Knowledge of the relationship between light and plants is common. Perhaps, less known, is the complexity of the biological systems involved. Light is a form of electromagnetic radiation (EMR). Its impact on plants goes well beyond the harnessing of energy. Plant cycles, flowering, dormancy, and many other morphogenetic effects are mediated by light. The strength of light, length of exposure, and dominant frequency, are all factors that influence plant response. Below are some introductory ideas that provide a glimpse into the nature of plant / light interaction.
The electromagnetic spectrum includes various types of radiation, categorized by wavelength and frequency.
Visible light is a small part of the spectrum, including colors from violet to red.
In 1818, Pelletier and Caventou first used the term Chlorophyll (Chl) to describe the green pigment that gives most plants their colour. The Chl porphyrin is one of the most important foundational components of life on earth. It forms the reaction centre of both photosystem I (PSI) and photosystem II (PSII) in higher plants, and is essential in creating chemical energy from light energy. An estimated 109 to 1012 tons of Chl is produced annually on earth (Grimm et al., 2006). Plants need Chl to live, and they also need to regulate levels of Chl as it is unstable and can cause photooxidative stress and possible death if excess energy absorbed by it cannot be safely dispersed (Ksas et al., 2015). A review by Reinbothe & Reinbothe (1996) offers a refined explanation of the complexities of how angiosperms are understood to produce Chl and illustrates the involvement of at least 13 separately identifiable stages in its biosynthesis; the control of Chl production depends on many internal and external factors some of which will be the subject of this introductory review.
Chl biosynthesis is a light dependent process in angiosperms, but not in most gymnosperms (Von Wettstein, 1958; Walles & Hudák, 1975). Apel (1979) discovered two separate light reactions were responsible for massively increasing the levels of Chl in the thylakloid membranes during the morphogenetic process of de-etiolation: both processes appeared to be controlled by a photoreceptor named phytochrome (Phy) and more investigations were recommended.
While Apel (1979) found that exposure to full light increased Chl production, Aarti et al. (2007) found that high levels of light completely inhibited the biosynthesis of Chl in cotyledons of Cucumis sativus L. Skribanek et al. (2012) found that different levels of Chl greening occurred between stems and leaves of both gymnosperms and angiosperms, and that Chl production rates were also dependent on light intensity, and temperature in some species. More recent developments suggest a new light-signalling component has been isolated in Arabidopsis, HTL, which affects photomorphogenic responses including contributing to the control of levels of Chl production (Sun & Ni, 2011).
The light sensing capability of plants is complex and the length of exposure, the wavelength of light and the light intensity are all factors that have effects the control of Chl production.
Gehring et al. (1977) suggested that circadian rhythms were involved in Chl biosynthesis and recommended further research. Later studies have confirmed that circadian rhythms have an effect on the control of Chl production: Argyroudi-Akoyunoglou & Prombona (1996) found that the rhythms were light independent, and suspected germination was the initiating trigger. A later study by the same authors (Prombona & Argyroudi-Akoyunoglou, 2004) confirmed that imbibition was the initiating factor for the circadian rhythm, but also noted that the time of day when the germination took place had a significant effect on the strength of circadian oscillations, which also had a measurable effect on resulting Chl levels.
The production of Chl is a key process in the de-etiolation of seedlings. Phy is a pigment-protein complex which acts as a photoreceptor and has two key isomers, Pr and Pfra — a photoactivated version of Pr caused by exposure to red light (650-680nm) (Taiz & Zeiger, 1991; Lack & Evans, 2002). Pfr absorbs far red light (710-740nm) which returns it to the Pr state. The ratio of Pr:Pfr controls photomorphogenic responses including the production of Chl (Lack & Evans, 2002; Taiz & Zeiger, 1991).
Aarti, D., Tanaka, R., Ito, H. & Tanaka, A., 2007. High Light Inhibits Chlorophyll Biosynthesis at the Level of 5-Aminolevulinate Synthesis During De-etiolation in Cucumber (Cucumis sativus) Cotyledons. Photochemistry and Photobiology, 83(1), pp. 171-176.
Apel, K., 1979. Phytochrome-Induced Appearance of mRNA Activity for the Apoprotein of the Light-Harvesting Chlorophyll a/b Protein of Barley (Hordeum vulgare). European Journal of Biochemistry, 97(1), pp. 183-188.
Argyroudi-Akoyunoglou, J. H. & Prombona, A., 1996. Light-independent endogenous circadian rhythm in the capacity for chlorophyll formation. Journal of Photochemistry and Photobiology B:Biology, 36(3), pp. 271-277.
Gehring, H., Kasemir, H. & Mohr, H., 1977. The Capacity of Chlorophyll-a Biosynthesis in the Mustard Seedling Cotyledons as Modulated by Phytochrome and Circadian Rhythmicity. Planta, 133(3), pp. 295-302.
Grimm, B., Porra, R. J., Rudiger, W. & Scheer, H., 2006. Chlorophylls and Bacteriochlorophylls. 1st ed. Arizona: Springer.
Ksas, B., Becuwe, N., Chevalier, A. & Havaux, M., 2015. Plant tolerance to excess light energy and photooxidative damage relies on plastoquinone biosynthesis. [Online]
Available at: http://dx.doi.org/10.1038/srep10919
[Accessed 12 11 2016].
Lack, A. J. & Evans, D. E., 2002. Plant Biology. 2nd ed. Oxford: Bios Scientific Publishers Ltd.
Prombona, A. & Argyroudi-Akoyunoglou, J., 2004. Diverse signals synchronise the circadian clock controlling the oscillations in chlorophyll content of etiolated Phaseolus vulgaris leaves. Journal of Photochemistry and Photobiology B: Biology, 167(1), pp. 117-127.
Reinbothe, S. & Reinbothe, C., 1996. Regulation of Chlorophyll Biosynthesis in Angiosperms. Plant Physiology, 111(1), pp. 1-7.
Skribanek, A. et al., 2012. The effect of abiotic stressors (light and temperature) on chlorophyll biosynthesis. Sopron, University of West Hungary.
Sun, X.-D. & Ni, M., 2011. Hyposensitive to Light, an Alpha/Beta Fold Protein, Acts Downstream of Elongated Hypocotyl 5 to Regulate Seedling De-Etiolation. Molecular Plant, 4(1), pp. 116-126.
Taiz, L. & Zeiger, E., 1991. Plant Physiology. 1st ed. New York: The Benjamin/Cummings Publishing Company Inc.
Von Wettstein, D., 1958. The photochemical apparatus, its structure and function.. Brookhaven Symposia in Biology, 11(195.8), pp. 138-159.
Walles, B. & Hudák, J., 1975. A comparative study of chloroplast morphogenesis in seedlings of some conifers (Larix decidua, Pinus sylvestris and Picea abies. Studia forestalia Suecica, 127(1), pp. 2-22.