Basic Guide to Light Use When Growing Plants


Science and Education Series

Basic Guide to Light Use When Growing Plants



Written by Kevin Zhao (Greenhound Volunteer, Montreal, Canada)

Editors: Dr. Brian Nguyen, PhD (Montreal, Canada)



It is well known today that having plants in a living space or spending time in a garden brings a wide range of health benefits by reducing stress (Lee et al., 2015), favoring positive emotions (Nieuwenhuis et al., 2014) and even improving productivity (Park et al., 2017)! In this article, we shall discuss how to illuminate their world and caring for them in return. We will briefly look at the science behind light, then examine the best light intensities and colours for plant


growth and finally look at some tips on how to use this information when raising our own.



What is Light?

Light is made of a discrete particle called a “photon” that also behaves like a wave. Photons carry energy which is reflected (no pun intended) by their colour and measured by their wavelengths.



  1. FIGURE 1 [Comparison of wavelength, frequency and energy for the electromagnetic spectrum.]. (2013, March). Retrieved February 05, 2021, from https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html


The more energy a photon carries, the smaller its wavelength and in terms of visible light, the “bluer” the colour. The opposite also applies for red light. The average plant uses light from the visible spectrum (400nm to 700nm) (McCree, K. J., 1971). Wait a second, isn’t this just a recap of high school physics you say? Absolutely correct! And now we will see how we can use this knowledge to make sure that the cathedral windows (Calathea makoyana) in your living room can be adequately “enlightened”.


Light Intensity

Light intensity refers to the number of photons a plant receives. The chloroplasts, the organelles in plant cells where photosynthesis takes place, absorb photons with their antenna pigments (molecules that capture photons). However, each pigment molecule can only absorb a single photon at a time, regardless of how much energy the photon carries.



FIGURE 2 RAINFOREST CANOPY LAYERS [RAINFOREST CANOPY LAYERS]. (n.d.). Retrieved February 05, 2021, from https://animalcorner.org/rainforests/canopy-layers/








Most house plants come from the understory layer of tropical rainforests and thus evolved to be shade-tolerant. That means they do not fare well in direct sunlight. So how much light does the average houseplant need for optimal growth? The light intensity (measured using PPFD* in this case) range of the typical tropical forest understory varies from 0.15mol*m-2d-1 to 1.00mol*m-2d-1 (Chazdon R.L., Fetcher N., 1984). There are very popular house plants that are notable exceptions like the aloe vera and the jade plant which require much higher light intensities so make sure to perform your own little research about the different plants you plan to grow.



Plants can tolerate a range of different light intensities and fluctuations of said intensities. For example, we know that Arabidopsis thaliana, basically the lab rat of plants and that shares many similarities with our crop plants, can grow in any combinations of environments with constant and fluctuating light intensities, and high and low light intensities. However, it was found to have grown much more in an environment with a relatively high and constant light intensity and all the while staying within its tolerance range (Vialet-Chabrand et al., 2017).


What happens when we get too zealous decide to give our plant more photons than it can handle? While the average house plants can adapt to varying levels of light, exposing them to constant direct sunlight, even indoors, will still likely be too much (remember they are from the understory layers of a rainforest). A bit like when humans exposed to too much light get sunburns, plants can receive too much light and suffer from photoinhibition (slower photosynthesis and thus slower growth) and photobleaching (where the pigment molecules of a plant are damaged by the indirect effects of light and the leaves turn white) (Lingvay et al., 2020).

FIGURE 3 Pugliesi, D. (2012, August 2). [Absorbance spectra of free chlorophyll a (blue) and b (red) in a solvent. The spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions]. Retrieved from https://upload.wikimedia.org/wikipedia/commons/2/23/Chlorophyll_ab_spectra-en.svg






Light Colour

Chlorophyll a and chlorophyll b are the main antenna pigments of plant cells. While plants use most of the spectrum of visible light (400nm to 700nm), their main antenna pigments absorb mostly blue light of a wavelength of approximately 450nm and red light at around 650nm as can be seen from FIGURE 3. This has led to the development of numerous types of monochromatic red and blue grow lights that are commercially available for plant lovers everywhere.


Not everything is as it seems, however. In 2017, a team of researchers from the University of Toronto found that despite the well known absorbance spectrum of chlorophyll, the productivity of the cyanobacterium (an organism with the same chlorophyll a as in plants) Synechococcus elongatus was maximized when it was exposed to green light of 534nm: “Here we demonstrated that contrary to conventional wisdom, highly absorbed red light is not necessarily the wavelength of choice for monochromatic cultivation, particularly for high density cultures typical of industrial operations. Instead, depending on culture density, depth and irradiance, weakly absorbed green light may provide the best option for achieving high productivity with increased efficiency because it is able to dilute the light over a larger portion of the reactor” (Ooms et al., 2017). According to the authors, even though the absorbance of green light is low, a photon will still be absorbed if given enough chances (i.e., by passing through enough leaves). There you have it folks, the colour of the light doesn’t matter THAT much seeing that green light (the wavelength least absorbed by plants) can compete with red light in productivity. It is interesting to note however from this study that blue light may damage the plant due to its photons having an excess of energy.


Tips

Now that we have examined all the generalities of how “more photons and more energy is not always better”, we can now look at ways to apply this newfound knowledge to improve the ways we raise our plants. Firstly, and most importantly, we should measure our light intensity! Remember the “PPFD” mentioned earlier? Well “PPFD” is one way of measuring it. It stands for “photosynthetic photon flux density” and measures the number of photons hitting a surface area over a period of time. When we measure our light in PPFD or any other units (like “lux” or “PPF” or “lumen”), we won’t need to know the intricacies of their definitions, only the number and the unit behind it will matter. By knowing one of these measurement units, we can find/calculate the others if we also know the light source (e.g. monochromatic LED 450nm blue, sunlight, etc.) and the use an online converter/calculator, like:

https://www.waveformlighting.com/horticulture/convert-lux-to-ppfd-online-calculator.


You for some reason don’t like PPFD? Well no worries, cheap or even free Lux meters from the store or on the app store of our smartphones can be used to measure the energy of the light shining onto an area in lux!

FIGURE 4 [The primary difference between the three bulbs aside from the internal engineering is really just the angle of the beam]. (n.d.). Retrieved February 06, 2021, from https://www.viribright.com/comparing-par-br-mr-light-bulbs/





There are numerous ways to adjust light intensity. Supplementing natural light with grow lights can be helpful, especially if you live in a northern country and would like your plant to thrive in the winter (Mortensen & Gislerød, 1990). There is typically no need to get expensive “grow” lights, regular LED bulbs will do the trick. Some of our members here at Greenhound prefer the “PAR” format floodlight.


As for measuring light colour, our naked eyes should be more than enough to approximate the wavelength of the light our plants are receiving.





Conclusion

We now hopefully have a better understanding of the intricate relationship between light and plant growth. We have done this through our discussions about the nature of light, on the importance of its intensity (whether it’s 0.15mol*m-2d-1, 1.00mol*m-2d-1, or higher) and on the counter-intuitiveness of its green wavelengths. In conclusion, now that we are equipped with the tips on light measurement, we can, with much more confidence, start “brightening” the day of our little green friends!



Lighting Products That We Currently Recommend


Note: If you are purchasing the products, a click-thru provides 5-10% referral earning to our Greenhound Foundation Mission.



  • Recommended beam angle for light bulbs and coverage: Sylvania Home Lighting 79333 90W Equivalent-Led PAR38 Light Bulb

  • Slightly narrow range for more focused light spread: AmazonBasics 50W Equivalent, Daylight, PAR20 LED Light Bulbs


REFERENCES

  1. Chazdon R.L., Fetcher N. (1984) Light Environments of Tropical Forests. In: Medina E., Mooney H.A., Vázquez-Yánes C. (eds) Physiological ecology of plants of the wet tropics. Tasks for vegetation Science, vol 12. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-7299-5_4

  2. Lee, M.-, Lee, J., Park, B.-J., & Miyazaki, Y. (2015). Interaction with indoor plants may reduce psychological and physiological stress by suppressing autonomic nervous system activity in young adults: a randomized crossover study. Journal of Physiological Anthropology, 34(1), 21. https://doi.org/10.1186/s40101-015-0060-8

  3. Lingvay, M., Akhtar, P., Sebők-Nagy, K., Páli, T., & Lambrev, P. H. (2020). Photobleaching of Chlorophyll in Light-Harvesting Complex II Increases in Lipid Environment. Frontiers in plant science, 11, 849. https://doi.org/10.3389/fpls.2020.00849

  4. McCree, K. J. (1971). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191–216. https://doi.org/10.1016/0002-1571(71)90022-7

  5. Mortensen, L. M., & Gislerød, H. R. (1990). Effects of air humidity and supplementary lighting on foliage plants. Scientia Horticulturae, 44(3–4), 301–308. https://doi.org/10.1016/0304-4238(90)90130-7

  6. Nieuwenhuis, M., Knight, C., Postmes, T., & Haslam, S. A. (2014). The relative benefits of green versus lean office space: Three field experiments. Journal of Experimental Psychology: Applied, 20(3), 199–214. https://doi.org/10.1037/xap0000024

  7. Ooms, M. D., Graham, P. J., Nguyen, B.,