What is the impact of light on hemp production?

Cannabis is a crop that differs from other horticultural crops. This is primarily because its yield cannot be evaluated based solely on the weight or number of flowers or leaves. The chemical compounds produced by Cannabis are a very important product of this crop. In order to produce high-value Cannabis, it is necessary to understand how environmental factors can be manipulated to enhance final product quality.

Controlled environment production
Consistent yields and quality uniformity between production cycles are important to Cannabis growers. This has resulted in more Cannabis growers operating in controlled environment facilities. CEA production in which temperature, humidity, light intensity, light spectrum and carbon dioxide (CO2) levels can be controlled offers the ability to grow Cannabis year round. Under stable environmental conditions, growers can produce up to six harvests per year. This makes indoor cropping 15-30 times more productive than outdoor cultivation (Magagnini et al., 2018).

Impact of light
There are several environmental factors that play key roles in controlling yield, flowering, sex of the plants, and cannabidiol (CBD) and tetrahydrocannabinol (THC) concentrations in Cannabis. These factors include photoperiod, light intensity and light quality.

Tournois (1912) was the first researcher to demonstrate that Cannabis flowering is hastened by short days and delayed under long days. Hall (2013) also concluded that environmental factors, including temperature and photoperiod, can influence Cannabis’ reproductive abilities. In many plants, flowering occurs when the meristematic tissue receives signals produced by changes in temperature and/or light duration and/or quality.

Sexual expression in Cannabis is largely genetic (Flachowsky et al. 2001), but can be altered by environmental influences. Tournois was the first to report sexual reversal in the Cannabaceae family when he observed the effects of short photoperiods on Cannabis. Tournois reported a tendency toward maleness in Cannabis despite most dioecious plants showing a female tendency under similar light conditions.

Many factors contribute to the sexuality of flowering Cannabis plants. Under average conditions with a normal inductive photoperiod, Cannabis plants flower and produce approximately equal numbers of pure male and female plants with a few hermaphrodites (both sexes on the same plant).

Under conditions of extreme stress, such as nutrient excess or deficiency, mutilation and altered light cycles, Cannabis plant populations have been shown to depart greatly from the expected one-to-one male to female plant ratio (Clarke, 1999). Clarke (1999) also found that photoperiods of less than 14-16 hours can promote premature floral transition concluding Cannabis displays a quantitative short-day response.
There are three distinct phases in Cannabis cultivation: propagation, vegetative growth and flowering. Recent applications for research of production purposes recommends the use of long photoperiods (18 hours) for the propagation and vegetative phases and short photoperiods (12 hours) for the flowering phase when using artificial lighting (Magagnini et al., 2018).

Light intensity
Several studies with Cannabis have demonstrated that increasing the light intensity can have a positive effect on plant photosynthesis and/or growth (Chandra et al. 2008; Potter and Duncombe, 2012; Vanhove et al., 2011).

However, there is not always a correlation between a higher photosynthesis rate and more vegetative growth with higher flower yields. For example, Potter and Duncombe, (2012) using different light intensity treatments, concluded no significant difference in mass foliage or total THC levels within the foliage. However, the same researchers reported an increase in flower yield and THC present in flowers with increasing light intensity.

In addition, Vanhove et al (2011) demonstrated that a higher light intensity with a plant density of 16 plants per square meter can increase yield and THC concentration in comparison to lower light intensity levels. This research also showed that the use of the same high light intensity, but under a higher planting density (20 plants per square meter), can affect yield. Chandra et al., (2008) also reported greater net photosynthetic and transpiration rates when the light intensity increased.

Light quality
Plant growth, morphology and metabolism can be manipulated by changing light quality. For example, it is known that blue light decreases internode length making plants more compact (Dong et al., 2014). Also, several studies have demonstrated that far-red and green wavelengths induce stem and leaf elongation (Franklin et al., 2005).

The same responses have been confirmed in a high number of crops including Cannabis. Lalge et al. (2017) concluded that in general, red and blue light spectrums cause shorter internodes, smaller leaf area and more compact morphology compared to a white light source. In addition, Hawley et al. (2018), reported a significant increase in yield and concentration of total THC when using intra canopy red and blue lighting compared to the sunlight control treatment.

Livadariu et al. (2018) tested the use of green or blue light in Cannabis production compared to sunlight. Results showed a significant increase in yield and THC content when using green light and an increase in polyphenols, flavonoids and protein under blue light treatments compared to the control sunlight treatment.

Lyndon et al. (1987) demonstrated that THC content of some Cannabis plants can be increased by irradiating them with UV-B light. Marti et al. (2014) also corroborated the same response in a more recent study. However, no other Cannabis chemical compounds have been confirmed to increase under UV treatments.

Comparing LEDs and HPS lamps
For many years Cannabis production was done mainly with high pressure sodium (HPS) lamps. It is now possible to control and improve Cannabis quality using LED lamps. A recent study by Magagnini et al. (2018) compared the production of Cannabis using HPS lamps and two different LEDs lamps (LED lamp 1: 12 percent blue, 19 percent green, 61 percent red and 8 percent far-red. LED lamp 2: 1 percent UV light, 20 percent blue, 39 percent green, 35 percent red and 5 percent far-red). The same light intensity of 450 μmol m-2 s-1 was used under all treatments.

Study results showed HPS lamps produced taller and higher stem dry weight compared to LED treatments. However, HPS treatments resulted in a significant decline in THC concentration in flowers compared to both LED treatments. Under LED light treatments plants were shorter and compact, but showed higher CBD and THC content. This shows that the optimized light spectrum improves the value and quality of Cannabis.

More research is required to fully understand Cannabis’ behavior under artificial lighting. However, enough research information is available to show LED lamps are the best light source to improve Cannabis production and quality.

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