Photobiology is the scientific study of the beneficial and harmful interactions of light (technically, non-ionizing radiation) in living organisms.[1] The field includes the study of photophysics, photochemistry, photosynthesis, photomorphogenesis, visual processing, circadian rhythms, photomovement, bioluminescence, and ultraviolet radiation effects.[2]

The division between ionizing radiation and non-ionizing radiation is typically considered to be a photon energy greater than 10 eV,[3] which approximately corresponds to both the first ionization energy of oxygen, and the ionization energy of hydrogen at about 14 eV.[4]

When photons come into contact with molecules, these molecules can absorb the energy in photons and become excited. Then they can react with molecules around them and stimulate "photochemical" and "photophysical" changes of molecular structures.[1]


This area of Photobiology focuses on the physical interactions of light and matter. When molecules absorb photons that matches their energy requirements they promote a valence electron from a ground state to an excited state and they become a lot more reactive. This is an extremely fast process, but very important for different processes.[5]


This area of Photobiology studies the reactivity of a molecule when it absorbs energy that comes from light. It also studies what happens with this energy, it could be given off as heat or fluorescence so the molecule goes back to ground state.

There are 3 basic laws of photochemistry:

1) First Law of Photochemistry: This law explains that in order for photochemistry to happen, light has to be absorbed.

2) Second Law of Photochemistry: This law explains that only one molecule will be activated by each photon that is absorbed.

3) Bunsen-Roscoe Law of Reciprosity: This law explains that the energy in the final products of a photochemical reaction will be directly proportional to the total energy that was initially absorbed by the system.

Plant Photobiology

Plant growth and development is highly dependent on light. Photosynthesis is one of the most important biochemical processes for life on earth and its possible only due to the ability of plants to use energy from photons and convert it into molecules such as NADPH and ATP, to then fix carbon dioxide and make it into sugars that plants can use for their growth and development.[7] But photosynthesis is not the only plant process driven by light, other processes such as photomorphology and plant photoperiod are extremely important for regulation of vegetative and reproductive growth as well as production of plant secondary metabolites.[8]


Photosynthesis is defined as a series of biochemical reactions that phototrophic cells perform to transform light energy to chemical energy and store it in carbon-carbon bonds of carbohydrates.[9] As it is widely known, this process happens inside of the chloroplast of photosynthetic plant cells where light absorbing pigments can be found embedded in the membranes of structures called thylakoids.[9] There are 2 main pigments present in the Photosystems of higher plants: chlorophyll (a or b) and carotenes.[7] These pigments are organized to maximize the light reception and transfer, and they absorb specific wavelengths to broaden the amount of light that can be captured and used for photo-redox reactions.[7]

Photosynthetically Active Radiation (PAR)

Main article: Photosynthetically active radiation

Due to the limited amount of pigments in plant photosynthetic cells, there is a limited range of wavelengths that plants can use to perform photosynthesis. This range is called "Photosynthetically Active Radiation (PAR)". This range is interestingly almost the same as the human visible spectrum and it extends in wavelengths from approximately 400-700 nm.[10] PAR is measured in μmol s−1m−2 and it measures the rate and intensity of radiant light in terms of micro-moles per unit of surface area and time that plants can use for photosynthesis.[11]

Photobiologically Active Radiation (PBAR)

Photobiologically Active Radiation (PBAR) is a range of light energy beyond and including PAR. Photobiological Photon Flux (PBF) is the metric used to measure PBAR.


This process refers to the development of the morphology of plants which is light-mediated and controlled by 5 distinct photoreceptors: UVR8, Cryptochrome, Phototropin, Phytochrome r and Phytochrome fr.[12] Light can control morphogenic processes such as leaf size and shoot elongation.

Different wavelengths of light produce different changes in plants.[13] Red to Far Red light for example, regulates stem growth and straightening of the seedling shoots that are coming out of the ground.[14] Some studies also claim that red and far red light increases the rooting mass of tomatoes[15] as well as the rooting percentage of grape plants.[16] On the other hand, blue and UV light regulate the germination and elongation of the plant as well as other physiological processes such as stomatal control[17] and responses to environmental stress.[18] Finally, green light was thought not to be available to plants due to the lack of pigments that would absorb this light. However, in 2004 it was found that green light can influence stomatal activity, stem elongation of young plants and leaf expansion.[19]

Secondary Plant Metabolites

These compounds are chemicals that plants produce as part of their biochemical processes and help them perform certain functions as well as protect themselves from different environmental factors. In this case, some metabolites such as anthocyanins, flavonoids, and carotenes, can accumulate in plant tissues to protect them from UV radiation and very high light intensity[20]


See also


  1. ^ a b Smith, Kendrick C. (2014). "What Is Photobiology?". Retrieved 2018-08-02.
  2. ^ Smith, Kendric (2013-03-08). The Science of Photobiology. Springer Science & Business Media. ISBN 9781461580614.
  3. ^ Robert F. Cleveland, Jr.; Jerry L. Ulcek (August 1999). "Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields" (PDF) (4th ed.). Washington, D.C.: OET (Office of Engineering and Technology) Federal Communications Commission. Archived (PDF) from the original on 2011-10-20. Retrieved 2018-08-02.
  4. ^ Jim Clark (2000). "Ionisation Energy". Archived from the original on 2011-11-26. Retrieved 2018-08-02.
  5. ^ a b "BASIC PHOTOPHYSICS". Retrieved 2019-11-24.
  6. ^ "BASIC PHOTOCHEMISTRY". Retrieved 2019-11-24.
  7. ^ a b c Eichhorn Bilodeau, Samuel; Wu, Bo-Sen; Rufyikiri, Anne-Sophie; MacPherson, Sarah; Lefsrud, Mark (2019-03-29). "An Update on Plant Photobiology and Implications for Cannabis Production". Frontiers in Plant Science. 10: 296. doi:10.3389/fpls.2019.00296. ISSN 1664-462X. PMC 6455078. PMID 31001288.
  8. ^ Lefsrud, Mark G.; Kopsell, Dean A.; Sams, Carl E. (December 2008). "Irradiance from Distinct Wavelength Light-emitting Diodes Affect Secondary Metabolites in Kale". HortScience. 43 (7): 2243–2244. doi:10.21273/hortsci.43.7.2243. ISSN 0018-5345.
  9. ^ a b Cooper, Geoffrey M.. (2018). The cell : a molecular approach. ISBN 9781605357072. OCLC 1085300153.
  10. ^ McCree, K.J. (January 1971). "The action spectrum, absorptance and quantum yield of photosynthesis in crop plants". Agricultural Meteorology. 9: 191–216. doi:10.1016/0002-1571(71)90022-7. ISSN 0002-1571.
  11. ^ Young, Andrew John (December 1991). "The photoprotective role of carotenoids in higher plants". Physiologia Plantarum. 83 (4): 702–708. doi:10.1034/j.1399-3054.1991.830426.x. ISSN 0031-9317.
  12. ^ Pocock, Tessa (September 2015). "Light-emitting Diodes and the Modulation of Specialty Crops: Light Sensing and Signaling Networks in Plants". HortScience. 50 (9): 1281–1284. doi:10.21273/hortsci.50.9.1281. ISSN 0018-5345.
  13. ^ Scandola PhD, Sabine. "Photobiology: Plant Light Matters". G2V Optics.
  14. ^ McNellis, Timothy W.; Deng, Xing-Wang (November 1995). "Light Control of Seedling Morphogenetic Pattern". The Plant Cell. 7 (11): 1749–1761. doi:10.2307/3870184. ISSN 1040-4651. JSTOR 3870184. PMC 161035. PMID 8535132.
  15. ^ Vu, Ngoc-Thang; Kim, Young-Shik; Kang, Ho-Min; Kim, Il-Seop (February 2014). "Influence of short-term irradiation during pre- and post-grafting period on the graft-take ratio and quality of tomato seedlings". Horticulture, Environment, and Biotechnology. 55 (1): 27–35. doi:10.1007/s13580-014-0115-5. ISSN 2211-3452. S2CID 16250222.
  16. ^ Poudel, Puspa Raj; Kataoka, Ikuo; Mochioka, Ryosuke (2007-11-30). "Effect of red- and blue-light-emitting diodes on growth and morphogenesis of grapes". Plant Cell, Tissue and Organ Culture. 92 (2): 147–153. doi:10.1007/s11240-007-9317-1. ISSN 0167-6857. S2CID 24671493.
  17. ^ Schwartz, A.; Zeiger, E. (May 1984). "Metabolic energy for stomatal opening. Roles of photophosphorylation and oxidative phosphorylation". Planta. 161 (2): 129–136. doi:10.1007/bf00395472. ISSN 0032-0935. PMID 24253600. S2CID 12539218.
  18. ^ Goins, G.D.; Yorio, N.C.; Sanwo, M.M.; Brown, C.S. (1997). "Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting". Journal of Experimental Botany. 48 (7): 1407–1413. doi:10.1093/jxb/48.7.1407. ISSN 0022-0957. PMID 11541074.
  19. ^ Folta, Kevin M. (July 2004). Green Light Stimulates Early Stem Elongation, Antagonizing Light-Mediated Growth Inhibition1. American Society of Plant Biologists. OCLC 678171603.
  20. ^ Demmig-Adams, Barbara. (2014-11-22). Non-Photochemical Quenching and Energy Dissipation in Plants, Algae and Cyanobacteria. ISBN 978-94-017-9032-1. OCLC 1058692723.