Harold W. Gausman

Interaction of Isotropic Light with a Compact Plant Leaf

A transparent plate with rough plane-parallel surfaces is used as a theoretical model to explain the interaction of diffuse light with a compact plant leaf. Effective optical constants of a corn leaf have been determined from leaf reflectance and transmittance measured over the spectral range 0.5–2.5 μ with a recording spectrophotometer. The effective index of refraction at 0.5 μ for the corn leaf is not inconsistent with the refractive index of epicuticular wax. The effective absorption spectra of the corn leaf appears to be a superposition of the absorption coefficients of chlorophyll and pure liquid water. Residual spectral data from other leaf constituents are at the resolution limit of the spectrophotometer. The plate model of a leaf is also used to determine moisture content of the corn leaf from reflectance and transmittance measurements.

Reflectance of leaf components

Abstract The reflectance of leaf components was evaluated over the 370 to 1100 nm wavelength interval. Kodak high speed, black-and-white infrared photographs at 850 nm showed that: (1) Leaf epidermises of Elodea (Anacharis canandensis, Planch.) and Lemna L, diffused incoming infrared light; (2) infrared light was reflected from surfaces inside leaves of Rhoeo discolor Hance, through stomatal apertures; and (3) crystals and chloroplasts in the expressed sap of Zebrina pendula Schnizl. reflected infrared light. Scans (370 to 1 100 nm) showed that reflectance from complex cell walls of Agave americana L. compared with that of the adjacent cytoplasm was significantly greater ( p=0.01 ) than the reflectance of simpler cell walls of Heliconia humile L. compared with that of the adjacent cytoplasm. The reflectance of Vicia faba L. nuclei was larger (significant, p=0.10 ) than that of adjacent cell areas. Results show that refractive index discontinuities in leaves cause the reflectance of near-infrared light.

Reflectance of cotton leaves and their structure.

Abstract Cotton plants were grown hydroponically with low-, medium-, and high-salinity substrate levels formulated with sodium chloride. Leaves were sampled from third and fourth nodes down from apexes of cotton plants, simulating what an overhead remote sensor would see. A spectrophotometer was used to measure reflectance and transmittance of light impinging on upper surfaces of individual leaves. Total reflectance of light in the 750- to 1300 -mμ spectral range was greater from leaves of cotton plants grown in medium- and high-salinity substrates than from those grown in low-salinity substrates. This increase in reflectance and a lessening in absorptance were consistent with the observed thicker leaves of the saline substrate-grown plants which had larger palisade cells and loosely arranged spongy mesophyll. These structural changes resulted in more intercellular spaces, thus supporting the premise that internal scattering of light is increased by cell wall—air cavity interfaces.

Refractive index of plant cell walls

Air was replaced with media of higher refractive indices by vacuum infiltration in leaves of cucumber, blackeye pea, tomato, and string bean plants, and reflectance of noninfiltrated and infiltrated leaves was spectrophotometrically measured. Infiltrated leaves reflected less light than noninfiltrated leaves over the 500-2500-nm wavelength interval because cell wall-air interfaces were partly eliminated. Minimal reflectance should occur when the average refractive index of plant cell walls was matched by the infiltrating fluid. Although refractive indices that resulted in minimal reflectance differed among the four plant genera, an average value of 1.425 approximates the refractive index of plant cell walls for the four plant genera.

Visible light reflectance, transmittance, and absorptance of differently pigmented cotton leaves

Abstract Spectrophotometrically measured light reflectance and transmittance, and calculated absorptance over the 0.45–0.70 μm waveband (WB) of four differently pigmented cotton ( Gossypium hirsutum L.) leaves are compared. Leaf color appearances were green (G), common red (CR), light red or bronze (B), and yellow-green (YG). The G and YG leaves (chlorophylls and carotenoid) had about the same reflectance and transmittance peaks at the 0.55-μm wavelength (WL), although YG leaves had much higher reflectance and transmittance than G leaves. Anthocyanin pigment manifestation in CR, B, and YG leaves occurred at about the 0.60-μm WL. The red light (0.65-μm) absorptances of leaves in decreasing magnitude were: CR, G, YG, and B. There was little effect on blue light (0.45-μm) absorptance. Spectral responses of cotton leaves were very sensitive to either their masked or predominant pigmentation over the 0.45–0.70-μm WBs. The possible importance of leaf pigmentation to LANDSAT-Ds thematic mapper, visible light WBs is discussed.

Mean Effective Optical Constants of Cotton Leaves

The plate description of a typical, thin, compact plant leaf, introduced previously, has been generalized to the noncompact case and applied to experimental data including average reflectance and transmittance measurements on 200 mature, field-cotton leaves. A compact leaf has few and a noncompact leaf has many intercellular air spaces in the mesophyll. No statistically significant difference was found between the average leaf thickness and the mean effective water thickness of the leaves. The Kubelka–Munk scattering coefficient s for a typical leaf, measured at the 1-μ spectral region, is approximated by the relation s = r/t, where r and t, respectively, are the reflectance and transmittance of the leaf. The approximate equality of r and t, noted by previous investigators, is explained on the basis of the scattering of diffuse light within the leaf by critical internal reflections. Predictions from the plate (P) model for cotton leaves compare favorably with those of the Kubelka–Munk (K–M) and Melamed (M) theories. Applied to vegetation, all three theories predict a characteristic linear dimension related to the cellular structure of the leaf.

Willstätter-Stoll Theory of Leaf Reflectance Evaluated by Ray Tracing

The widely accepted Willstätter-Stoll (W-S) theory of leaf reflectance has been investigated by extensive ray tracing through a model (W-S model) in which the leaf cellular structure is approximated by circular arcs. Calculations were performed on an IBM 1800 computer. The W-S model is treated as a two-dimensional, uncentered optical system consisting of a single medium and air. Optical properties of the medium are specified by a complex index of refraction. Given an incident ray, new reflected and transmitted rays are produced at each interface with properties determined by the laws of Snell, Fresnel, and Lambert. Calculations indicate that the W-S model, as exemplified by their artists conception, is too transparent, that is, the magnitude predicted for transmittance is too high. Transmittance is still too high if each interface is treated as a diffusive instead of a smooth surface. The W-S model can be easily improved, however, by introduction of more intercellular air spaces. The modified W-S model promises to be an excellent representation of physical reality. Accurate predictions, however, require an inordinate amount of computer time.

Radiometric estimation of biomass and nitrogen content of Alicia grass

Abstract Hand-held MARK-II radiometric measurements were used to estimate biomass yields and nitrogen (N) content of Alicia grass (Cynodon spp.) plots having five levels of nitrogen fertilization. The radiometric RED (630- to 690-nm) and NIR (760- to 900-nm) measurements obtained from the plots were converted to reflectance factors and to perpendicular (PVI) and ratio (RVI = NIR/RED) vegetation indices and then correlated with grass biomass yield and N content. The coefficients of determination (r2) in estimating biomass yield for the RED and NIR reflectance factors were 0.04 and 0.73, 1 respectively, and for PVI and RVI they were 0.68 1 and 0.61, 1 respectively. The correlations for N content were 0.02, 0.71, 1 0.69, 1 and 0.60, 1 respectively. Since beef cattle protein needs are related to grass N content these results may be useful to operational rangeland remote sensing programs for estimating animal carrying capacity using satellite data.

Mean Effective Optical Constants of Thirteen Kinds of Plant Leaves

Plant leaves grown in a greenhouse and leaves collected from the field have been analyzed to obtain mean effective optical constants based upon diffuse reflectance and transmittance measurements taken over the 0.5-2.5-micro spectral range. These optical constants are used in a generalized flat-plate model to describe the phenomena of leaf reflectance. Analysis procedures developed led to measures of the amount of water and intercellular air spaces in the leaves. Over the 1.4-2.5-micro spectral range, the absorption spectra of leaves are not statistically different from that of pure liquid water. Leaf reflectance differences among the plant leaves over the 0.5-1.4 micro range are caused principally by Fresnel reflections at external and internal leaf surfaces and by plant pigment absorption. Reflectance over the 1.4-2.5-micro range results largely from Fresnel reflections and absorption by water. Data are presented in the form of dispersion curves with 95% confidence bands and tabulated plant leaf absorption spectra. The dispersion curves were assumed to be cubic equations of the form n = Sigmaa(i)lambda(i) (i = 0, 1, 2, 3), where lambda is wavelength. Reflectance measurements at 1.65 micro have been associated with the equivalent water thickness and the intercellular air spaces in the leaf. Accuracy of the plate theory based upon a cubic dispersion curve is shown to be within experimental error.

The influence of ammonia-induced cellulardiscoloration within cotton leaves (Gossypium hirsutum L.) on light reflectance, transmittance, and absorptance

Abstract Cellular discoloration within cotton leaves, caused by ammoniatreatment, reduced spectrophotometrically measured near-infrared light reflectance over the wavelength interval 750–900 mμ, where rounding of the plateau occurred. Results with ammonia-treated leaves are compared with other studies in which reduced reflectance occurred over the 750–900 mμ wavelength interval. It is possible that this rounding of the plateau caused by internal cellular discoloration may become useful in remote sensing for identifying certain nonvisual symptoms of plant leaf stresses caused by cellular discoloration.