Lighting in Plant Growth Chambers: Part 1

Why this topic, why now?

Lighting is one of the most critical variables in plant science research and controlled environment agriculture, directly influencing plant physiology, morphology, and productivity. As plant scientists evaluate growth chamber options, understanding the nuances of lighting is essential for designing experiments that yield reliable and reproducible results. This first installment in our two-part series explores the practical considerations and scientific principles behind spectrum choices for plant growth chambers, helping researchers align their choices with both experimental goals and biological needs.

Plant scientists need to have the ability to grow plants in accurately controlled environments, either stable or dynamic. Temperature, relative humidity, irrigation and CO₂ systems have been well integrated into growth chambers which allow flexibility. Light is also integrated, however the rapid growth of the horticultural lighting industry has resulted in lamps with diverse spectral distributions and system capabilities. This offers more choice and adds to the plant scientist’s tool kit, but it adds complexity within and across growth facilities. Today, there are over 1,400 different DesignLights Consortium (DLC) qualified LED horticultural luminaires as well as many non-qualified LED luminaires on offer.

Electric lamps have been used to grow plants for approximately 150 years as electrohorticulture began in 1880 using carbon arc lamps ^[1]. Throughout history, plant lighting has always followed human lighting technologies. By the 1970s, the combination of incandescent and fluorescence lamps were the luminaires of choice in plant growth chambers. The combination of the red and far-red enriched spectrum of the incandescent with the blue and green enriched spectrum of the fluorescent lamp provided a good broad spectrum for plant growth.

Figure 1. Fluorescent and incandescent spectral power distributions (W/m2/nm) of an incandescent and a compact fluorescent lamp (CFL). The combination of these two light sources spans the spectrum and resulted in relatively good plant growth and development [2].

Lighting for controlled environment plant research remained unchanged until the 1990s when red light emitting diodes (LEDs) were first used at the University of Wisconsin and NASA ^[1]. Since then, more efficient blue, green, white and far-red LEDs have been developed. The advancement and adoption of LEDs in controlled environment chambers have resulted in higher efficiencies and efficacies with lower heat emission compared to incandescent and fluorescent lamps. They are typically dimmable and if the different LED channels are tunable, the researcher can adjust the spectral ratios for different crops, developmental stages or desired research goals. What appears to be an insignificant difference in spectral distributions between lamps can have significant effects on plant growth, morphology, biochemistry and development.

Figure 2. Different direct emission LED wavebands. There are many different colors of LEDs that lighting manufacturers can use to fabricate horticultural fixtures. This leads to an assortment of spectral distributions on the market. (Arrow.com)

In addition to the large selection of direct emission LEDS, there are many different white LEDS. White LEDs are typically narrow band blue LEDs whose lenses are coated with different mixtures of phosphors. These phosphors absorb the blue light and re-emit some or most of the light into the green, red and sometimes far-red regions of the spectrum.

Figure 3. Three examples of different white LEDs used in horticultural fixtures. Spectral distribution of warm, cool and neutral white LEDs at 200 µmol/m2/s. The corresponding color temperatures for these examples are 2500-3500 K (warm white), 3500–4500 K (neutral white) and 4500–5500 K (cool white) [3].

What light do plants need?

It is well known that plants need and use light across a broad spectrum. As such, there are many different horticultural luminaires on the market, and they can have varying spectral distributions. A common topic among growers, researchers and lighting companies is what spectrum is best for plants. What spectrum should tunable LED luminaires be set at? The answers will depend on what the user is trying to achieve.

To determine what parts of the spectrum a plant truly requires, a good starting place is to understand the absorptance of light and the action spectrum of photosynthesis of whole plants or leaves ^[4,5]. Both of these span the entire spectrum from blue to green to red light. Simultaneously, the process of photomorphogenesis uses the information carried in light from UV to far-red (700-750 nm) to activate or deactivate many plant processes while guiding the development of the plant from seed to maturity.

The McCree curves

The graphs below reference the relative quantum yields (Φ) (a), relative action (b), and absorptance (c) spectra for 22 crop species grown in the field and growth chambers ^[4].

Most green plants grow best when photosynthesis is active and there are no limitations in the environment. From a physiological and plant growth perspective, plants would benefit from a spectral distribution somewhere between the action spectrum and the quantum yield of photosynthesis. See the McCree curves below.

Figure 4.

Figure 4 panel a depicts the relative quantum yield of photosynthesis. This quantifies the maximum photosynthetic efficiency and is where the rate of photosynthesis is equal to the photons absorbed. Panel b depicts the relative action spectrum of photosynthesis. This indicates the relative effectiveness of different wavelengths on the overall rate of photosynthesis and is defined as the CO2 taken up per incident energy (light). Panel c depicts the relative absorptance of leaves. This indicates the fraction of incident energy (light) absorbed.

The Multifaceted Role of Light

Light is crucial for photosynthesis. Apart from providing the energy required for plant growth, it simultaneously provides information or cues for photomorphogenesis which directs the development of plants. These light-induced developmental responses depend on the crop, cultivars and the plasticity of flexibility of the plant.

Light across the spectrum controls developmental and physiological processes usually through photoreceptors that act as “on/off” switches. For instance, phytochrome is activated by red light (660 nm) and de-activated by far-red light (740 nm) ^[6]. Cryptochrome is activated by blue light and reversed by green light ^[7]. The optimal broad LED spectrum for plant growth and development requires a balanced ratio of all wavebands. Some examples of the many spectrally induced plant processes are outlined in Figure 5 below.

Figure 5.

Figure 6. Glossary of horticultural light terms ^[8,9].

References

[1] Wheeler, R. (2008) A Historical Background of Plant Lighting: An Introduction to the Workshop. HortSci. 43:(7): 1942-1943.

[2] Konstantinos Papamichael, Michael Siminovitch, Jennifer A. Veitch & Lorne Whitehead (2015): High Color Rendering Can Enable Better Vision without Requiring More Power, LEUKOS: The Journal of the Illuminating Engineering Society of North America, DOI: https://doi.org/10.1080/15502724.2015.1004412

[3] Cope, K. R., & Bugbee, B. (2013). Spectral Effects of Three Types of White Light-emitting Diodes on Plant Growth and Development: Absolute versus Relative Amounts of Blue Light. Hortsci. 48(4):504-509. DOI: 10.21273/HORTSCI.48.4.504

[4] McCree, K.J. (1971). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agric Meteorol. Vol. 9, pp. 191-216.

[5] Hogewoning, S.W. et al. (2012). Photosynthetic quantum yield dynamics: from photosystems to leaves. Plant Cell. Vol. 24, pp. 1921-1935.

[6] Franklin, K.A. and quail, P.H. (2010) Phytochrome functions in Arabidopsis development. J. Exp. Bot. 61(1): 11-24.

[7] Folta,K.M. and Maruhnich, S.A. (2007) Green light: A signal to slow down or stop. J. Exp. Bot. 59(2): 3099-3111.

[8] Bugbee, B. (2017) Economics of LED lighting. In: Light Emitting Diodes for agriculture. Smart Lighting. Springer Nature. Ed: S.D. Gupta pp 81-99.

[9] Runkle, E. Light Terminology: From a plant perspective.