Part 1: Which Wavelengths do Chlorophylls Absorb?

christmas cactus light sunshine rainbow wavelength

In order to choose the best light for growing your plants, it’s essential to understand which wavelengths of light are required for normal plant growth.  Plants are experts at capturing light energy and converting it into sugars through the process of photosynthesis.  The first step of photosynthesis is the absorption of light by specialized molecules called pigments that are found in plant cells.  In addition to pigments, plants have a number of other light receptor molecules known as photoreceptors.  We will explore the range and function of key plant pigments and photoreceptors and identify the wavelengths of light they absorb and respond to.  This article, which will cover chlorophyll pigments is the first in a 4-part series.  Click here to read about xanthophylls and carotenes.  Here to read about anthocyanins and betalains.  And here to read about photoreceptors.

Plant cells have chloroplasts that convert light energy into sugar.  Each chloroplast has many light harvesting complexes (LHC) that absorb this light energy.  LHC have two main parts: the reaction center and the antenna.  The reaction center is a single chlorophyll A molecule (Figure 1).  The antenna is a mix of many pigments such as chlorophylls, xanthophylls, and carotenes (Figure 1).  A packet of light energy (photon) is captured by the pigments in the antenna and transferred to the reaction center where it is converted into two electrons.  These electrons are essential for photosynthesis.  Most pigments in the LHC are chlorophylls (about 65% of them)!  There are also xanthophylls (about 29%) and carotenes (about 6%)1.

Words of caution: a complex network of factors control plant growth and development.  This article focuses on just one of these factors: light spectrum.  When deciding which wavelengths of light will be best for your plants, consider how all factors (light intensity, temperature, soil, etc.) interact together.  It’s also important to remember that most of what we know about pigments and photoreceptors is derived from studies with the model plant Arabidopsis (the plant equivalent of the lab mouse) and much remains to be learned about other species.  Different plant species have variations in the chemical composition of their pigments and photoreceptors.  For this reason, pigments and photoreceptors from different species can have slightly different absorption peaks than the values listed here.

light harvesting complex with a beam of light energy
Figure 1: Diagram of a light harvesting complex (LHC) within the chloroplast of a plant cell. The LHC is made up of the reaction centre (a single chlA molecule) and the antenna (a mix of chlorophylls, xanthophylls, and carotenes).

Light Wavelengths for: Chlorophylls

About 10 different kinds of chlorophylls exist in plants and each has a unique absorption spectrum1.  Chlorophyll A and B (chlA and B) are pigments that vary in colour from yellow-green to green to blue-green.  There is usually more chlA than chlB because chlA is the only pigment that makes up the reaction center.  The ratio of chlA to chlB (chlA:B) ranges from about 1 to 6, depending on the species and environmental conditions.  For example, tomato has a chlA:B of 2.14, lettuce is about 3.30, and chrysanthemum is about 1.202–4.  Under shade or stress conditions, the abundance of chlB can increase to supply more light to chlA.  Chlorophyll is vital for photosynthesis, so it is important to provide your plants with the wavelengths of light that both chlorophyll A and B can absorb.

So which wavelengths of light do chlA and chlB absorb? Chlorophyll A has the highest absorption at 430 nm and 660 nm while chlorophyll B has the highest absorption at 450 nm and 640 nm (Figure 2).  These wavelengths correspond to the blue and red parts of the spectrum, respectively.  When plants are given a combination of both blue and red light, they grow better than if they were given just red or blue light5.  The red-blue light combination increases chlorophyll content, photosynthetic rate, leaf area and number, and total plant biomass (dry mass) for many different species4–8.

In addition to absorption in the red and blue wavelengths, chlA and chlB absorb small amounts of other light colors, such as ultraviolet, green, and yellow (Figure 2).  The addition of small amounts of green and yellow light to red-blue light enhances plant growth compared to just red-blue light7,9.  To optimize plant yields, it is important to grow your plants using a light that satisfies as many of the chlA and chlB wavelength requirements.  This means that you should choose a light that has high amounts of both blue and red light.  The spectrum from URSA Lighting’s most popular grow light, the Optilux, has high levels of blue and red light.

Figure 2: Light absorption for various plant pigments and photoreceptors. Chlorophyll A is the most abundant pigment and chlorophyll B is the second most abundant. Xanthophylls (lutein and vioxanthan) and carotenes (beta-carotene) are the next most abundant pigments. Anthocyanins (malvidin, pelargonidin, and chrysanthemin) and photoreceptors (phytochrome) are also essential for how a plant senses its environment.


Citations:

  1. Gates, D. M., Keegan, H. J., Schleter, J. C. & Weidner, V. R. Spectral Properties of Plants. Appl. Opt. 4, 11 (1965). 
  2. Bartley, G. E. Plant Carotenoids: Pigments for Photoprotection, Visual Attraction, and Human Health. Plant Cell Online 7, 1027–1038 (1995). 
  3. Kurilčik, A. et al. In vitro culture of Chrysanthemum plantlets using light-emitting diodes. Cent. Eur. J. Biol. 3, 161–167 (2008).
  4. Johkan, M., Shoji, K., Goto, F., Hashida, S. nosuke & Yoshihara, T. Blue light-emitting diode light irradiation of seedlings improves seedling quality and growth after transplanting in red leaf lettuce. HortScience 45, 1809–1814 (2010).
  5. Li, Q. & Kubota, C. Effects of supplemental light quality on growth and phytochemicals of baby leaf lettuce. Agric. Food Sci. 67, 59–64 (2009).
  6. Hernandez, R., Eguchi, T. & Kubota, C. Growth and morphology of vegetable seedlings under different blue and red photon flux ratios using light-emitting diodes as sole-source lighting. Proc. VIII Int. Symp. Light Hortic. 8, (2016).
  7. Darko, E., Heydarizadeh, P., Schoefs, B. & Sabzalian, M. R. Photosynthesis under artificial light: the shift in primary and secondary metabolism. Philos. Trans. R. Soc. B Biol. Sci. 369, 20130243–20130243 (2014). 
  8. Ouzounis, T. et al. Blue and red LED lighting effects on plant biomass, stomatal conductance, and metabolite content in nine tomato genotypes. Proc. VIII Int. Symp. Light Hortic. 8, (2016). 
  9. Wang, Y. & Folta, K. M. Contributions of green light to plant growth and development. Am. J. Bot. 100, 70–78 (2013). 

Dr. Vanessa Nielsen completed her Ph.D. at the University of Toronto, Canada in plant biology and physiology.  She uses this background and fascination with novel lighting technologies to research the impact of light on plant yield at URSA Lighting.  Her background and extensive experience in a plant biotechnology lab offers a unique perspective on lighting for the cannabis industry.  Vanessa is always happy to share the best industry practice in cannabis growth and the latest discoveries of how to optimize lighting conditions for your plants.

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