Spectrophotometric transmittance and reflectance curves were recorded for wavelengths from 0.45 (in some cases 0.34) to 2.7 micrometers for faces and backs of leaves and for stacked leaves of several plant species. Measurements were made at different angles of illumination. Leaf spectrophotometric curves were compared with curves for leaf extracts, potato tuber tissue, glass beads in water, and frozen leaves to demonstrate the physical bases for the leaf curves. Leaves were infiltrated with liquids of different refractive indices for further comparison of spectrophotometric curves. Goniophotometric reflectance curves were recorded, giving visible reflectance and degree of polarization as functions of viewing angle for two different angles of illumination.No retroreflection was observed, and no phenomena were observed which could be attributed to interference because of similarity between leaf structural sizes and wavelengths used. Figure 1 illustrates typical leaf reflectance and transmittance curves for incident light nearly normal to the leaf surface. If given qualities of sunlight or skylight are assumed, as in Figure 1, the light intensity can be multiplied by the reflectance and transmittance at any given wavelength to give the fate of the radiant energy. In any particular case, however, information is needed concerning the specific ways in which the features of the reflectance curves depend on leaf composition and orientation. The purpose of this work is to provide such information for use in remote sensing and energy balance studies. MATERIALS AND METHODSThe term "light" will be used in this paper to indicate electromagnetic radiation regardless of whether this radiation is ultraviolet, visible, or infrared.Reflectance

Light penetration through a Drummer silty clay loam and a Broomfield sand was measured spectrophotometrically and biologically. The spectrophotometric measurements showed that less than 1% of the incident fight penetrated 2.2 millimeters at any wavelength between 350 and 780 nanometers for ped sizes up to 1 milimeter. Biological measurements with lightsensitive lettuce (Lactuca sativa) seeds In soil showed that an exposure to Ught equivalent to about I sunny day induced some germination of seeds which were 2 milmeters below the surface, but did not affect seeds 6 millimeters below the surface.The known light sensitivity of many seeds (11) has led to the hypothesis that light may be important in inducing the germination of weed seeds in cultivated fields. Dormant seeds, brought close to the surface by cultivation, could receive light, either directly during cultivation or later through the soil, and could therefore germinate. Experiments have tended to confirm this hypothesis (3,7,10,12). Still, there has been no attempt to measure light penetration into soil or to find out how near a seed must be to the surface to be affected. Nor has there been an attempt to consider the role of temperature in such germination, even though light sensitivity of seeds is known to be highly dependent on temperature (1-3, 6, 10, 11). We therefore measured light penetration through soil spectrophotometrically and also biologically, controlling temperature, and using light-sensitive lettuce seeds buried in the soil as our bioindicator. At temperatures below about 20 C, Grand Rapids lettuce (Lactuca sativa L.) seeds do not require light treatment for germination, and will germinate when allowed to imbibe water. At higher imbibition temperatures, some of the seeds will germinate without light, but others need a light treatment for germination. At field sand. The Drummer soil, when dry, is dark gray with a Munsell color notation of 5YR 4/1, and when moist is black (5YR 2.5/1). The two Drummer ped sizes, 0.42 to 0.50 mm and 0.84 to I mm, were separated by sifting. This soil maintains the integrity of its aggregates well through wetting and drying cycles. Dry Broomfield sand is yellowish brown (IOYR 5/4), and the moist sand is dark yellowish brown (IOYR 3/4). The sand grain size range was 0.3 to 0.5 mm.To determine light transmittance spectrophotometrically, we used a Beckman DK-2A spectroreflectometer, with soil samples in acrylic plastic cuvettes.To determine light transmittance through soils biologically, we placed light-sensitive lettuce seeds at various depths in the soils, then exposed the soils to light. Subsequent germination of the seeds indicated light penetration into the soil.In the preliminary experiment to determine the exact characteristics of our seeds and to establish the optimum temperature, time, and light regimes, we cultured seeds in Petri dishes, which were kept dark at the desired inbibition and incubation temperature for the desired time, then exposed to light and again placed in the dark at a desired tem...

Common cocklebur (Xanthium pensylvanicumWallr. ♯ XANPE), velvetleaf (Abutilon theophrastiMedic. ♯ ABUTH), and jimsonweed (Datura stramoniumL. ♯ DATST) are weeds that grow taller than soybeans [Glycine max(L.) Merr.] and compete for limited resources such as light. These weeds have an average area of influence of 0.5 m2, thus a density of 1 to 2 plants/m2forms a full canopy of weed leaves above the soybeans that intercept 44 to 56% of the sunlight. Shade of 44 to 56% without weeds, placed above soybeans late in the season corresponding to times of weed shading, reduced soybean yield 19 to 26%. Since reported soybean yield reductions at weed densities of 0.7 to 2.5 plants/m2were 18 to 54% for common cocklebur and 12 to 31% for jimsonweed and velvedeaf, it was concluded that most of the interference from jimsonweed and velvetleaf infestations in soybeans could be ascribed to competition for light, but only about one-half the interference was due to competition for light for common cocklebur. Soybean photosynthesis in the field was reduced about twice as much as soybean yield at equivalent levels of shading.

Attraction of the greenhouse whitefly Trialeurodes vaporariorum Westw. to reflected and to transmitted light of various wavelength compositions was measured by trapping and counting adults on greased, colored surfaces. The wavelength composition of the reflecting or transmitting surfaces was spectrophotometrically determined. The whiteflies showed a strongly positive response to surfaces with maximum reflectance or transmittance in the “yellow‐green” region (520–610 nm), and a moderately positive response to ultra‐violet (< 400 nm). Light in the “blue‐violet” region seemed to inhibit the response, and “red” (610 to ca. 700 nm) might also be moderately inhibitory. The response to the most excitatory hues sharply decreases with darker shades or with less saturated tints of the same hues. The implications of these responses in the mechanisms of host selection by the greenhouse whitefly are discussed. Zusammenfassung SPEKTRALSPEZIFISCHE REAKTIONEN BEIM VISUELLEN VERHALTEN DER WEISSEN FLIEGE, TRIALEURODES VAPORARIORUM (HOMOPTERA: ALEYRODIDAE) Die Attraktion der Weißen Fliege zu remittiertem und transmittiertem Licht verschiedener Wellenlängen‐Kombinationen (spektrometrisch bestimmt) wurde gemessen durch Fang und Auszählen von angeflogenen Adulten an geleimten, farbigen Oberflächen. Die Weißen Fliegen zeigten eine starke positive Reaktion zu Oberflächen mit maximaler Remission oder Transmission im Gelb‐Grün‐Bereich (520–610 nm) und eine mäßige positive Reaktion zu UV (< 400 nm). Licht im Blau‐Violett‐Bereich scheint die Reaktion zu hemmen und Rot (610 bis etwa 700 nm) scheint ebenfalls hemmend zu sein. Die Reaktion zu den Farbtönen mit höchster Wirksamkeit nimmt stark ab bei geringerer Intensität (Schwarzverhüllung) oder geringerer Sättigung (Weissverhüllung) der Farbtöne. Die Bedeutung dieser Reaktionen für den Mechanismus der Wirtswahl der Weißen Fliege wird diskutiert.

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The chlorophyll‐deficient, near‐isogenic lines of the soybean [Glycine max (L.) Merr.j genotype ‘Clark’, Clark y9 and Clark y11, have similar photosynthetic CO2‐exchange rates (CER) per unit leaf area as the wild‐type at full sun photosynthetic photon flux densities (PPFD). We hypothesized that, because of increased leaf reflectance and transmittance in such lines, more PPFD may penetrate the canopy, causing an increase in canopy CER. A 2‐yr field study on a Flanagan silt loam (Aquic Argiudoll) was undertaken to compare canopy CERs of the chlorophyll‐deficient isolines and the normal pigmented wild‐type. Canopy CER measurements were taken throughout the majority of the daylight period for several days in both growing seasons. For an 11‐d period in 1986, average CER values for Clark y9 were 14% higher than for Clark and during early pod fill average CER values for Clark y11 were 28% higher than for Clark in 1987. The canopy extinction coefficients for Clark y9 and Clark were not found to differ significantly. Clark and Clark y, were similar in maturity, but Clark y11 was on average 9 d earlier, which could have been a factor contributing to its higher CER. These results indicate that chlorophyll‐deficient isolines can, at times, photosynthetically out‐perform the wild‐type, possibly because of increased PPFD penetration into the canopy.