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. This pigment system consists of two interconvertible forms: the Pr form absorbs red light and upon absorption is transformed into the Pfr form which absorbs far-red light and is transformed into the Pr form. Of the two forms, the Pfr form is assumed to be the active form that controls signal transduction and plant response.
Photon ratios between the red and far-red region of the spectrum (R:FR ratio) and in vitro estimates of phytochrome photoequilibrium (f) [amount of phytochrome in the Pfr form relative to total phytochrome (Pfr:Ptot at photoequilibrium)] have been commonly used to quantitatively describe the phytochrome-mediated responses such as stem elongation. In general, f depends largely on the absorption of red and far-red wavelengths by the plant and therefore, f decreases with decreasing R:FR ratio. A hyperbolic relationship exists between R:FR ratio and f indicating that a small change in R:FR ratio can result in a large change in f in the natural environment. Stem elongation rate and height of a range of herbaceous plants have been shown to be inversely proportional to the f (i.e. higher the f shorter the plant). Therefore, by manipulating the red and far-red light in the greenhouse to establish a high f, height of greenhouse crops can be controlled with minimum chemical applications.
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. Incandescent lamps, which are low in R:FR ratio, frequently lead to stem elongation while fluorescent sources, which are high in R:FR ratio, produce short and compact plants. Radiation filters, both liquid and rigid, for improving greenhouse crop productivity and reducing greenhouse temperature gained attention in the 1970s and considerable progress has been made since then.