A Call for More Representative Horticultural Lighting Metrics for Cannabis Cultivation

Optimizing horticultural lighting requires a comprehensive understanding of plant-light interactions, yet current metrics fail to capture the full complexity of photobiological responses. This article explores fundamental principles of photobiology, including Photosynthetically Active Radiation, photoreceptor-mediated responses, and the spectral influences on plant morphology and development. Conventional horticultural lighting metrics—such as Photosynthetic Photon Flux, Photosynthetic Photon Flux Density, and Daily Light Integral—provide useful benchmarks for light intensity but fail to account for spectral quality and plant-specific physiological responses. While the Design Lights Consortium Horticultural Lighting Program establishes industry standards for lighting efficacy, its metrics primarily emphasize energy efficiency rather than biological relevance. To bridge this gap, this article highlights the need for more biologically representative indicators of light efficacy. Physiological metrics such as photosynthetic rate, chlorophyll content, leaf area expansion, stem elongation, biomass accumulation, and gene expression offer deeper insights into plant-light interactions. By integrating these indicators into lighting assessments, researchers and cultivators can refine their strategies to enhance growth, optimize plant health, and maximize phytochemical production in controlled environment agriculture. A comprehensive approach that integrates plant physiology, photobiology, and spectral engineering is vital for advancing precision lighting methodologies, especially for high-value crops like cannabis.

Introduction to Photobiology

Light, a form of electromagnetic radiation, plays a pivotal role in plant growth and development. While the electromagnetic spectrum is vast, plants primarily utilize wavelengths within the visible spectrum (approximately 400-700 nm) for photosynthesis. However, it’s important to note that plants respond differently to various wavelengths within this range, and even to some extent outside of it. For example, ultraviolet (UV) (280-400 nm) and far-red (700-750 nm) radiation are also important, as these wavelengths can influence plant development and stress responses. Different spectral compositions can significantly affect plant growth, morphology, and even phytochemical content. The ideal spectrum often varies by crop and growth stage. Photosynthetically Active Radiation (PAR) is a critical concept in horticultural lighting. It encompasses the range of light wavelengths that drive photosynthesis, typically between 400-700 nm. PAR is quantified in µmol (micromoles) per square meter per second, representing the number of photons within the PAR range reaching a given area in a specific time frame.

Understanding PAR is essential for evaluating the efficacy of horticultural lighting systems, as it directly correlates with the light available for photosynthesis. However, it’s crucial to remember that not all wavelengths within the PAR range are equally efficient for photosynthesis, which leads us to the importance of spectral quality.

Plants have evolved sophisticated systems to detect and respond to various aspects of light. These systems, known as photoreceptors, include chlorophyll, phytochromes, and cryptochromes. Chlorophyll is the primary pigment responsible for photosynthesis, absorbing light most efficiently in the red and blue portions of the spectrum. This absorption pattern is a key factor in the design of many horticultural grow lights (1). Phytochromes are red and far-red light receptors that play crucial roles in various plant processes, including seed germination, stem elongation, and flowering. The ratio of red to far-red light can significantly influence plant morphology and development (2). Cryptochromes are sensitive to blue and UV-A light, cryptochromes are involved in regulating circadian rhythms and photomorphogenesis. They influence processes such as stem elongation, leaf expansion, and the transition to flowering (3).

Additionally, understanding plant responses to lighting is crucial for optimizing growth and development in controlled environment agriculture (CEA). These responses are complex and multifaceted, involving various physiological and morphological changes. Photomorphogenesis refers to light-mediated developmental changes in plant form, such as stem elongation, leaf expansion, and chlorophyll synthesis, which are influenced by the red to far-red ratio, spectral composition, and blue light intensity, respectively (4-6). Photoperiodism is the response of plants to the relative lengths of light and dark periods, which regulates various developmental processes, including flowering. Phototropism is the directional growth response of plants to light. While primarily studied in relation to blue light, recent research suggests that red light may also play a role. In indoor growing environments, phototropism can influence canopy structure and light interception efficiency (7). Shade avoidance responses are adaptive strategies plants use to compete for light. These responses are primarily mediated by changes in the red to far-red ratio and overall light intensity. Typical shade avoidance responses include increased stem elongation, upward leaf movement, and reduced branching. In controlled environment agriculture, unintended triggering of shade avoidance responses can lead to undesirable plant morphology. Careful management of the light spectrum, particularly the red to far-red ratio, is essential to avoid these effects (8).

Current Metrics

As of today, there are multiple horticultural lighting metrics that have been established to help guide growers in terms of photobiological efficacy. The metrics listed below are easy to understand and implement; however, are they the most representative of what one can expect to achieve in their cultivation environment? We explore the pros and cons of each and explore more physiologically-derived metric in the proceeding section.

Photosynthetic photon flux (PPF) quantifies the total amount of PAR produced by a lighting system per second, measured in micromoles per second (µmol/s). This metric provides insight into the overall light output of a fixture but does not account for the distribution of that light (9). Photosynthetic photon flux density (PPFD) represents the best lighting metric available to date. PPFD measures the amount of PAR that actually reaches the plant canopy, expressed in micromoles per square meter per second (µmol/m²/s). This metric is crucial as it indicates the usable light intensity at the plant level. PPFD can vary across the growing area, necessitating the use of PPFD maps to evaluate light distribution (10). It is essentially, ‘the number of photons in the 400-700nm waveband incident per unit time on a unit surface’. Photosynthetic photon efficacy (PPE) is another pertinent horticultural lighting metric that is measured in µmol/J and determines how well your lighting system is converting electrical energy into photons. PPE is calculated by taking the PPF of the luminaire or system, and dividing it by input electric power. PPE is represented as micromoles per second per electric watt (µmol × s-1 × We-1 ), or micromoles per joule (µmol × J-1 ) and the theoretical maximum PPE for light emitting diodes (LEDs) is 4.6 to 5.1 µmol/J-1, depending on the composition of the LEDs used in an array. Unfortunately, none of these metrics account for a plant’s spectral requirements, and all three metrics can be ‘gamed’ to present elevated metric values that are not representative of horticultural efficacy.

Daily Light Integral (DLI) is another available variable and is defined as the cumulative amount of the total number of photons that land on a particular surface during the daily photoperiod and is measured in mol/m2/d. It is ‘the amount of PAR received each day as a function of light intensity and duration.’ DLI plays a pivotal role in plant growth, yield and development, and it can be carefully managed and controlled in variable conditions to ensure that you get the best possible results. Different plant species have varying DLI requirements, making this metric valuable for tailoring lighting strategies to specific crops (11). DLI is a popular metric in the world of horticultural lighting, particularly for greenhouse growers using supplemental lighting; however, as with the aforementioned PPF, PPFD, and PPE metrics, it’s not the whole story. DLI measures the total amount of photosynthetically active radiation (PAR) over 24 hrs, but it’s blind to the light spectrum. This is important because different plants require different lighting recipes and while plants might be getting enough light, the spectrum may not be optimized. DLI also ignores when light is delivered during the day, which can be problematic as some plants are early birds (For example, hibiscus) and others are night owls (Such as, evening primrose). Also, DLI provides the summation of total daily light exposure but does not calculate relative intensity at any given time throughout the day. Furthermore, pursuing an elevated DLI without considering intensity can lead to photoinhibition, where excess light actually reduces photosynthesis. Lastly, DLI oversimplifies the fact that plants are complex beings with constantly changing needs. Case in point, the variances in lighting intensity and spectra that are appropriate/optimal for seedlings compared to plants in vegetative growth, the stretch phase, and final flower. With all of that being said, DLI is still a useful starting point and should be coupled with careful considerations of spectral composition, light intensity at different growth stages, photoperiod, and any other plant-specific requirements.

What about DLC Metrics?

The Design Lights Consortium (DLC) Horticultural Lighting Program has established a Qualified Products List (QPL) that uses these metrics to evaluate and list high-performance lighting fixtures for horticultural applications. This list is becoming increasingly important for regulatory compliance and utility rebate programs. While the DLC metrics for horticultural lighting provide valuable standardization, there are some drawbacks to consider. As with the other aforementioned metrics, DLC metrics have a limited scope of measurement that is primarily focused on energy efficiency and light output; not lighting spectra or the nuances of different growth stages. This has the potential for misinterpretation as growers might overemphasize DLC ratings without considering other crucial factors, such as lighting distribution. And while an emphasis on PPE is certainly important in terms of electrical efficiency, it certainly doesn’t tell the whole story with respect to plant-light interactions, which is the what we are all trying to optimize during cultivation. DLC metrics do provide a useful baseline, but they are not the be-all and end-all of horticultural lighting. Successful cultivation requires a holistic approach that considers your specific plants, environment, and goals. As we will discuss in the proceeding section, data-driven decision making is critical but only when the metrics that you are using to quantify success are representative of photobiology and the terminal goals that you are aspiring to achieve. It’s important to ‘listen’ to your plants and observe their response to different lighting conditions.

Potentially More Relevant Physiological Indicators

Physiological indicators of light efficacy in horticulture are crucial for understanding and optimizing plant growth and development, providing valuable insights into plant responses to different lighting conditions. By manipulating light quality, intensity, and duration using advanced light technologies, such as full-spectrum LEDs, researchers and growers can optimize plant growth, development, and quality for various applications in controlled environment agriculture. Here are ten key physiological indicators, along with the resulting technical findings, and the corresponding analytical tools utilized in each investigation:

Photosynthetic Rate: The Engine of Growth

Photosynthesis is the cornerstone of plant growth, and LED lights are proving to be powerful tools for optimizing this process. Recent studies have shown that higher levels of green light increased photosynthetic rate at higher photon flux densities (PFDs) in lettuce (12). These findings underscore the importance of considering the entire spectrum, not just red and blue light, in horticultural lighting strategies. To measure photosynthetic rates accurately, researchers employ gas exchange systems. These sophisticated instruments measure CO₂ uptake and water vapor exchange, providing precise data on the plant’s photosynthetic performance. It’s worth noting that while PAR is crucial, it’s merely a measure of photons delivered over a given area and time, expressed in micromoles per square meter per second (µmol·m⁻²·s⁻¹). PAR alone doesn’t tell us how efficiently plants are using that light for photosynthesis.

Chlorophyll Content: The Light-Harvesting Powerhouse

Chlorophyll content plays a critical role in a plant’s ability to absorb light for photosynthesis, directly influencing its photosynthetic efficiency. Light quality significantly affects chlorophyll biosynthesis and degradation, with species- and cultivar-specific responses. Research on various plant species, including Coleus cultivars, has demonstrated that different light spectra can modulate chlorophyll accumulation, potentially impacting overall plant health, growth, and productivity (13). To quantify chlorophyll content, researchers use a variety of tools. Soil Plant Analysis Development (SPAD)meters provide a non-destructive way to estimate relative chlorophyll content in leaves. For more precise measurements, High-Performance Liquid Chromatography with Ultraviolet Detection (HPLC-UV) or UV-Vis spectrophotometry can be employed to measure chlorophyll a and b concentrations. These methods allow for a nuanced understanding of how different lighting conditions affect the plant’s light-harvesting capabilities.

Leaf Area and Expansion: Maximizing Light Capture

Leaf area and expansion are critical factors in optimizing light capture for photosynthesis. Research indicates that far-red radiation enhances plant growth by stimulating leaf expansion and increasing whole-plant net assimilation (14). Precise quantification of leaf area and morphological traits is achieved using leaf area meters, such as the LI-COR LI-3100C, while image analysis software provides a non-destructive approach for tracking leaf growth over time by digitally assessing leaf area, shape, and color.

Stem Extension: Reaching for the Light

Stem extension rate reflects how plants respond to light quality and quantity. Research has shown that incorporating 6% green light into the lighting spectrum enhances stem elongation rate in microgreens compared to red and blue light alone (15). To accurately measure these responses, time-series image analysis software tracks stem growth dynamics under varying light conditions. Digital calipers and laser displacement sensors offer precise manual and automated measurements, while automated phenotyping systems provide real-time monitoring in controlled environments, enabling high-throughput data collection for advanced growth analysis.

Biomass Accumulation: The Bottom Line of Growth

Biomass accumulation serves as the fundamental measure of overall plant growth and productivity. Research indicates that lighting recipes containing green light significantly enhance biomass accumulation in lettuce (12). Gravimetric analysis (dry weight measurement) remains the standard for biomass quantification, providing a direct measure of plant growth. Additionally, HPLC-UV is particularly useful for separating and quantifying specific compounds, while Near-Infrared Spectroscopy (NIRS) provides rapid, non-destructive analysis of chemical composition, offering insights into the quality of the accumulated biomass.

Anthocyanin Content: Nature’s Stress Indicator

Anthocyanin content serves as a natural indicator of plant stress, contributing not only to pigmentation but also reflecting the plant's response to varying environmental conditions. In studies with Coleus, it was observed that different light intensities led to increased anthocyanin levels, resulting in noticeable changes in plant coloration (13). To accurately quantify anthocyanin content, researchers employ spectrophotometers (UV-Vis at approximately 530 nm) and HPLC, which offer precise measurements of these vital pigments. Although quantum sensors are used to measure PAR, they do not directly measure anthocyanins but are essential for correlating light intensity with anthocyanin production.

PPFD Response: Dialing in the Right Intensity

Different plant species exhibit diverse growth responses to varying levels of light intensity, quantified as PPFD (16). This variability underscores the necessity for species-specific light recipes to optimize growth. Quantum sensors play a crucial role in directly measuring PAR and PPFD, providing vital data on light intensity at the plant level. Additionally, spectroradiometers offer detailed insights into the light spectrum and intensity, presenting a comprehensive view of the light environment.

Chlorophyll Fluorescence: A Window into Plant Health

Chlorophyll fluorescence serves as a critical indicator of photosystem II (PSII) efficiency and overall plant health. In Coleus, fluorescence measurements revealed significant variations in response to different light qualities, highlighting the impact of spectral composition on photosynthetic performance (13). To assess these effects, chlorophyll fluorometers are employed as powerful analytical tools. These sophisticated, non-invasive instruments provide real-time measurements of PSII efficiency and plant stress responses, enabling precise monitoring of plant health under diverse lighting conditions.

NDVI: Remote Sensing of Plant Health

The Normalized Difference Vegetation Index (NDVI) serves as a critical remote sensing metric for evaluating plant biomass and physiological status in a non-invasive manner. Variability in NDVI values observed in Coleus under different light spectra suggests significant modulation of plant health and biomass accumulation as a function of spectral composition (13). This assessment relies on advanced analytical instrumentation, including specialized NDVI sensors and multispectral imaging systems, which facilitate high-throughput, non-destructive monitoring of vegetative health across extensive cultivation areas. The integration of NDVI in controlled environment agriculture enhances precision crop management by enabling real-time assessment of plant vigor and early detection of stress responses.

Gene Expression: The Molecular Response to Light

Plants modulate gene expression in response to light conditions, influencing key physiological processes at the molecular level. Notably, exposure to green light was found to alter the expression of nitrate assimilation genes, including (NR and NiR), which are integral to protein biosynthesis and chlorophyll production (15). These molecular adaptations were quantified using real-time quantitative PCR (qPCR), a highly sensitive analytical technique that enables precise measurement of gene expression dynamics under varying light treatments. By elucidating the genetic mechanisms governing plant responses to spectral composition, qPCR facilitates deeper insights into optimizing lighting strategies for enhanced growth and metabolic efficiency.

For controlled experiments across all these indicators, environmental control chambers are crucial for maintaining consistent light, temperature, and humidity conditions. These chambers provide the precise control necessary for isolating the effects of light on plant physiology, ensuring the reliability and reproducibility of research findings.

Conclusion

The future of horticultural lighting depends on shifting from traditional efficiency metrics to biologically relevant, data-driven evaluations. Standard measures like PPF and DLI fail to capture the complexity of plant-light interactions, limiting their effectiveness in optimizing cultivation strategies. To maximize growth and phytochemical production, lighting systems must be designed with plant physiology in mind rather than focusing solely on electrical efficiency. Advanced analytical tools are essential for this transition. By accurately measuring plant responses, cultivators can tailor lighting strategies to specific growth stages, refining spectral composition, intensity, and duration for improved productivity and sustainability. The next generation of horticultural lighting must be adaptive and data-driven, moving beyond static performance metrics. As research advances, the industry must embrace flexible lighting solutions as integral components of controlled environment agriculture. This shift is critical to unlocking the full potential of horticultural lighting, driving improvements in yield, quality, and sustainability.

References

1. Lichtenthaler, H. K.; and Burkart, S. Photosynthesis and high light stress. Bulgarian Journal of Plant Physiology, 1999, 25(3-4), 3-16.
2. Smith, H. Phytochromes and light signal perception by plants—an emerging synthesis. Nature, 2000, 407(6804), 585-591.
3. Cashmore, A. R.; Jarillo, J. A.; Wu, Y. J.; & Liu, D. Cryptochromes: blue light receptors for plants and animals. Science, 1999, 284(5415), 760-765.
4. Thomas, B.; and Vince-Prue, D. Photoperiodism in plants. Academic Press, 1997.
5. Massa, G. D.; Kim, H. H.; Wheeler, R. M.; and Mitchell, C. A. Plant productivity in response to LED lighting. HortScience, 2008, 43(7), 1951-1956.
6. Casal, J. J. Photoreceptor signaling networks in plant responses to shade. Annual review of plant biology, 2013, 64, 403-427.
7. Choi, H. G.; Moon, B. Y.; and Kang, N. J. Effects of LED light on the production of strawberry during cultivation in a plastic greenhouse and in a growth chamber. Scientia Horticulturae, 2015, 189, 22-31.
8. Magagnini, G.; Grassi, G.; and Kotiranta, S. The effect of light spectrum on the morphology and cannabinoid content of Cannabis sativa L., Medical Cannabis and Cannabinoids, 2018, 1(1), 19-27.
9. Demotes-Mainard, S.; Péron, T.; Corot, A.; Bertheloot, J.; Le Gourrierec, J.; Pelleschi-Travier, S.; and Sakr, S. Plant responses to red and far-red lights, applications in horticulture. Environmental and Experimental Botany, 2016, 121, 4-21.
10. Huché-Thélier, L.; Crespel, L.; Le Gourrierec, J.; Morel, P.; Sakr, S.; and Leduc, N.Light signaling and plant responses to blue and UV radiations—Perspectives for applications in horticulture. Environmental and Experimental Botany, 2016, 121, 22-38.
11. Kim, H. H.; Goins, G. D.; Wheeler, R. M.; and Sager, J. C. Green-light supplementation for enhanced lettuce growth under red-and blue-light-emitting diodes. HortScience, 2004, 39(7), 1617-1622.
12. Puccinelli, M.; Maggini, R.; Angelini, L. G.; Santin, M.; Landi, M.; Tavarini, S.; Castagna, A.; and Incrocci, L. Can Light Spectrum Composition Increase Growth and Nutritional Quality of Linum usitatissimum L. Sprouts and Microgreens?, Horticulturae, 2022, 8(2), 98. https://doi.org/10.3390/horticulturae8020098
13. Light Quality Influence on Growth Performance and Physiological Parameters of Coleus (Plectranthus scutellarioides) Cultivars. MDPI, 2024, https://www.mdpi.com/2037-0164/15/3/58
14. Park, Y.; and Runkle, E. S. Far-Red Radiation Promotes Growth of Seedlings by Increasing Leaf Expansion and Whole-Plant Net Assimilation. Environmental and Experimental Botany, 2017, 136, 41-49. https://doi.org/10.1016/j.envexpbot.2016.12.013
15. Sen, S.; Kumari, S.; Kumar, V.; and Husen, A. Light emitting diode (LED) lights for the improvement of plant performance and production: A comprehensive review. Current Research in Biotechnology, 2024, 7, 100184. https://doi.org/10.1016/j.crbiot.2024.100184
16. Runkle, E. S.; Meng, Q.; and Park, Y. LED Applications in Greenhouse and Indoor Production of Horticultural Crops. Acta Horticulturae, 2019, 1263, 17-29. https://doi.org/10.17660/ActaHortic.2019.1263.2

About the Columnist

Dr. Zacariah Hildenbrand is a research Professor at the University of Texas at El Paso, the principal founder of Inform Environmental, a partner of Medusa Analytical, and is a director of the Curtis Mathes Corporation (OTC:CMCZ). Direct correspondence to: zlhildenbrand@utep.edu.

About the guest Co-Author

Hannia Mendoza -Dickey has a MS degree in Chemistry and is the founder of Green Matter Consulting. hemendoza0816@gmail.com.

About the guest Co-Author

Robert Manes is the CEO and CTO of the publicly-traded Curtis Mathes Corporation, where he specializes in the creation and manufacture of frequency-specific lighting for horticulture and phototherapy purposes.

How to Cite this Article

Hildenbrand, Z., Mendoza-Dickey, H., Manes, R., A Call for More Representative Horticultural Lighting Metrics for Cannabis Cultivation, Cannabis Science and Technology, 2025, 8(1), 14-19.