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Source: Adapted from Baumgartner, 1973.

near-infrared (near-IR) reflectance of leaves is caused not by chlorophyll but by the internal cell structure. Near the border of visible light, absorption by the plant decreases but then increases again in the infrared. Infrared radiation greater than 3 |.im is completely absorbed by the plants.

It can be summed up that the plant leaf strongly absorbs blue and red wavelengths, less strongly absorbs the green, very weakly absorbs the near infrared, and strongly absorbs in the far-infrared wavelengths. Because the absorption of the near-infrared wavelengths (which contain the bulk of energy) by the leaf is limited, by discarding this energy it prevents the internal temperature from becoming lethal. At the infrared wavelengths, the plant leaf is an efficient absorber, but in these wavelengths the energy at the surface is small, with the result that the plant is a good absorber in the far-infrared. It is an equally a good radiator at these wavelengths.

The quality of radiation affects flowering, germination, and elongation. Red light with a wavelength of 0.66 |j.m is by far the most effective inhibitor of flowering in the case of long-day plants. Red light helps mature apples to turn red. Germination of seeds is inhibited when they are exposed to green, blue, and other short wavelength colors. However, germination is induced when seeds are exposed to the red portion of the spectrum. The red and infrared parts of the spectrum have reversible effects on seed germination. Stem elongation is promoted by exposure to far-red wavelengths, whereas the red part of the spectrum suppresses the elongation (Butler and Roberts, 1966; Takaichi et al., 2000).

The visible part of the spectrum also influences the orientation of shoots, phenomenon known as phototropism (Stowe-Evans, Luesse, and Liscum, 2001; Koller, Ritter, and Heller, 2001; Jin, Zhu, and Zeiger, 2001). When shoots turn toward the light, the phenomenon is known as positive photo-tropism. With increasing intensity of light, positive phototropism turns into negative phototropism. The strongest influence on phototropism is by the blue part of the spectrum (0.5 ^m) and the weakest influence is by red rays. The phototropism action of the visible spectrum increases from the red to the blue part; subsequently, it declines again in the ultraviolet part. However, Ruppel, Hangarter, and Kiss (2001) have demonstrated that, in addition to the previously described blue-light-dependent negative phototropic response in roots, roots of wild-type and mutant (ACG 21) Arabidopsis thaliana display a previously unknown red-light-dependent positive photo-tropic response.

The ultraviolet and gamma part of the spectrum has only a slight effect on the plant. This may be partly because very little of this part of the spectrum reaches the earth's surface. However, it is well known that these rays have biological effects (Skorska, 2000; Predieri and Gatti, 2000). These rays may kill microorganisms, disinfect the soil, and eradicate diseases (Sharp and Polavarapu, 1999). Ultraviolet rays also influence the germination and quality of seeds. These rays lead to many irregularities in the growth and development of plants (Caldwell, 1981). Ultraviolet radiation leads to a strong in hibition of photosynthesis and metabolism (Karsten et al., 1999; Correia et al., 2000).

The solar spectrum can be divided into the following eight broad bands on the basis of the physiological response of plants:

1. Wavelength greater than 1.000 ^m: Most of this radiation absorbed by plants is transformed into heat without interfering with the biochemical processes.

2. Wavelength 1.000 to 0.700 ^m: Elongation effects on plants.

3. Wavelength 0.700 to 0.610 ^m: Very strong absorption by chlorophyll, the strongest photosynthetic activity, and in many cases strong photo-periodic activity.

4. Wavelength 0.610 to 0.510 ^m: Low photosynthetic effectiveness in the green segment and weak formative activity.

5. Wavelength 0.510 to 0.400 ^m: Strong chlorophyll absorption, strong photosynthetic activity, and strong formative effects.

6. Wavelength 0.400 to 0.315 ^m: Produces fluorescence in plants and a strong response by photographic emulsions.

7. Wavelength 0.315 to 0.280 ^m: Significant germicidal action. Practically no solar radiation of wavelengths shorter than 0.29 ^m reaches the earth's surface.

8. Wavelength shorter than 0.280 ^m: Very strong germicidal action. It is injurious to eyesight and when below 0.26 ^m can kill some plants. No such radiation reaches the earth's surface.

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