Light quality, also called spectral composition and spectral energy distribution (SED),
refers to the composition of light as to wavelengths that are
effective in photosynthesis and other plant growth processes.
The wavelengths with primary importance in photobiology are the ultraviolet (UV), visible light, and infrared (IR) (Hopkins 1999). According to Devlin (1975), the wavelengths between 300 nm to 900 nm are capable of affecting plant growth. However, it is not light quality alone that affects plant growth processes. Other properties of light including light intensity and light duration, as well as other climatic factors, are also involved.
Sunlight consists of about 4% ultraviolet radiation, 52% infrared radiation, and 44% visible light (Moore et al. 2003). Light can be reflected, transmitted, or absorbed. Only light that is absorbed can have an effect, and the visible light that is reflected is the one that is perceived by the naked eye. But most of the sunlight that reaches the upper atmosphere is filtered or reflected back to outer space so that only a small portion reaches the earth.
The light quality reaching the plant and the absorbing organs varies according to many factors. These include the time of the day, season, geographic location, atmospheric gases and moisture, clouds, smoke, dust, and other pollutants in the air, topography, presence of barriers including other plants, plant architecture, and location of absorbing plant organs within the canopy.
The visible wavelengths of light lie from about 390 nm to 760 nm which is just a small section of the entire electromagnetic spectrum of solar radiation. Visible light corresponds roughly to the photosynthetically active radiation (PAR) from about 400 nm to 700 nm. These wavelengths contain the right amounts of energy for the biochemical processes and their relative proportion in the available light is of prime importance in determining light quality. Applying the formula as illustrated, the energy contents of PAR range from about 171 to 299 kJ per mole of photon.
Arranged from shortest to longest wavelengths, visible light corresponds to the colors violet, blue, green, yellow, orange and red (VBGYOR). These colors are used as standards in describing rainbow and light quality. The extreme part of the red region in excess of 700 nm is often referred to as infrared or far-red light. White light is that in which all the wavelengths of visible light are present while dark has no visible wavelength.
But the ultraviolet radiation is packed with excessive amounts of energy. It can break bonds and destroy organic molecules. It is absorbed by atmospheric O2 and O3 or ozone, and glass (Hopkins 1999).
In contrast, the infrared radiation contains minimal energy and only participates indirectly in photosynthesis and other plant reactions. Nevertheless, it is responsible for the warm temperature. It makes the Earth comfortable and livable for man and most other organisms as well as plants.
Infrared light is absorbed by cells, but
the energy that it contains is too low to excite electrons. Most of
this energy is immediately converted to heat. Significant amounts of
infrared radiation are absorbed by water vapor, CO2, and other gases in
the atmosphere but it can pass through glass (Hopkins 1999; Moore et al. 2003; Whiting et al. 2010).
Normal daylight is enriched with blue wavelength, the reason for the blue skies. At twilight, that part of the day between sunrise or sunset and the time when the true position of the sun may be 10 degrees or less below the horizon, the light is enriched with the longer red and far-red wavelengths. Hopkins (1999) explains that at twilight, the distance from the sun to an observer on earth is several times longer than when the sun is directly overhead. As a result, much of the violet and blue light is scattered out of line of sight, making the longer orange and red visible to the observer.
In addition, only a minimal amount of blue and red light reach the plants that grow under a canopy because it is absorbed by the chlorophyll-containing leaves at the higher level.
- The violet-blue and red wavelengths are most effective in photosynthesis. The absorbing pigment is the green-colored chlorophyll. Green light is least effective; it is mostly transmitted or reflected by the plant. It is the reflection of the green wavelength from most leaves that gives the visual sensation of green color.
- The blue, red, and far-red are active in photomorphogenesis, the regulation of plant development by light. The pigments involved in light absorption are the phytochrome, the carotenoids, and flavins. The pigments that are responsible for phototropism, the directional response of plants to unilateral light, absorb light in the violet, blue, and green regions (Poincelot 1980).
- Vegetation absorbs red but transmits far-red light so that light under a canopy or reflected from nearby stems has more of the far-red light or has a low red to far-red (red/far-red) light ratio. This signals a plant of the presence of neighboring plants. It induces stem elongation, suppressed branching, and early flowering of plants to outgrow competitors and to complete their life cycle before they are deprived of sufficient light (Arnold Arboretum 2010). These responses are characteristic of the shade-avoidance syndrome (Cerdan and Chory 2003).
-Both red and far-red can be converted to the other, and the effect depends on which is last absorbed. This phenomenon is called photoreversibility in which two forms of phytochrome, a red-absorbing form called Pr and a far-red absorbing form called Pfr, are involved. Pr absorbs maximally at 660 nm while Pfr absorbs at about 730 nm. The absorption of red light induces plant processes, including seed germination in light-sensitive species like lettuce, while far-red is inhibitory. However, Hopkins (1999) explains that phytochrome-mediated effects and the occurence of photoreversibility vary according to energy requirement.
Other plant growth processes that are mediated by the phytochrome pigment are photoperiodic floral induction, nyctinastic leaf movements, phototropic sensitivity, leaf and stem elongation, synthesis of chlorophyll, carotenoid, and anthocyanin, enzyme activation, protein synthesis, root development, rhizome and bulb formation, formation of leaf primordia, leaf abscission, and auxin catabolism (Poincelot 1980; Hopkins 1999).
Light quality is also important in indoor gardening. The discovery about the role of blue and red light in photosynthesis has been widely applied in artificial lighting.