Light is a major environmental factor in controlled-environment agriculture, as crop growth and development is dependent upon photosynthetic and photomorphogenic capacities, which are driven by light. The success of a cultivation venture depends on maximizing the yield/ft3, minimizing cost per pound, increasing crop quality and consistency, and more. All of these goals can be achieved with a proper lighting system.
A thorough understanding of the environmental conditions for cultivation, including lighting, is essential to achieve business and cultivation goals. For instance, if you are selling a premium cannabis variety based on the unique taste and purple pigment content, the specific cultivation practices for special lighting treatments and their costs must be considered when selecting the appropriate lighting system(s).
A variety of lighting fixtures are currently available on the market, however, most of these fixtures are not suitable for ideal crop growth. There are many reasons for this, among them is the industry’s use of misleading metrics. In this article, we will focus on the proper metrics to measure the performance of a horticultural lighting system:
- Photosynthetic Photon Flux (PPF)
- Photosynthetic Photon Flux Density (PPFD)
- Electrical Energy Input (Fixture Input Wattage)
- Photon Efficacy
- Coefficient of Utilization (CU)
- Form Factor
Basics of Photosynthetic Lighting
Light has both wave and particle nature, a fact which helps us understand and describe its quality and quantity. The term photon is used to describe the particle nature of light, an “energy packet.”
Plant responses are dependent on light intensity and spectrum. Crops like cannabis require high light levels focused on the canopy to drive photosynthesis during early to mid-flower stages. Conversely, cannabis clones require significantly lower light intensities when they are in the establishment stage. The lighting system you choose should be determined by your needs.
The amount of light intensity required depends on a plant’s photoperiod, the illuminated period per day for a specific crop. Crop growth and development is dependent on the cumulative amount of photons received over the photoperiod, and it is called the daily light integral (DLI, units: mol/m2/d).
Light spectrum plays a significant role in plant growth and development. Plant photosynthetic pigments, such as chlorophylls and carotenoids, and other photoreceptors such as phytochromes, cryptochromes, and phototropins absorb different wavelengths of light. Receptors such as the ultraviolet-B (UV-B) receptor, UVR8, and phytochrome far-red form absorb light outside of the PAR region and have been shown to regulate important plant responses such as UV-induced secondary metabolite expression and photoperiodic flowering. For example, application of UV-B has shown to increase cannabinoids in cannabis leaves and flowers (Lydon et al., 1987). Because spectrum plays a significant role in modulating plant responses, a grow-lighting company should publish its light spectra for every lighting system. This demonstrates an understanding that there is no single, ideal spectrum that suffices all growing needs.
Both light intensity and spectrum are important for plant growth and development. Plant responses vary significantly due to changes in either variable. For example, if cannabis flowers are illuminated with conventional red/blue LED spectrum at high light intensities, the tips of buds can turn white due to photobleaching. Under broad spectrum LED lights, white tips are not produced due to the plant’s ability to better utilize the light energy, even at higher light intensities.
In order to measure light spectra, a spectroradiometer is used. Spectroradiometers measure the spectral power distribution (SPD), which shows radiant flux at each wavelength. In simple terms, spectroradiometers show the “shape of the light spectrum” .
- Lydon, J., Teramura, A. H. and Coffman, C. B. (1987), UV-B RADIATION EFFECTS ON PHOTOSYNTHESIS, GROWTH and CANNABINOID PRODUCTION OF TWO Cannabis sativa CHEMOTYPES. Photochemistry and Photobiology, 46: 201–206. doi:10.1111/j.1751-1097.1987.tb04757.x
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About the Author
Tharindu Weeraratne, PhD, is a horticulture scientist and a plant physiologist working with Fluence Bioengineering.