In the first of this four-part series, we take a look at how light intensity, duration and spectrum impact the yields, potency and morphology of Cannabis.
Glossary of Essential Terms
Colour Rendering Index (CRI)
A measure of a light source’s ability to show object colours realistically or naturally.
Measured between 1-100, the higher the value the more a light fixture can render all frequencies of its colour spectrum when compared to a perfect reference light e.g sunlight.
Luminous Efficiency
A measure of how well a light source produces light that is visible to humans.
Photobiology
The science of how light impacts living organisms.
Photosynthesis
A light-driven chemical reaction in plants that converts carbon dioxide and water into glucose and oxygen. Glucose is a simple sugar that is used as a store of chemical energy to drive essential processes like respiration, building cell walls and making proteins.
Photoreceptors
Proteins in plants that sense changes in the spectrum, intensity and direction of light and allow plants to adjust their growth accordingly. The major groups are cryptochromes, phototropins and phytochromes.
PAR (Photosynthetic Active Radiation)
The wavelength of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis.
All wavelengths between 400 and 700nm contribute to photosynthesis and also transmit information to the plant about its surroundings.
PBAR (Photobiologically Active Radiation)
Describes the wavelengths between 280-800nm but beyond the PAR region but which also transmit critical information to plants. These occur in the UV range below 400nm and in the far-red range above 700nm and strongly affect different aspects of plant growth.
PPFD (Photosynthetic Photon Flux Density)
The amount of light (amount of photons) within the PAR range that land on a given surface per second. Measured in µMol/m2/S.
This is the most accurate measure for growers to determine the light intensity that is reaching their plant canopy.
Secondary Metabolites
Non-essential bioactive compounds that give plants aroma, colour, and taste and provide resistance against external biotic and abiotic stress.
Cannabis contains over 500 secondary metabolites with the most well-known comprising cannabinoids, terpenes and flavonoids.
White Light
A combination of all the colours in the spectrum and the light naturally provided by the sun. White light has the highest colour rendering index of any type of light.
Why Research Light?
Light is one of the most critical factors in determining the yields and quality of commercially grown Cannabis. Plants use light as a source of energy for photosynthesis and for information about their environment. Combined with temperature, nutrients and CO2 concentration, light interacts with all aspects of cultivation to direct plant growth and development (Fig.1).
Fig.1. Factors that affect plant growth can be summarised into nine main parameters. Of these, light is the parameter which can affect the behaviour of all of the other eight so can be used effectively to modify all other parameters to optimise plant growth. Source: Crop Physiology Lab, Utah State University.
An increase in controlled environment agriculture has driven progress in our understanding of how light affects different aspects of plant growth. In particular, research with LED lighting continues to reveal how different light spectrums can affect plant morphological and physiological traits and how growers can harness this information to get the most out of their crops.
Plant Light Requirments
In the 1970s Keith McCree calculated the average efficiency of different light colours at stimulating photosynthesis in 22 plant species (McCree 1971). His curve showed the critical wavelengths of light were in the 400-700nm zone and showed peaks at 440nm (blue) and 620nm (red) wavelengths. This work defined the earliest interpretations of the range of photosynthetic active radiation (PAR) (Fig.2).
Fig.2: PAR is the main driver of photosynthesis in plants. The different wavelengths of light drive photosynthesis at distinct efficiencies. From the graph, red and blue wavelengths are the most efficient and explain why red-dominant HPS lighting and blue-red LEDs are the most popular spectrums used by cultivators. However, an over-interpretation of the graph led to the mistaken conclusion that green light is not used by plants due to its decreased efficiency compared to other wavelengths. Source: Fluence Bioengineering
However, McCree’s research was limited due to the single-leaf treatments and measurements he used and the technology he had available at the time. Although it showed the most abundant forms of chlorophyll, (chlorophyll a and chlorophyll b), absorbed wavelengths in the red and blue spectrum, it has been misinterpreted.
One of the most common errors concluded from the data includes the oversight of the importance of green wavelengths for stimulating photosynthesis. Many people still believe plants do not utilise green wavelengths based on mistakenly thinking most green light is reflected when in fact plants absorb around 85% of green light and transmit around 5-10% through the leaf surface (Runkle 2017). A minority of green light is reflected (around 5-10%) and it is actually not reflected by chlorophyll pigments as is another misconception but is instead reflected by cell walls (Virtanen et al 2020).
One of the first trials to demonstrate the importance of green light in photosynthesis showed green wavelengths penetrated deeper into the leaf than red or blue wavelengths (Sun et al 1998). Another trial on sunflower leaves showed adding green light to white light increased photosynthesis more than the addition of red or blue light. In other experiments, green wavelengths of light were also shown to drive photosynthesis more successfully than other wavelengths depending on the light intensity (Runkle 2017).
We will go more in-depth into the impact of different wavelengths in Part II and III of the series.
LED Technology - A New Dawn
A new era of LED lights that allow precise control of both spectrum and intensity is paving the way for a new frontier of photobiology insights. LED lighting studies have revealed the role of wavelengths of light beyond the PAR range in driving photosynthesis. This extended range of wavelengths from 280-800nm and which includes UV and far-red light is termed plant biologically active radiation (PBAR) (Fig.3). Control of the spectrum has also permitted trials on the effects of wavelengths in isolation, deepening the understanding of the impacts of UV, blue, green, red and far-red on plant growth and development.
Fig.3: Research showing the role of wavelengths of light outside of the PAR spectrum (400-700nm) has shown the importance of UV light and light in the Far-Red spectrum in impacting photosynthesis. Access to better technology has allowed the manipulation of the plant spectrum and improvements in our abilities to more accurately measure changes in plant photosynthetic response have also driven this progress. Source: Heliospectra
Dr Bruce Bugbee and his team from Utah State University are renowned for LED light research which tests whole-plant responses to light spectrums as opposed to McCree’s work which tested single leaf responses. Dr Bugbee has said, “LEDs are causing paradigm shifts in our understanding of photobiology.” He explains wavelengths work synergistically and that wavelengths within and beyond PAR are essential components of a “balanced diet” of light (Utah State University).
Earlier research in the late 1950s had already established the importance of supplying plants with varying wavelengths of light. Exposing plants to wavelengths above and below 680nm activated both photosystems and thereby improved photosynthetic efficiency and plant yields (Emerson,1957). Significantly, this synergy drove rates of photosynthesis to a greater extent when combined compared to the rates of the sum of their parts individually, a result known as the “Emerson Effect.”
Before digging deeper into the research on the effects of the light spectrum on plant growth and behaviour Part I of this series defines some important terminology relevant to the topic of light.
Qualities of Light
Light Duration (Photoperiod)
Light duration or, photoperiod, is the length of time plants are exposed to light. This influences reproductive behaviour and in short-day plants like Cannabis, determines when plants enter the flowering phase.
Growers can manipulate the photoperiod to maintain mother plants in their vegetative stage by exposing plants to at least 18 hours of light. The flower-producing generative phase in cannabis is triggered by shortening the photoperiod to at least 12 hours of uninterrupted darkness.
Light Intensity
Light intensity refers to the amount of light a plant receives. It directly influences the amount of biomass produced, which has been shown to increase linearly between a range of 120-1,800μmol·m−2·s−1 (Rodriguez et al 2021, Eaves et al 2020).
It is important to note that increasing PPFD after approximately 500-550μmol·m−2·s−1 requires C02 enrichment and that no other limiting factors such as water, nutrients and temperature etc, are present (Chandra 2011).
Cannabis has a high degree of plasticity meaning it has the ability to tolerate and adapt its shape and physiological characteristics to light intensities far higher than other commercial crops like tomatoes and cucumbers. This is often thought of as a legacy of its native environment in which some landrace strains in the Himalayas grow at heights of upwards of 3,000m where the solar radiation is much greater than at sea level (Pate, 1983). It is argued that during the history of domestication, plants were bred outdoors so this tolerance was conserved once they were brought in and grown indoors.
Light Intensity and Yields
When it comes to light, intensity is the most important determinant of final plant yields and is adjusted throughout the growth stage to correspond with and drive the growth potential of larger plants. A guideline range for plants at their different growth stages is provided below.
| Target PPFD (µmol/m2/s) | |
|---|---|---|
Seeds & Clones | Vegetative | Flower |
80-200 | 300-600 | 600+ |
Although mainly dependent on photoperiod, as in many other herbaceous plants, light intensity in cannabis also favours biomass partitioning in favour of bud production resulting in a larger ratio of flowers produced compared to other biomass (Hawley et al 2018). Further, intensity not only increases the number of flowers produced but also the density of these apical buds. This is relevant for increasing the yields of marketable products and has been exploited by researchers who are experimenting with sub-canopy lighting to maximise the production of marketable biomass (Hawley et al 2018).
Light Intensity and Secondary Metabolite Production
Studies suggest intensity also has a role in influencing terpene concentration. The levels of myrcene, limonene and caryophyllene were found to increase linearly by 25% as PPFD levels rose from 130-1,800μmol·m−2·s−1 (Rodriguez et al 2021). This could be beneficial for enhancing the aroma and the quality of flowers thereby producing not only better quality dried bud but also better extracts.
The issue of whether intensity can increase cannabinoid concentration remains rather more controversial. The same study that found linear increases in overall yields and terpene concentration with an increasing light intensity found no effects on cannabinoid potency (Rodriguez et al 2021). This confirmed earlier findings from another trial that found a negative relationship between intensity and cannabinoid concentration (Potter and Duncombe 2012).
A review of research on the impact of light intensity and spectrum highlights that there have been instances where intensity can increase potency (Brousseau et al 2021).
Fig.4: A review of the effects of light intensity and spectrum on secondary metabolite production has shown much research is contradictory. Most current studies have only tested intensities up to 500μmol·m−2·s−1 and there is minimal research using narrow-spectrum light.
Researchers often conclude the role of factors including chemotype and strain, the range and the duration of spectrums and intensities tested and the light source as majorly impacting experimental results.
Often this is used to discuss why experiments conclude different things and highlight some of the difficulties in drawing valid comparisons between studies and interpreting their results. Source: Brousseau et. al 2021
The difficulty in reproducing results between studies is due to the genetic variability of many cannabis cultivars, their high degree of plasticity and the wealth of factors e.g., nutrients, C02, substrate, plant training techniques, infrastructure etc that could all impact experimental results. There is also a lack of standardisation in how secondary metabolite concentration is measured with some researchers measuring secondary metabolite accumulation in leaves and others measuring it in the flowers.
Light Spectrum
Light spectrum describes the quality of light that is received by plants and although it can also impact biomass production it has a greater influence on plant morphology.
Natural sunlight produces white light since it contains all the colours of the spectrum. The sun has a high luminous efficiency meaning the white light it produces can be easily perceived by humans. Sunlight is also the standard used to determine a light's ability to accurately reproduce the colours of an object it illuminates, known as the "colour rendering index."
Studies manipulating the plant spectrum during the growth cycle of various crops and growing crops under sole wavelengths have advanced our understanding of photobiology.
We are learning more about the positive health benefits of some wavelengths of UV, the importance of a well-balanced red-blue light ratio for well-functioning photosynthesis and the role of red and far-red lighting on flowering. More on that in Part II of the series.
Light Spectrum and Plant Photoreceptors
Summary
Here's a handy summary of how the qualities of light mentioned so far in this introductory article affect different aspects of Cannabis plant growth.
Fig.5: Decades of light research has shown how three different properties of light can impact three main properties of plants including Cannabis. Interestingly, every property of light can impact biomass, morphology and flowering. Source: Heliospectra
In Part II of our lighting series, we take a look at the impact of UV, blue, and green light on plant growth and development.
References:
Brousseau, V.D., Wu, B.S., MacPherson, S., Morello, V. and Lefsrud, M., 2021. Cannabinoids and Terpenes: How Production of Photo-Protectants Can Be Manipulated to Enhance Cannabis sativa L. Phytochemistry. Frontiers in plant science, 12.
Chandra, S., Lata, H., Mehmedic, Z., Khan, I.A. and ElSohly, M.A., 2010. Effect of Light Intensity on Photosynthetic Characteristics of High Δ9-THC Yielding Varieties of Cannabis sativa L. Planta Medica, 76(05), p.P11.
Eaves, J., Eaves, S., Morphy, C. and Murray, C., 2020. The relationship between light intensity, cannabis yields, and profitability. Agronomy Journal, 112(2), pp.1466-1470.
Emerson, R. (1957, January). Dependence of yield of photosynthesis in long-wave red on wavelength and intensity of supplementary light. In Science (Vol. 125, No. 3251, pp. 746-746).
Hawley, D., Graham, T., Stasiak, M. and Dixon, M., 2018. Improving cannabis bud quality and yield with subcanopy lighting. HortScience, 53(11), pp.1593-1599.
McCree, K. J. (1971). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191-216.
Pate, D.W., 1983. Possible role of ultraviolet radiation in evolution of Cannabis chemotypes. Economic Botany, 37(4), pp.396-405.
Potter, D.J. and Duncombe, P., 2012. The Effect of Electrical Lighting Power and Irradiance on Indoor‐Grown Cannabis Potency and Yield. Journal of forensic sciences, 57(3), pp.618-622.
Rodriguez-Morrison, V., Llewellyn, D. and Zheng, Y., 2021(A). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in plant science, 12, p.456
Runkle 2017, "Growing Plants with Green Light." Available at: https://gpnmag.com/article/growing-plants-with-green-light/
Sun, J., Nishio, J. N., & Vogelmann, T. C. (1998). Green light drives CO2 fixation deep within leaves. Plant and Cell Physiology, 39(10), 1020-1026.
Terashima, I., Fujita, T., Inoue, T., Chow, W. S., & Oguchi, R. (2009). Green light drives leaf photosynthesis more efficiently than red light in strong white light: revisiting the enigmatic question of why leaves are green. Plant and cell physiology, 50(4), 684-697.
Utah State University, Crop Physiology Lab. College of Agriculture and Applied Science. Available at: chttps://caas.usu.edu/labs/cpl/
Virtanen, O., Constantinidou, E., & Tyystjärvi, E. (2020). Chlorophyll does not reflect green light–how to correct a misconception. Journal of Biological Education, 1-8.