The previous white paper focused on seasonal and daily changes to light quality (i.e., spectrum) and quantity (i.e., intensity and photoperiod), which significantly impact plant growth and development. While the impact of light quantity may appear more obvious, that of light quality is more abstract due to the nanoscopic nature of light. With light quantity, we can easily discern whether light is more intense and whether the day is longer or shorter. With light quality, we are considering the spectrum thus the decomposition of light into different colors, their wavelengths, and the relative amounts of each wavelength.
Light quality varies both seasonally and daily as weather conditions and the atmospheric path traveled by light impacts the absorption and scattering of different wavelengths, resulting in a discrepancy between pure sunlight and what reaches the plant canopy. Considering spectrum in terms of photon ratios has made it easier to study the impacts of light quality on plant growth and development, with the main ratios of interest being red to blue (R:B) and red to far-red (R:FR). Researchers are also beginning to study blue to green (B:G) and blue to far-red (B:FR), among others.
Understanding the morphological impacts of different photon ratios allows a grower to harness these ratios as a tool to guide plant development. For example, the ratio of R:FR light has been extensively studied due to its impacts on stem elongation, apical dominance, leaf expansion, photosynthetic efficiency, and flowering response in short-day and long-day plants. Controlling the R:FR ratio thus allows control over plant morphology architecture. With Sollum Technologies’ dynamic LED lighting solution, growers can adjust the light spectrum at any point to achieve target ratios that result in desired morphological traits. To better understand the significance of this innovation, this white paper focuses on plant photoreceptors and the R:FR ratio.
Plants receive light as a source of energy for photosynthesis and information for metabolic processes, both of which are mediated by photoreceptors. Photoreceptors are a complex of proteins, and a pigment called a chromophore, the latter which selectively absorbs photons of specific wavelengths. Upon absorption of a photon, the chromophore triggers a change in the protein structure of the photoreceptors which in turn elicits an array of cellular responses in the plant.
Chlorophyll is perhaps the most well-known photosynthetic photoreceptor as its two main forms, a and b, are the most important photosynthetic pigments. Chlorophyll is responsible for the leaves’ green color due to the preferential absorption of red and blue light and the reflection of green light. The importance of chlorophyll is highlighted by the famous McCree Curve, which depicts a maximum photosynthetic efficiency induced by red and blue light. These maxima correlate with the absorption peaks of chlorophyll a (642 nm and 372 nm) and chlorophyll b (626 nm and 392 nm). However, photoreceptors absorb a range of wavelengths and non-chlorophyll photoreceptors contribute to photosynthesis to a lesser degree and/or indirectly. Research has also demonstrated that while far-red light falls outside chlorophyll’s peak absorption range, it enhances the photosynthetic efficiency of red light through the Emerson effect. Red light is particularly efficient at driving photosynthesis because it is absorbed by both chlorophylls and phytochrome. Further, it is widely accepted that stomatal opening is partially regulated by blue-light photoreceptors, with water and carbon dioxide also strongly impacted stomatal functioning
Besides their important role in photosynthesis, photoreceptors strongly impact photomorphogenesis, phototropism, and photoperiodism.
Photomorphogenesis: light-mediated plant development, such as stem elongation, leaf thickening, etc. Strongly impacted by far-red and blue wavelengths.
Phototropism: growth of plants towards a light source. Strongly impacted by the direction of
As signaling molecules, photoreceptors communicate the ambient light conditions to the plant which then responds accordingly. For example, photoreceptors receiving low levels of red light communicate to the plant that it is being shaded. Depending on the species, plants react to low light levels through shade avoidance syndrome (SAS) which includes stem elongation, apical dominance and leaf expansion to grow past the obstruction and access more light.
Photoreceptors are present in all plant tissues: leaves, shoots and even roots. Moreover, the site of photoreceptor does not always correspond to the site of response, which means that the plant tissues intercepting light may be far away from the tissues or organs that react to it[i]. Not including chlorophylls and related pigments, there are five classes of photoreceptors for three categories of light. Phytochromes absorb primarily red (600- 700 nm) and far-red (700-750 nm) while cryptochromes, phototropins and zeitlupes absorb blue and UVA wavelengths. The photoreceptor UVR8 absorbs UVB light.
The importance of phytochromes will be further elaborated upon in subsequent sections. Cryptochromes play a role in plant shade avoidance, flowering induction and possibly temperature-dependent hypocotyl elongation and responses to changes in magnetic fields, as research studies suggest. Phototropins, which also respond to blue light, are responsible for light-mediated stomatal opening, chlorophyll translocation in plant tissues, and phototropism responses. Zeitlupes, the third class of photoreceptors that react to blue light, play an important role in maintaining the circadian rhythm, and can induce time-dependent flowering in some species. UVR8 photoreceptors are unique in that their activity is mediated not by a chromophore but by an aromatic molecule that absorbs UV-B light and triggers shade avoidance responses[ii].
Phytochromes were first discovered in the 1950s but research into their role in plant development is ongoing. Phytochrome preferentially absorbs red and far-red light and exists in two forms: Pr and Pfr. It is the relative concentration of Pr to Pfr that determines which physiological processes are triggered in the plant.
Phytochrome is a photoreversible molecule that switches between its active Pfr form and inactive Pr form based on the ratio of red to far-red light. The inactive form Pr quickly absorbs red light and transforms into the active Pfr form. This process is reversible, with the absorption of far-red light leading to the conversion of Pfr into Pr. The Pfr form also slowly reverts to the Inactive Pr form under dark conditions. Due to this reversibility, the phytochrome mechanism is often referred to as a molecular switch that clicks “on” in the presence of red light and “off” in the presence of far-red light. The on/off mechanism can have various implications in plant development including germination, chloroplast translocation and shade avoidance[iii].
During the day, high levels of red light in sunlight results in the conversion of most Pr into Pfr which signals to the plant that it is receiving significant light. During the nighttime without supplemental lighting, Pfr slowly converts back into Pr. This mechanism is how plants detect whether they are under long-day or short-day conditions, which is critical for the flowering of certain species (Figure 1).
In the case of short-day plants, long nights are necessary to complete the slow conversion of Pfr to Pr. The plant interprets the near-zero concentration of Pfr as a signal that the dark period is long enough, then begin its flowering process. In other words, the low levels of Pfr signals a lack of red light thus darkness. As such, Pfr inhibits flowering in short-day plants. Examples of short-day plants include poinsettia, Christmas cactus and chrysanthemum.
In long-day plants, Pfr encourages flowering as the short nighttime period results in incomplete conversion of Pfr to Pr, with the leftover Pr signalling to the plant that the nights are short enough to begin flowering[iv]. Examples of long-day plants include hibiscus, spinach, and potato.
Galvão, V. C., Fankhauser, C. (2015). Sensing the light environment in plants: photoreceptors and early signaling steps. Current Opinion in Neurobiology, 34, 46–53. https://doi.org/10.1016/j.conb.2015.01.013
Paik, I., Huq, E. (2019). Plant photoreceptors: Multi-functional sensory proteins and their signaling networks. Seminars in Cell & Developmental Biology, 92, 114– 121. https://doi.org/10.1016/j.semcdb.2019.03.007
Sharrock, A. (2008). The phytochrome red/far-red photoreceptor superfamily. Genome Biology, 9(8), 230. https://doi.org/10.1186/gb–2008–9–8–230
Stephanie (2017). 9.4 The Phytochrome System. Retrieved from https://www.youtube.com/watch?v=tf3avaZCCQo
Pierik, , de Wit, M. (2014). Shade avoidance: phytochrome signalling and other aboveground neighbour detection cues. Journal of Experimental Botany, 65(11), 2815–2824. https://doi.org/10.1093/jxb/ert389
Runkle, E. (2016, May). A Closer Look at Far-Red Retrieved December 3, 2021, from https://gpnmag.com/article/a-closer-look-at-far-red-radiation/
Demotes-Mainard, S., Péron, T., Corot, A., Bertheloot, J., Le Gourrierec, J., Pelleschi-Travier, S., … Sakr, S. (2016). Plant responses to red and far-red lights, applications in horticulture. Environmental and Experimental Botany, 121, 4–21. https://doi.org/10.1016/j.envexpbot.2015.05.010
Kalaitzoglou, , van Ieperen, W., Harbinson, J., van der Meer, M., Martinakos, S., Weerheim, K., … Marcelis, 322. F. M. (2019). Effects of Continuous or End-of-Day Far-Red Light on Tomato Plant Growth, Morphology, Light Absorption, and Fruit Production. Frontiers in Plant Science, 10, 322. https://doi.org/10.3389/fpls.2019.00322
Ji, Y., … Heuvelink, E. (2019). Far-red radiation increases dry mass partitioning to fruits but reduces Botrytis cinerea resistance in tomato. Environmental and Experimental Botany ,168. https://doi.org/10.1016/j.envexpbot.2019.103889
Legendre, R., van Iersel, M.W. (2021). Supplemental far-red light stimulates lettuce growth: disentangling morphological and physiological effects. Plants, 10, 166. https://doi.org/10.3390/plants10010166