Light is one of the most essential environmental signals which influence the growth of plants. Phototropism, or plant growth in response to unidirectional light, is an adaptive response to these signals of light (Lariguet et al 2006). The interaction of plants and tropisms is important in determining the growth form of the plant body in the presence of a given light signal (Kiss et al 2003). In order to positively respond to the light signals, plants have evolved multiple photoreceptor systems to perceive these signals over a broad range of wavelength and intensity (Takano et al 2009). These photoreceptors include plant phytochromes, cryptochromes, and phototropins (Takano et al 2009). Phytochrome is a photoreceptor and a pigment that plants use to detect light.  Phytochromes are sensitive to light in the red and far-red region of the visible spectrum, and they are the red and far-red light absorbing receptors (Chory 1993). Phytochrome pigments play a role in many plant responses, such as photoperiodism, plant development, changes in plastids, the production of anthocyanin pigments, and the detection of shading by other plants (Kneissl et al 2008). While phytochromes mainly perceive red and far-red light, cryptochromes and phototropins recognize ultraviolet and blue light (Takano et al 2009).

            Since phytochromes perceive red and far-red light, they then exist in two forms: Pr (red-absorbing form, also known as phytochrome A) and Pfr (far-red absorbing form, also known as phytochrome B) (Chory 1993). When either form absorbs light, it is converted to the other form. Stated more clearly, Pr becomes Pfr when it absorbs red light and Pfr becomes Pr when it absorbs far-red light.  Under normal light, there is an equal amount of Pr and Pfr (Kneissl et al 2008). Red light perception is usually attributed only to the light-stable phytochromes, which are phytochromes B-E; phytochromes C, D, and E have been identified as weak red light sensors (Kneissl et al 2008).           

            In a study conducted on Arabidopsis plant roots by the Department of Botany at Miami University, it was found that plant roots exposed to blue or white light exhibited negative phototropism mediated by the phototropin family of photoreceptors, while plant roots exposed to red light exhibited positive phototropism (Kiss et al 2003). In a 2009 study conducted at the National Institute of Agrobiological Sciences in Japan, a phytochrome-dependent phototropism was observed in cucumber (Cucumis sativus) shoots. When exposed to far-red light, a positive response was produced, partly mediated by phytochrome B (Takano et al 2009). When exposed to red light, a positive phototropic response was also observed to be induced, mediated by phytochrome A and phytochrome B (Takano et al 2009). The phototrophic actions observed to be mediated by phytochromes in the abovementioned studies serve as the evidence behind the baseline hypothesis for the current study.