Solar radiation in the higher-than-visible wavelength segment, referred to as infrared radiation, has thermal effects on plants. In the presence of water vapors, this radiation does not harm plants; rather, it supplies the necessary thermal energy to the plant environment.
The third spectrum, lying between the ultraviolet and infrared, is the visible part of solar radiation and is referred as light. This segment of solar radiation plays an important part in plant growth and development through the processes of chlorophyll synthesis and photosynthesis and through photosensitive regulatory mechanisms such as phototropism and photoperiodic activity. Light of the correct intensity, quality, and duration is essential for normal plant development. Poor light availability is frequently responsible for plant abnormalities and disorders. Virtually all plant parts are directly or indirectly influenced by this part of the spectrum. It affects the production of tillers; the stability, strength, and length of the culms; the yield and total weight of plant structures; and the size of leaves and root development (Rodriguez et al., 1999). The length of day or the duration of the light period determines flowering and has a profound effect on the content of soluble carbohydrates present. The majority of plants flower only when exposed to certain specific photoperiods. It is on the basis of this response that the plants have been classified as short-day plants, long-day plants, and day-neutral plants. When other environmental factors are not limiting it, photosynthesis increases with longer duration of the light period (Salisbury, 1981).
Reflection, Transmission, and Absorption
Reflection and transmission from the leaves have similar spectral distributions as shown in Figures 2.3 and 2.4. The maxima for both are in the green light as well as in the infrared region. The impression of the green color of the plants depends on the high reflectivity, the relatively high intensity of solar radiation, and the greater sensitivity of the human eye for green light. The strong infrared reflection from plants is an important natural device for protection of plant life against damage due to overheating. On average, the plant canopy absorbs about 75 percent of the incident radiation, with about 15 percent reflected and 10 percent transmitted.
Due to their chemical components or physical structures, plants absorb selectively in discrete wavelengths (Figure 2.5). The transparent epidermis allows the incident sunlight to penetrate into the mesophyll, which consists of two layers: (1) the palisade parenchyma of closely spaced cylindrical cells and (2) the spongy parenchyma of irregular cells with abundant interstices filled with air. Both types of mesophyll cells contain chlorophyll, n
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