What is light and what it is composed of?
Light is the sole source of energy for plant growth and development. Terrestrial sunlight has been considered to consist of shortwave ultraviolet light (UV-B, 290-320 nm), long-wave ultra-violet light (UV-A, 320-400 nm), visible light (400-700 nm) and infra-red light (700-2500 nm). Plants have specialized pigment systems that can capture radiant energy in different regions of the electromagnetic spectrum. For example, photosynthetically active radiation (400-700 nm), captured by chlorophyll pigments, provides the energy for photosynthesis, the process by which plants combine carbon dioxide and water to produce oxygen and carbohydrates. Carbon assimilated during photosynthesis provides the energy to sustain life on earth.
Light also acts as a signal of environmental conditions surrounding the plants. There are photoreceptors that function as signal transducers to provide information that controls physiological and morphological responses. Through these pigments, plants have the ability to perceive subtle changes in light composition for initiation of physiological and morphological changes. This ability of light to control plant morphology is independent of photosynthesis and is known as photomorphogenesis. In photomorphogenesis, photons in specific regions of the spectrum are perceived by the photoreceptors present in smaller quantities. Known photomorphogenic receptors include phytochrome (the red and far-red light sensor that has absorption peaks in red and far-red regions of the spectrum, respectively) and "cryptochrome" (the hypothetical UV-B and blue light sensor). Phytochrome is the most intensively studied sensory pigment that controls photomorphogenesis. Phytochrome is capable of detecting wavelengths from 300 to 800 nm with maximum sensitivity in red (R, 600 to 700 nm with peak absorption at 660 nm) and far-red (FR, 700 to 800 nm with peak absorption at 730 nm) wavelengths of the spectrum.
The relationship between light and plant growth can be demonstrated by exposing leaves to various colors of light. Light supplies the power to carry on photosynthesis, the food-making process in leaves. But the spectrum of light most utilized by a leaf is limited to three distinct colors, red, blue and yellow. For example, leaves appear green because green is the color most leaves reflect rather than absorb and use. Greenhouse light quality manipulation can be achieved either with supplemental electric lighting systems with relatively high red and low far-red light or by spectral filters that can alter red and far-red light balance of sunlight.;