The quality of short-wave radiation within plant canopies

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In this paper, a mathematical attempt is made to predict the effects of leaf area index, leaf angle, and leaf spectral properties on changes in the relative composition of short-wave radiant fluxes as they penetrate plant canopies. Results of this theoretical analysis indicate that a sizeable change in the quality of visible radiation will only occur if the canopy is sufficiently dense to intercept at least 98% of the incident flux one or more times. By contrast, a significant increase in the proportion of infrared radiation is predicted within plant communities, even those of a low effective leaf area index. For natural plant communities, the results would indicate a minimal change in the composition of penetrating radiation at solar noon and a maximal change at sunrise or sunset.The implications of these phenomena to plant morphogenesis and to radiation-measuring techniques are discussed.

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... The intensity in the shade is largely dependent on the fraction of the sky seen from the point of measurement and the intensity of diffuse "sky" radiation exterior to the canopy. Some long wave radiation is transmitted through the leaves, and reflected and radiated by the leaves but this radiation is of practically no photo synthetic value (5,11,15). ...
... The direct solar radiation, constituting about 83% of the total incident radiation (4) passes through the unobstructed area between leaves at the same intensity as the incident radiation at the exterior of the tree. Reflected radiation from leaf surfaces also has little photosynthetic value (5,11,15). ...
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The percentage of solar radiation passing through the foliage canopy of ‘Mcintosh’ hedgerows, A) pruned annually by cutterbar and B) pruned by slotting saw, was measured continuously from pre-bloom until the completion of foliage growth in late July. Assuming the spur leaf canopy (Sp), emerges first and virtually completes growth before the shoot leaf canopy (Sh), emerges, the relative extent of Sp and Sh, and their respective interception of radiation is calculated at approximately 4-day intervals during the growing season. Even though A produced more total foliage and intercepted more total radiation, the spurs of B produced more flowers and more fruit. Spur leaf canopy B intercepted more solar radiation because of less shading by Sh. The maximum ratio Sh/Sp for adequate illumination of Sp is estimated to be about .8. The relative importance of sunlight on Sp and the time of interception of light during the growing season in relation to flower bud initiation and fruit development are discussed.
A general review of geometrical and statistical light models is presented in this paper. In the geometrical approach plant shapes are simulated by various geometrical forms described by characteristic dimensions. Geometrical models may be divided into two classes — those which consider individual shapes and those which consider an arrangement of shapes. In the statistical approach the location of plant elements is parameterized by various distributions. The type of leaf dispersion in space is the most important consideration in statistical models. Four different types of leaf dispersion are considered in this review: regular leaf dispersion, clumped leaf dispersion, random leaf dispersion, and variable leaf dispersion.Hypotheses which underlie the various random and more generalized types of statistical light models are presented. Several models in the literature are discussed in terms of these assumptions.Although, in many cases, required plant data and actual light measurements in the field are grossly inadequate for experimental verification of light models, it appears that the light regime in plant canopies can be adequately described by those models already available. However, for the most part, these models are very complex and a synthesis of these fundamental models into workable expressions that can be used by agronomists, crop ecologists and others concerned with breeding plants for more efficient interception of light is needed.
Spectral distributions of shade light between 400 and 740 mm were measured under corn, sugar maple, oaks, pines, and spruce with a portable recording spectrophotometer. Differences were found between hardwoods and softwoods and between clear cloudy days. An energy maximum at 550 nm, a minimum at 670 to 680 nm, and a very high maximum in the near infrared occurred under all species. Four components of light within a plant canopy can be distinguished: both beam solar radiation and diffused sky radiation are transmitted both directly and indirectly (reflected and scattered). Separate consideration of each of these components leads to great understanding of similarities and differences between light regimes in different stands.
‘Relative frequency’ recorded by point quadrats measures not the actual area of foliage but the area projected in the direction in which the quadrat lies. Accordingly the relative frequency varies both with the slope of the foliage and also—when inclined quadrats are used—with the inclination of the quadrat. A theoretical study reveals that variation in relative frequency resulting from differences in foliage angle is greatest for vertical quadrats, is considerably reduced when (as suggested by Tinney, Aamodt and Ahlgren) quadrats are inclined at 45°, and is least when quadrat inclination is 32.5°. Accordingly the usual, vertical position for point quadrats is the worst possible one, since it results in the most erroneous estimates of percentage contribution (area basis); while with quadrats inclined at 32.5° errors are greatly reduced and are of an order acceptable in general survey work.
Synopsis Synopsis A spectrophotometer with a sensitive range lying between 0.3 and 1.0 μ and its calibration and use are described. The spectrophotometer was used to measure the spectral characteristics of 0.3- to 10-μ radiation penetrating a dense stand of corn 300 cm. high (26,000 plants/acre, 29-inch rows, LA1 = 4.3) planted in north-south oriented rows. Transmission was quite low for the visible range, but was 30 to 40% from 0.75 to 0.9 μ. Efficiency of photosynthesis was 0.42 times the yield obtained from laboratory-grown Chlorella. Please view the pdf by using the Full Text (PDF) link under 'View' to the left. Copyright © . .
Light relations of terrestial plant communities and their measurement Stand structure and light penetration. 11. A theoretical analysis
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Downloaded from by Texas A&M University on 11
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Can. J. Bot. Downloaded from by Texas A&M University on 11/12/14 For personal use only.