only optical mediums. The values of the reflection and transmission found from the ray tracing tests agree closely with the experimental results obtained using a Beckman Dk-2A Spectroreflector.

Kumar, R.↗

NASA NTRS1973

Light ray tracing through a leaf cross section

A light ray, incident at about 5 deg to the normal, is geometrically plotted through the drawing of the cross section of a soybean leaf using Fresnel's equations and Snell's law. The optical mediums of the leaf considered for ray tracing are air, cell sap, chloroplast, and cell wall. The above ray is also drawn through the same leaf cross section considering cell wall and air as the only optical mediums. The values of the reflection and transmission found from ray tracing agree closely with the experimental results obtained using a Beckman DK-2A spectroreflectometer.

Kumar, R.↗

NASA NTRS1982

Linkage between gravity perception and response in the grass leaf-sheath pulvinus

Cellulose synthesis inhibitors 2,6-dichlorobenzonitrile and coumarin inhibit the normal gravitropic response of the leaf-sheath pulvini in several grasses. These inhibitors induce thickenings that are mostly distributed in the radial walls. Their distribution patterns follow a previously published equation that describes the geometry of asymmetric growth in the leaf-sheath pulvinus. It is proposed that radial transport of growth promotors may account for the observed asymmetric growth rather than the classical concept of lateral downward transport in an horizontally placed shoot. Electron microscopic observation of an interaction between the tonoplast membrane and statoliths suggests that such an interaction might be a basis for gravity perception in grass leaf-sheath pulvini.

Dayanandan, P.↗

NASA NTRS1982

Reflectance measurements of cotton leaf senescence altered by mepiquat chloride

Spectrophotometric reflectance measurements were made on plant-attached leaves to evaluate growth chamber-grown cotton leaf (Gossypium hirsutum L.) senescence (chlorophyll degradation as criterion) that was delayed by mepiquat chloride (1,1-dimethylpiperidinium chloride) rates of 0, 10, 40, 70, and 100 g a.i./ha. Mepiquat chloride (MC increased both chlorophyll and leaf water contents as compared with that of untreated leaves. Reflectance was inversely and linearly correlated (r = -0.873**) with eater content at the 1.65 micrometer wavelength and was inversely correlated (r = -0.812**) with chlorophyll concentration at the 0.55 micrometer wavelength but best fit a quadratic equation. Either wavelength measurement might be useful to remotely detect cotton leaf senescence or fields of MC-treated cotton plants.

Gausman, H. W.↗

NASA NTRS1984

Remote sensing of the leaf area index of temperate coniferous forests

To estimate the one-sided leaf area index (LAI) of temperate coniferous forests using data acquired from the Daedalus Airborne Thematic Mapper, an empirical model is developed. The study area follows an environmental gradient across west-central Oregon, where leaf development varies in response to temperature and moisture. The relationship between the ratio of thematic-mapper simulator channels four and three and the leaf area index for selected closed canopy or fully stocked forest stands along the gradient is analyzed. Results show that a good relationship exists between the LAI and the IR/red ratio for conifers and that a conifer species-independent asymptotic relationship is observed between LAI and near IR/red reflectance, with near radiometric saturation occurring at an LAI of about 7-8.

Spanner, M. A.↗

NASA NTRS1987

Radiation transfer in plant canopies - Transmission of direct solar radiation and the role of leaf orientation

Understanding the details of the interaction between the radiation field and plant structures is important climatically because of the influence of vegetation on the surface water and energy balance, but also biologically, since solar radiation provides the energy necessary for photosynthesis. The problem is complex because of the extreme variety of vegetation forms in space and time, as well as within and across plant species. This one-dimensional vertical multilayer model describes the transfer of direct solar radiation through a leaf canopy, accounting explicitly for the vertical inhomogeneities of a plant stand and leaf orientation, as well as heliotropic plant behavior. This model reproduces observational results on homogeneous canopies, but it is also well adapted to describe vertically inhomogeneous canopies. Some of the implications of leaf orientation and plant structure as far as light collection is concerned are briefly reviewed.

Verstraete, Michel M.↗

NASA NTRS1987

Relative water content of Spruce needles determined by the leaf water content index

Leaf relative water content (RWC) is defined as the volume of water in a leaf divided by the volume at full turgor. Using reflectance factors of wavelengths 0.83 micron and 1.6 microns, a Leaf Water Content Index (LWCI) was derived from the Lambert-Beer Law such that LWCI should equal RWC; LWCI was equal to RWC for Picea pungens, Picea rubens, Liquidambar styraciflua, and Quercus agrifolia. Algebraic manipulation shows that R(1.6)/R(0.83) termed the Moisture Stress Index (MSI), is near-linearly correlated to RWC and to the Equivalent Water Thickness (EWT). Five species tested so far had the same relationship between MSI and EWT, but EWT is not a measure of plant water status.

Hunt, E. Raymond, Jr.↗

NASA NTRS1988

Effect of curvature on the backscattering from a leaf

Using a model previously developed for the backscattering cross section of a planar leaf at X-band frequencies and above, the effect of leaf curvature is examined. For normal incidence on a rectangular section of a leaf curved in one and two dimensions, an integral expression for the backscattered field is evaluated numerically and by a stationary phase approximation, leading to a simple analytical expression for the cross-section reduction produced by the curvature. Numerical results based on the two methods are virtually identical, and in excellent agreement with measured data for rectangular sections of coleus leaves applied to the surfaces of styrofoam cylinders and spheres of different radii.

Sarabandi, K.↗

NASA NTRS1989

Detection of changes in leaf water content using near- and middle-infrared reflectances

A method to detect plant water stress by remote sensing is proposed using indices of near-IR and mid-IR wavelengths. The ability of the Leaf Water Content Index (LWCI) to determine leaf relative water content (RWC) is tested on species with different leaf morphologies. The way in which the Misture Stress Index (MSI) varies with RWC is studied. On test with several species, it is found that LWCI is equal to RWC, although the reflectances at 1.6 microns for two different RWC must be known to accurately predict unknown RWC. A linear correlation is found between MSI and RWC with each species having a different regression equation. Also, MSI is correlated with log sub 10 Equivalent Water Thickness (EWT) with data for all species falling on the same regression line. It is found that the minimum significant change of RWC that could be detected by appying the linear regression equation of MSI to EWT is 52 percent. Because the natural RWC variation from water stress is about 20 percent for most species, it is concluded that the near-IR and mid-IR reflectances cannot be used to remotely sense water stress.

Hunt, E. Raymond, Jr.↗

NASA NTRS1990

Environmental modification of yield and food composition of cowpea and leaf lettuce

Cowpea (Vigna unguiculata (L.) Walp.) and leaf lettuce (Lactuca sativa L.) are candidate species to provide ligume protein and starch or serve as a salad base for a nutritionally balanced and psychologically satisfying vegetarian diet in the Controlled Ecology Life Support System (CELSS). Various nutritional parameters are reported. Hydroponic leaf lettuce grew best under CO2 enrichment and photosynthetic photon flux (PPF) enhancement. Leaf protein content reached 36 percent with NH4(+) + NO3 nutrition; starch and free sugar content was as high as 7 or 8.4 percent of DW, respectively, for high PPF/CO2 enriched environments.

Mitchell, Cary A.↗

NASA NTRS1991

Airborne remote sensing of canopy water thickness scaled from leaf spectrometer data

The reflectance ratio of the middle-infrared band (MIR) to the near-infrared band (NIR) is linearly related to the log(10) equivalent water thickness (EWT) for single leaves of different morphologies, whereas the MIR/NIR radiance ratio is correlated with the leaf area index (LAI). The hypothesis that the MIR/NIR ratio measures canopy EWT was tested by reanalyzing airborne Thematic Mapper Simulator and field data obtained across a large gradient of LAI in western Oregon, U.S.A. The measured airborne MIR/NIR reflectance ratios for canopies were not significantly different from the predicted ratios using leaf data for canopy EWT, except for two desert woodland sites. The interpretation of the MIR/NIR ratio is scale-dependent, because leaf EWT is determined primarily by variations in LAI.

Hunt, E. Raymond, Jr.↗

NASA NTRS1991

Temporal versus spatial variation in leaf reflectance under changing water stress conditions

Leaf reflectance changes associated with changes in water stress were analyzed in two separate experiments. Results indicate that the variation in reflectance among collections of leaves of a given species all at the same level of water stress is at least as great as the variation in reflectance associated with changes in water stress for a given leaf collection of that species. The implications is that results from leaf reflectance-water stress studies have only limited applicability to the remote sensing of plant canopy water stress.

Cohen, Warren B.↗

NASA NTRS1991

Transport theory for a leaf canopy of finite-dimensional scattering centers

A formalism for photon transport in leaf canopies with finite-dimensional scattering centers that cross shade mutually is developed. Starting from first principles, expressions for the interaction cross sections are derived. The problem of illumination by a monodirectional source is studied in detail using a successive collisions approach. A balance equation is formulated in R3 and the interaction between a leaf canopy and the adjacent atmosphere is discussed. Although the details are those relating to a leaf canopy, the formalism is equally applicable to other media where the constituents cross shade mutually such as planetary surfaces, rings and ridged-ice in polar regions, i.e., media that exhibit opposition brightening.

Myneni, Ranga B.↗

NASA NTRS1993

Leaf Areas And Spectral Properties Of Slash Pine

NASA technical memorandum describes experiments to test feasibility of estimating seasonal leaf areas of forest canopies from spectral radiances measured by remote instruments. Accurate estimates of seasonal leaf areas of forests necessary for studies of seasonal exchanges of energy between forest canopies and atmosphere. Potential use of remote sensing in estimating seasonal changes in leaf area index (LAI).

Curran, Paul J.↗

NASA NTRS1988

Growth and yield characteristics of 'Waldmann's Green' leaf lettuce under different photon fluxes from metal halide or incandescent + fluorescent radiation

Growth of 'Waldmann's Green' leaf lettuce under metal halide radiation was compared with that under In = Fl at the same photosynthetic photon flux (920 micromol/s/sq m) to evaluate the influence of lamp type on growth. No differences in leaf dry weight, leaf area, relative growth rate or photosynthesis occurred after 8 days of exposure to these radiation treatments for 20 h/day.

Knight, Sharon L.↗

NASA NTRS1992

Oregon transect: Comparison of leaf-level reflectance with canopy-level and modelled reflectance

The Oregon Transect Ecosystem Research (OTTER) project involves the collection of a variety of remotely-sensed and in situ measurements for characterization of forest biophysical and biochemical parameters. The project includes nine study plots located along an environmental gradient in west-central Oregon, extending from the Pacific coast inland approximately 300km. These plots represent a broad range in ecosystem structure and function. Within the OTTER project, the sensitivity of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) signal to absorption by foliar biochemicals is being examined. AVIRIS data were acquired over all plots in conjunction with the four OTTER Multi-sensor Aircraft Campaigns spanning the growing season. Foilage samples were gathered during each campaign for biochemical determination (at Ames Research Center), to estimate stand-level constituency at each plot. Directional-hemispheric leaf reflectance throughout the 400-2400nm region was measured in the laboratory as an aid to interpreting concurrent AVIRIS data. Obtaining leaf spectra in this manner reduces or eliminates the confounding influences of atmosphere, canopy architecture, and reflectance by woody components, understory, and exposed soils which are present in airborne observations. These laboratory spectra were compared to simulated spectra derived by inverting the PROSPECT leaf-level canopy reflectance derived from AVIRIS data by use of the LOWTRAN-7 atmospheric radiative-transfer model.

Johnson, Lee F.↗

NASA NTRS1999

Leaf Optical Properties in Higher Plants: Linking Spectral Characteristics with Plant Stress

A number of studies have addressed responses of leaf spectral reflectance, transmittance, or absorptance to physiological stress. Stressors included dehydration, ozone, herbicides, disease, insufficient mycorrhizae and N fertilization, flooding and insects. Species included conifers, grasses, and broadleaved trees. Increased reflectance with maximum responses near 700 nm wavelength occurred in all cases. Varying the chlorophyll content in leaves or pigment extracts can simulate this effect. Thus, common optical responses to stress result from decreases in leaf chlorophyll contents or the capacity of chloroplasts to absorb light. Leaf optic can be quite sensitive to any stressor that alters soil-plant-atmosphere processes.

Carter, Gregory A.↗

NASA NTRS1998

Changes in Arabidopsis leaf ultrastructure, chlorophyll and carbohydrate content during spaceflight depend on ventilation

Leaf structure and function under spaceflight conditions have received little study despite their important implications for biological life support systems using plants. Previous reports described disruption of the membrane apparatus for photosynthesis and a general decrease in carbohydrate content in foliage. During a series of three short-duration experiments (Chromex-03, -04, -05) on the US space shuttle (STS-54, STS-51, STS-68), we examined Arabidopsis thaliana leaves. The plants were at the rosette stage at the time of loading onto the space shuttle, and received the same light, temperature, carbon dioxide and humidity regimes in the orbiter as in ground controls. The experiments differed according to the regime provided in the headspace around the plants: this was either sealed (on mission STS-54); sealed with high levels of carbon dioxide (on mission STS-51) or vented to the cabin air through a filtration system (on mission STS-68). Immediately post-flight, leaf materials were fixed for microscopy or frozen in liquid nitrogen for subsequent analyses of chlorophyll and foliar carbohydrates. At the ultrastructural level, no aberrations in membrane structure were observed in any of the experiments. When air-flow was provided, plastids developed large starch grains in both spaceflight and ground controls. In the experiments with sealed chambers, spaceflight plants differed from ground controls with regard to measured concentrations of carbohydrate and chlorophyll, but the addition of airflow eliminated these differences. The results point to the crucial importance of consideration of the foliage microenvironment when spaceflight effects on leaf structure and metabolism are studied.

STS-51 Shuttle Project↗