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Cannabis sativa L.: Crop Management and Abiotic Factors That Affect Phytocannabinoid Production

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Abstract

The main characteristic of Cannabis sativa L. is the production of compounds of medicinal interest known as phytocannabinoids. Environmental factors and crop management practices are directly related to the yield of these compounds. Knowing how these factors influence the production of phytocannabinoids is essential to promote greater metabolite yield and stability. In this review, we aim to examine current cannabis agronomic research topics to identify the available information and the main gaps that need to be filled in future research. This paper introduces the importance of C. sativa L., approaching state-of-the-art research and evaluating the influence of crop management and environment conditions on yield and phytocannabinoid production, including (i) pruning; (ii) light and plant density; (iii) ontogeny; (iv) temperature, altitude, and CO2 concentration; (v) fertilization and substrate; and (vi) water availability, and presents concluding remarks to shed light on future directions.
Citation: Trancoso, I.; de Souza,
G.A.R.; dos Santos, P.R.; dos Santos,
K.D.; de Miranda, R.M.d.S.N.; da
Silva, A.L.P.M.; Santos, D.Z.;
García-Tejero, I.F.; Campostrini, E.
Cannabis sativa L.: Crop Management
and Abiotic Factors That Affect
Phytocannabinoid Production.
Agronomy 2022,12, 1492. https://
doi.org/10.3390/agronomy12071492
Academic Editor: Matteo Caser
Received: 27 May 2022
Accepted: 20 June 2022
Published: 22 June 2022
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4.0/).
agronomy
Review
Cannabis sativa L.: Crop Management and Abiotic Factors That
Affect Phytocannabinoid Production
Ingrid Trancoso 1,2, Guilherme A. R. de Souza 1, Paulo Ricardo dos Santos 3, Késia Dias dos Santos 1,
Rosana Maria dos Santos Nani de Miranda 1, Amanda Lúcia Pereira Machado da Silva 1, Dennys Zsolt Santos 4,
Ivan F. García-Tejero 5,* and Eliemar Campostrini 1, *
1Setor de Fisiologia Vegetal, LMGV, Centro de Ciências e Tecnologias Agropecuárias, Universidade Estadual
do Norte Fluminense, 2000, Alberto Lamego Avenue, Campos dos Goytacazes 28013-620, RJ, Brazil;
ingrid-trancoso@hotmail.com (I.T.); guilherme.rodrigues@edu.uniube.br (G.A.R.d.S.);
kesiadias.s@gmail.com (K.D.d.S.); ronani.uenf@gmail.com (R.M.d.S.N.d.M.);
amandaecofisio@gmail.com (A.L.P.M.d.S.)
2Canapse—Associação de Canabiologia, Pesquisa e Serviços, Maricá24900-000, RJ, Brazil
3Instituto Federal do Amapá, Campus Porto Grande, Br 210 Km, Sn, Porto Grande 68997-000, AP, Brazil;
prs_ufal@hotmail.com
4Cann10 Brazil—Chief Operations Officer, Regis Bitencourt Highway, 1962, Cooperativa,
Embu das Artes 06818-300, SP, Brazil; dennys@cann10.com
5Centro IFAPA “Las Torres-Tomejil”, Ctra. Sevilla-Cazalla, Km. 12, 2, Alcaládel Río, 41200 Sevilla, Spain
*Correspondence: ivanf.garcia@juntadeandalucia.es (I.F.G.-T.); campostenator@gmail.com (E.C.)
Abstract:
The main characteristic of Cannabis sativa L. is the production of compounds of medicinal
interest known as phytocannabinoids. Environmental factors and crop management practices are
directly related to the yield of these compounds. Knowing how these factors influence the pro-
duction of phytocannabinoids is essential to promote greater metabolite yield and stability. In this
review, we aim to examine current cannabis agronomic research topics to identify the available
information and the main gaps that need to be filled in future research. This paper introduces the
importance of
C. sativa
L., approaching state-of-the-art research and evaluating the influence of crop
management and environment conditions on yield and phytocannabinoid production, including
(i) pruning
; (ii) light and plant density; (iii) ontogeny; (iv) temperature, altitude, and CO
2
concentra-
tion;
(v) fertilization
and substrate; and (vi) water availability, and presents concluding remarks to
shed light on future directions.
Keywords: Cannabis sativa; crop management; CBD; THC; hemp; marijuana; environmental factors
1. Introduction
Legal restrictions in the last decades [
1
] have prevented the progress of academic
research involving Cannabis sativa L. These conditions have resulted in a scarcity of science-
based information on C. sativa L., which is peculiar considering that it is one of our oldest
crops with a rich usage history by humankind [
2
,
3
], and its domestication dates back to
prehistoric times [
3
5
]. Samples of microfossil pollens indicate that its origin was in Tibet
at least 19.6 million years ago [
6
]. Historically, several cultures and societies have reported
medicinal applications [
4
,
7
], and it is currently used for the treatment of several diseases,
such as epilepsy, Parkinson’s disease, and chronic pain [
7
], due to its therapeutic safety and
efficacy, as recent clinical trials have demonstrated [
8
]. This species has several uses due to
the production of fibers, seeds, and secondary metabolites that have industrial value for a
myriad of production chains [9].
In recent years, the medicinal applications of C. sativa L. have gained repercussions
worldwide. The United Nations (UN) recently removed the species from the highly dan-
gerous substances list and of low medicinal application [
10
], and this change recognizes
the plant’s medicinal value, which may allow additional scientific advances [
11
]. The
Agronomy 2022,12, 1492. https://doi.org/10.3390/agronomy12071492 https://www.mdpi.com/journal/agronomy
Agronomy 2022,12, 1492 2 of 30
medicinal value of this species is related to the production of secondary metabolites called
phytocannabinoids, molecules produced in abundance by this species, which are present
in their acidic form in plant tissue [
12
14
]. About 150 phytocannabinoids have been re-
ported in the literature, and the most prevalent in many varieties are CBDA, CBGA, and
THCA; this latest one, in its decarboxylated form (THC), being responsible for psychotropic
effects. [12,13,15].
Cannabis sativa L., popularly known as cannabis (canábis or cânabis in Portuguese),
is classified as hemp or marijuana based on its THC content. The USDA (United States
Department of Agriculture) stated that marijuana contains from 3% to 15% THC (dry
weight), whilst hemp has less than 1%. In the European Union (EU), although each
member-state has different cannabis legislation, all of the EU members basically distinguish
hemp from marijuana using the 0.2% threshold for THC concentration (Regulation (EU) n
º
1307/2013), although this level of THC in cannabis is insufficient to induce psychotropic
effects [
9
]. Although the classification between hemp and marijuana is based on legal
convention, different studies indicate that the discrimination between these varieties is not
limited to cannabinoid biosynthesis but can be monitored across the whole genome [15].
Cannabis is a herbaceous, annual [
13
,
16
], dioecious species that can produce mo-
noecious plants [
17
]. Morphologically, the inflorescences of male dioecious plants are
characterized by hanging panicles with few or no leaves, and inflorescences of female
plants bear racemes with leafy bracts [
17
,
18
]. Female plants have the highest phytocannabi-
noid production due to a higher density of glandular trichomes, where these compounds
are synthesized and stored [
19
22
]. In addition, male and hermaphroditic plants have
reduced floral biomass and, thus, reduced phytocannabinoid yield [23].
Male and female plants of the same variety have been reported to produce sim-
ilar amounts of cannabinoids [
24
28
]. However, other studies have shown the oppo-
site
[23,29,30]
. According to metabolomic analyses, the female floral tissue from different
genotypes averaged 3.5% phytocannabinoids, while male floral tissues from different
genotypes averaged less than about 1% total phytocannabinoids [
23
]. According to the
literature, male and female plants of some varieties may have similar concentrations of
cannabinoids; however, cannabis cultivation for medicinal purposes is carried out with
female plants. Furthermore, sowing male plants for phytocannabinoid production is un-
common due to the female plants’ pollination, diverting phytocannabinoid production to
seed development [21,31].
There is a misunderstanding that cross-pollination changes the chemotype of the
plant. However, this change only appears in seeds resulting from cross-pollination and
not in the pollinated plant. The problem with pollination occurring in crops intended for
the production of phytocannabinoids is that the energy is shifted to seed production, not
cannabinoid production [
32
]. Feder et al. (2021) [
31
] observed that pollination resulted in a
significant decrease in the overall total phytocannabinoid concentration in inflorescences.
The THC-rich chemovar female exhibited an average 75% decrease, while CBD-rich females
showed a 60% decrease in phytocannabinoid content after fertilization [31].
Pollination prevention stimulates the formation of new flowers, increasing phyto-
cannabinoid production [
33
,
34
]. Although male plants tend to be larger and bloom before
female plants, it is difficult to distinguish them during the vegetative phase [
21
]. The most
common way to differentiate female plants from male plants is by analyzing the anatomy
of the inflorescences, although some genotypes develop solitary internode flowers at early
stages of development, making it possible for early sexual differentiation [35].
To avoid pollination, one option is to remove male plants as they appear. Another
alternative is to prevent the presence of male plants by using vegetative propagation,
ensuring that the mother plant is female. When cultivation is carried out under ideal
conditions, it is unlikely that there is a change in the sexual expression of clones from the
mother plant. However, care should be taken with the prolonged life of the mother plant
due to the occurrence of mutations and somaclonal variations that can decline the vigor and
Agronomy 2022,12, 1492 3 of 30
phytocannabinoid content in clones compared to the original mother plant [
32
]. Another
option is to carry out seminiferous propagation with feminized seeds [36].
Although it is practically unanimous among scientists to consider flowers or inflores-
cences as the product of interest for medicinal use [
12
,
13
,
21
], recently a paper was published
reporting that these structures should be considered as fruits or infructescence, considering
that the flowers senesce and turn into fruits that, in turn, ripen [
37
]. However, it should be
noted that parthenocarpy consists of the growth of the ovary into seedless fruit without the
occurrence of pollination [
38
]. The structures produced by this crop have complex floral
structures (such as stigmas) (Figure 1) that support their identification as flowers. It should
be considered that, when the flower reaches the ideal harvest point it presents an advanced
stage of senescence although it does not mature.
Agronomy 2022, 12, x FOR PEER REVIEW 3 of 30
mother plant. However, care should be taken with the prolonged life of the mother plant
due to the occurrence of mutations and somaclonal variations that can decline the vigor
and phytocannabinoid content in clones compared to the original mother plant [32].
Another option is to carry out seminiferous propagation with feminized seeds [36].
Although it is practically unanimous among scientists to consider flowers or
inflorescences as the product of interest for medicinal use [12,13,21], recently a paper was
published reporting that these structures should be considered as fruits or infructescence,
considering that the flowers senesce and turn into fruits that, in turn, ripen [37]. However,
it should be noted that parthenocarpy consists of the growth of the ovary into seedless
fruit without the occurrence of pollination [38]. The structures produced by this crop have
complex floral structures (such as stigmas) (Figure 1) that support their identification as
flowers. It should be considered that, when the flower reaches the ideal harvest point it
presents an advanced stage of senescence although it does not mature.
Figure 1. Cannabis inflorescence with emphasis on stigmas.
The majority of researchers and growers work under the common misconception that
cannabis is a short-day plant [39–41]. However, short-day plants are ones that only
blossom after being exposed to light periods shorter than a certain critical length. Tournois
(1912) demonstrated that flowering in this species was hastened by short days and
delayed by long days [42], which is not in accordance with the definition of short-day
plants. According to Spitzer-Rimon et al. (2019) [35], the effect of short photoperiods on
cannabis florogenesis is not flower induction, but rather a dramatic change in shoot apex
architecture to form an inflorescence structure. The development of solitary flowers
clearly indicates that the plant at this stage cannot be defined as vegetative or
noninductive in the classical sense. Therefore, the flower induction of solitary flowers is
probably age-dependent and is controlled by internal signals, but not by photoperiod [35].
The purpose of this study is to conduct a thorough review of the crop management
and abiotic factors affecting phytocannabinoid production in cannabis. The review aims
to summarize the current scientific understanding of the factors influencing cannabinoid
productivity and, hopefully, to inform the development of management protocols that
favor quality and productivity, as well as new studies to address current knowledge gaps.
With this end, the paper is structured in five sections. Section 1 introduces the importance
and main characteristics of cannabis. Section 2 approaches the methodology to develop
this review and state-of-the-art research that evaluates the influence of management and
the environment on the productivity of phytocannabinoids. The Section 3 assesses the
factors influencing phytocannabinoid production and crop management techniques
Figure 1. Cannabis inflorescence with emphasis on stigmas.
The majority of researchers and growers work under the common misconception
that cannabis is a short-day plant [
39
41
]. However, short-day plants are ones that only
blossom after being exposed to light periods shorter than a certain critical length. Tournois
(1912) demonstrated that flowering in this species was hastened by short days and delayed
by long days [
42
], which is not in accordance with the definition of short-day plants.
According to Spitzer-Rimon et al. (2019) [
35
], the effect of short photoperiods on cannabis
florogenesis is not flower induction, but rather a dramatic change in shoot apex architecture
to form an inflorescence structure. The development of solitary flowers clearly indicates
that the plant at this stage cannot be defined as vegetative or noninductive in the classical
sense. Therefore, the flower induction of solitary flowers is probably age-dependent and is
controlled by internal signals, but not by photoperiod [35].
The purpose of this study is to conduct a thorough review of the crop management
and abiotic factors affecting phytocannabinoid production in cannabis. The review aims
to summarize the current scientific understanding of the factors influencing cannabinoid
productivity and, hopefully, to inform the development of management protocols that
favor quality and productivity, as well as new studies to address current knowledge gaps.
With this end, the paper is structured in five sections. Section 1introduces the importance
and main characteristics of cannabis. Section 2approaches the methodology to develop this
Agronomy 2022,12, 1492 4 of 30
review and state-of-the-art research that evaluates the influence of management and the
environment on the productivity of phytocannabinoids. The Section 3assesses the factors
influencing phytocannabinoid production and crop management techniques applied in
cannabis cultivation, which include (i) pruning; (ii) light and plant density; (iii) ontogeny;
(iv) temperature, altitude, and CO
2
concentration; (v) fertilization and substrate; and
(vi) water
availability. Finally, Section 4presents the concluding remarks and sheds light
on future directions.
2. State-of-the-Art Research That Evaluates Crop Management and Abiotic Factors
That Affect Phytocannabinoid Production
To develop this review, a search was carried out in the online databases of Google
Scholar, Web of Science, and Scopus in order to access research that evaluated factors
related to management and the environment in the production of phytocannabinoids. The
criterion used to search for the articles was based on the following keywords: (hemp or
cannabis + cannabinoids
+ pruning, nutrient, fertilizer, light, light quality, light intensity,
water availability, substrate, ontogeny, plant density, temperature, altitude, relative humid-
ity, CO
2
, management). In addition to searching the databases, the bibliographic references
of all the articles found were analyzed, and the cited articles that were related to this review
were included.
We selected 78 articles that evaluated phytocannabinoid production in relation to prun-
ing, ontogeny, light, water availability, temperature, altitude, nutrition, and substrate. The
criteria for including the articles in the review was based on the chemical analysis of phyto-
cannabinoids by the study in order to access the influence of management and abiotic factors
on the productivity of these compounds. However, some studies (41, 66, 88, 98 and 100)
that did not evaluate the effect of management or abiotic factors on the production of phyto-
cannabinoids were cited because the results obtained could contribute to the development of
future researchers. These studies were not counted in the 78 selected articles.
Among the 78 articles selected, 142 evaluated 2 concomitant factors (Figure 2A). The
results obtained in these articles are independently reported in the respective sections
of this review. It was observed that some articles carried out evaluations that were not
explicit in the title or in the keywords, such as studies 114, 115 and 125, whose main focus
was to evaluate the availability of nutrients in phytocannabinoid productivity but also
performed analyses related to ontogeny. The Figure 2B shows the number of publications
that evaluated the influence of a specific management or abiotic factor on the production of
phytocannabinoids. This Figure corresponds to the evaluations observed in the 76 selected
articles. According to Figure 2B, it is possible to observe that the studies that evaluated
the influence of management and abiotic factors on the production of phytocannabinoids
focused on evaluations of the influence of ontogeny, light, nutrition, and plant density.
Figure 2C shows the number of publications related to the influences of management and
abiotic factors on phytocannabinoid productivity over the years. According to the Figure, it
is possible to observe that the number of publications increased significantly from 2018. In
recent years, the legalization of medicinal cannabis in some countries has led to an increase
in the number of publications, such as in Canada, Israel, and some states of the EUA, places
that contributed the most to the number of publications (Figure 2D).
Agronomy 2022,12, 1492 5 of 30
Agronomy 2022, 12, x FOR PEER REVIEW 5 of 30
Figure 2. (A) Number of publications that evaluated two factors related to management and/or
environment in the production of cannabinoids; (B) Number of publications that evaluated the
management and/or environment in the production of cannabinoids; (C) Analysis of cannabis.
literature about crop management and abiotic factors that affect cannabinoid production in terms
of years and number of publications published; (D) Analysis of literature in terms of country and
number of publications published.
3. Phytocannabinoids Production and Management Techniques in Cannabis
Cultivation
Plant secondary metabolites are a set of molecules that are part of a mechanism in
plants that responds to variations in abiotic conditions. These conditions can influence the
biosynthesis of secondary metabolites, inducing changes in the yield, metabolite profile,
and metabolite concentration produced by the plant [43]. In addition, management
techniques can be an alternative to promote an increase in secondary metabolites.
However, a plant’s response to environmental factors and management varies according
to the species, genotype, and metabolic route responsible for the production of secondary
metabolites [43,44].
Thus, the cultivation of medicinal plants for the extraction of secondary metabolites
requires knowledge of the genotype responses under different conditions, as the abiotic
factors considered ideal for plant development may not be the same ones that promote an
increase in secondary metabolites synthesis. It is important to emphasize that, under
stress-induced conditions, the biosynthetic pathway can be stimulated, causing an
increase in the production of these molecules by the plant. According to Haney and
Kutscheid [45], the concentration of phytocannabinoids had a strong environmental
control, and stress conditions (such as nutrient deficiency, competition with other plants,
and reduced water availability) could increase phytocannabinoid production.
For centuries, humanity has explored such physiological adjustments in plants as a
way to increase the biosynthesis of secondary metabolites, which are the source of
pharmaceutically active ingredients used in the production of medications by the
pharmaceutical industry or in the direct use of plants as herbal medicines to treat and/or
0
3
6
9
12
15
18
21
Ontogeny + Nutrition
Ontogeny + Pruning
Ontogeny + Ligth
Ontogeny + Plant density
Nutrition + Substrat e
Ligth + nutrition + temp.
0123456
A
Number of publications
1968
1971
1970
1975
Ontogeny
Ligth
Nutrition
Plant density
Pruning
Temperature
Water availability
Altitude
Substrate
0 5 10 15 20 25 30
B
Number of publications
USA
Israel
Canada
Others
Italy
Germany
Australia
Belgium
Brazil
Greece
Japan
Spain
0 4 8 12 16 20 24
D
Number of publications
C
1968
2016
1988
1987
1985
1983
1981
1980
1978
1977
1970
1971
1975
1997
2020
2008
2011
2006
2012
2017
2018
2019
2021
Number of publications
Year
Figure 2.
(
A
) Number of publications that evaluated two factors related to management and/or
environment in the production of cannabinoids; (
B
) Number of publications that evaluated the
management and/or environment in the production of cannabinoids; (
C
) Analysis of cannabis.
literature about crop management and abiotic factors that affect cannabinoid production in terms
of years and number of publications published; (
D
) Analysis of literature in terms of country and
number of publications published.
3. Phytocannabinoids Production and Management Techniques in
Cannabis Cultivation
Plant secondary metabolites are a set of molecules that are part of a mechanism in
plants that responds to variations in abiotic conditions. These conditions can influence
the biosynthesis of secondary metabolites, inducing changes in the yield, metabolite pro-
file, and metabolite concentration produced by the plant [
43
]. In addition, management
techniques can be an alternative to promote an increase in secondary metabolites. How-
ever, a plant’s response to environmental factors and management varies according to
the species, genotype, and metabolic route responsible for the production of secondary
metabolites [43,44].
Thus, the cultivation of medicinal plants for the extraction of secondary metabolites
requires knowledge of the genotype responses under different conditions, as the abiotic
factors considered ideal for plant development may not be the same ones that promote
an increase in secondary metabolites synthesis. It is important to emphasize that, under
stress-induced conditions, the biosynthetic pathway can be stimulated, causing an increase
in the production of these molecules by the plant. According to Haney and Kutscheid [
45
],
the concentration of phytocannabinoids had a strong environmental control, and stress
conditions (such as nutrient deficiency, competition with other plants, and reduced water
availability) could increase phytocannabinoid production.
For centuries, humanity has explored such physiological adjustments in plants as a
way to increase the biosynthesis of secondary metabolites, which are the source of pharma-
ceutically active ingredients used in the production of medications by the pharmaceutical
industry or in the direct use of plants as herbal medicines to treat and/or cure diseases [
44
].
Agronomy 2022,12, 1492 6 of 30
Biotechnological tools (such as omics-based methods) can contribute to the understanding
of the biosynthesis of these compounds in plants cultivated under different stimuli, in
addition to assisting in the development of varieties with specific chemical characteris-
tics [11,15].
Considering the high therapeutic potential of phytocannabinoids, the knowledge of
how management and abiotic factors influence the productivity of these molecules can help
in the development of cultivation protocols that provide greater productivity. According to
Danziger and Bernstein (2021) [
46
], secondary metabolite production in cannabis was very
high compared to other plants. This crop presented up to 16% secondary metabolites in dry
weight, equivalent to 3.2% of the fresh weight, when compared to 0.046% in roses, 0.4% in
sage, 0.8% in sweet basil [
46
], and 0.1–2% in hops [
47
]. According to Bevan
et al. (2021)
[
48
],
the inflorescence yield had significant correlations with the aboveground plant fresh weight,
plant growth index, and root dry weight, which indicated that larger plant size could result
in a higher inflorescence yield.
Despite the inflorescences having the highest concentration of cannabinoids, higher
flower yield is not always related to higher yields of these compounds. When the final prod-
uct is a secondary metabolite, this prediction requires greater caution, given the significant
influence of the environment. There appears to be an inverse relationship between inflores-
cence and phytocannabinoid yield, with phytocannabinoid concentrations decreasing as
the plant inflorescence yield increases [
48
], probably due to the dilution effect. Coffman
and Gentner (1975) [
49
] observed that, as plant height increased, the concentration of THC
in the leaf tissue generally decreased. However, other floral characteristics besides the
inflorescence biomass (such as the number of flowers, length, width, uniformity, perimeter,
and distribution) should be explored, as they can influence the final productivity [50].
It is important to know the maximum performance of cannabis genotypes cultivated
under different environments to verify the interaction of genotypes and environments, as
well as whether the applied management is conditioning an adequate yield. The production
of secondary metabolites widely varies in their concentrations and phytochemical profiles,
even within the same genotype cultivated under controlled conditions, as well as even
between different seasons and cycles. In a study developed by Fischedick and Hazekamp
(2010) [
51
], under strictly controlled indoor growing conditions and in a standard lot, an
average variation was observed of 7.6% and 5.5% for phytocannabinoids and 11% and 4%
for terpenoids in Bedrocan and Bedica cannabis plant varieties, respectively.
One main difficulty in the production of plant secondary metabolites is the standard-
ization of these compounds. Herbal products can never be perfectly standardized for
active component content; however, variability in chemical composition may also be a
cause of concern for medicinal users [
51
] because the maintenance of the concentration and
profile of secondary metabolites is important to maintain the stability of the treatment. The
knowledge of how abiotic factors and management techniques influence the production of
phytocannabinoids can help in the development of management protocols that provide
greater productivity, quality and stability in the production of these compounds. Know-
ing the factors influencing the biosynthesis and formation of these compounds becomes
essential to provide appropriate conditions enabling high yield.
Although genotype has a significant influence on phytocannabinoid production
[26,5153]
,
its concentration is highly dependent on biotic and abiotic factors [
54
]. The majority of agro-
nomic research conducted under field conditions has focused mostly on fiber production and
yield [
55
,
56
]. Information that has been published on hemp production allows some parallels
to be drawn. Despite being used for phytocannabinoid production [
33
,
57
], hemp is a field-
grown crop that traditionally has been bred selectively for fiber or seed production, rather than
for flower and secondary metabolite production [
55
,
58
]. However, despite fiber and seeds
being the main products, a growing interest in the valorization of hemp phytocannabinoids
has appeared [
55
,
57
], which can be extracted from the leaves and the inflorescences of
plants cultivated for the production of fibers and seeds, although crop development for
Agronomy 2022,12, 1492 7 of 30
dual purposes initially would reduce the final concentration of cannabinoids or the fiber
quality [57,59].
Cannabis can be grown in open fields (outdoor cultivation), in protected environments
(such as greenhouses), or in controlled growing rooms (indoors) [
21
,
60
]. Field cultivation
with or without a protected environment has been proved to be efficient in promoting
increased phytocannabinoid yields, as demonstrated by García-Tejero et al. (2020) [
61
]
in cannabis cultivation covered by a polyethylene macrotunnel. The main advantage of
outdoor cultivation is the reduction in production costs, as indoor cultivation requires
artificial lighting, ventilation, and temperature control, making the costs quite high for
both implementation and maintenance. In protected cultivation, it is possible to produce
up to six harvests per year and to scale them up [
39
,
62
], making it possible to obtain fresh
material throughout the year [
41
]. In comparison, outdoor cultivation allows only one or
two harvests per year, depending on the environmental conditions and genotype [
16
]. In
addition, the greater stability of the chemical profile provided by controlling environmental
conditions is an important advantage of protected cultivation, and this fact should be
considered when the main product to be obtained is the phytocannabinoids [21].
Most commercial production is carried out in greenhouses or growth chambers with
supplementary or single-source electric lighting [
41
,
63
,
64
]. Even though greenhouses allow
the use of natural light, complementary lighting is required due to the plants’ photosensi-
tivity, although electricity consumption is still significantly lower when compared to the
opaque structures used for indoor cultivation [
40
]. In equatorial and tropical countries
such as Brazil, the demand for artificial lighting is lower due to the greater natural light
incidence, an essential factor for reducing production costs. In fact, 80% of the arable
land in Brazil is suitable for producing this crop [
65
]. When this crop is developed under
controlled conditions, it can be carried out in hydroponic cultivation, in pots of different
sizes, and to a lesser extent, directly in the soil. In this case, during crop development, it is
common to transplant into larger containers. The change to larger containers depends on
the final size of the plant; when the objective is to produce smaller plants, cultivation is
directly in the final container (<3 gallons) [66].
Among the techniques applied by industries and growers are, stand out pruning, de-
foliation, the use of techniques such as Sea of Green (SoG) or Screen of Green (ScroG), light
supplementation, transplanting plants to larger containers, increasing CO
2
and others. In
addition, knowledge of basic factors related to management, such as water availability, min-
eral nutrition, lighting, temperature, spacing, and substrate, helps in the development of
management protocols that promote the maximum productive performance of the genotypes.
The scarcity of basic information curbs the development of high-quality, standardized
production for the growing of cannabis for biomedical purposes [
2
]. In this agreement, few
studies have been focused on phytocannabinoid production, and hence, evaluating the
effects of external drivers and management techniques on phytocannabinoid production
is essential for understanding those agronomic treatments and methodologies that can be
applied to suit the final crop and products [67].
Several books on cannabis crop management techniques have been published. How-
ever, it is important to emphasize that many of the techniques described are based on
constructions of common knowledge, lacking systematic studies to prove their efficiency
and efficacy. Producers have access to internet-based horticulture guides and information
online, but most of this information is not based on scientific research [68].
The techniques known as Sea of Green (SoG) and Screen of Green (ScroG) are reported
in several cultivation manuals and books and are commonly applied to increase yield [
69
].
The Screen of Green technique, also known by the acronym ScroG, is widely used by
large companies and consists of using a screen on the surface of the plants to support the
branches, allowing the formation of a uniform canopy. As the plants grow, the branches
are guided through the screen to form a uniform cover. Flowering is induced after the
shoots cover the entire screen [
69
]. This technique allows more uniform light distribution
among the canopy of the plants since the tendency is for the lower branches to receive
Agronomy 2022,12, 1492 8 of 30
less incident light [
69
,
70
]. Therefore, these branches may have lower phytocannabinoid
concentrations [71].
The Sea of Green technique, or SoG, consists of a higher dense cultivation [
69
,
72
] that
aims to produce as many plants as possible per square meter, so small pots are used with
greater depth and less width. The objective of this technique is to accelerate the vegetative
phase and induce early flowering. As soon as the clones reach the desired height, they
are induced to bloom [
69
]. The association between SoG and pruning (removal of side
shoots) may favor the automatization of the harvest process (e.g., using electric tumbler
blade trimmers). In this case, a lower yield per plant can be overcome by a higher yield
per area [
72
]. The main differences between these techniques are the use of the screen, the
duration of the vegetative cycle (which is longer for the ScroG technique), and the number
of plants grown per area. In ScroG, larger spacing is used because the objective is to induce
the formation of lateral branches [
69
]. However, there is practically no research that has
evaluated the development of the plants and the impact on their productivity when using
these techniques.
3.1. Pruning
Crop management techniques that alter the morphology of plants are commonly
applied in the cultivation of cannabis, such as pruning the main stem once, twice, or even
three times; the removal of leaves, which can range from the removal of old leaves up to
total defoliation; the removal of branches; the use of trellises for conduction (such as the
ScroG technique); or a combination of all the previously described techniques [
46
]. Few
studies have evaluated the influence of pruning on the yield of secondary metabolites.
In general, the main objectives of pruning are to promote greater development of the
branches and limit the plants’ sizes [
40
], but it also provides other benefits (Figure 3).
Pruning induces hormonal changes in sink-to-source ratios that explain the physiological
and metabolic changes, alter the morphology of the plant, and consequently have an impact
on microclimatic conditions [46,73].
Agronomy 2022, 12, x FOR PEER REVIEW 8 of 30
support the branches, allowing the formation of a uniform canopy. As the plants grow,
the branches are guided through the screen to form a uniform cover. Flowering is induced
after the shoots cover the entire screen [69]. This technique allows more uniform light
distribution among the canopy of the plants since the tendency is for the lower branches
to receive less incident light [69,70]. Therefore, these branches may have lower
phytocannabinoid concentrations [71].
The Sea of Green technique, or SoG, consists of a higher dense cultivation [69,72] that
aims to produce as many plants as possible per square meter, so small pots are used with
greater depth and less width. The objective of this technique is to accelerate the vegetative
phase and induce early flowering. As soon as the clones reach the desired height, they are
induced to bloom [69]. The association between SoG and pruning (removal of side shoots)
may favor the automatization of the harvest process (e.g., using electric tumbler blade
trimmers). In this case, a lower yield per plant can be overcome by a higher yield per area
[72]. The main differences between these techniques are the use of the screen, the duration
of the vegetative cycle (which is longer for the ScroG technique), and the number of plants
grown per area. In ScroG, larger spacing is used because the objective is to induce the
formation of lateral branches [69]. However, there is practically no research that has
evaluated the development of the plants and the impact on their productivity when using
these techniques.
3.1. Pruning
Crop management techniques that alter the morphology of plants are commonly
applied in the cultivation of cannabis, such as pruning the main stem once, twice, or even
three times; the removal of leaves, which can range from the removal of old leaves up to
total defoliation; the removal of branches; the use of trellises for conduction (such as the
ScroG technique); or a combination of all the previously described techniques [46]. Few
studies have evaluated the influence of pruning on the yield of secondary metabolites. In
general, the main objectives of pruning are to promote greater development of the
branches and limit the plants’ sizes [40], but it also provides other benefits (Figure 3).
Pruning induces hormonal changes in sink-to-source ratios that explain the physiological
and metabolic changes, alter the morphology of the plant, and consequently have an
impact on microclimatic conditions [46,73].
Figure 3. Main pruning techniques applied in cannabis cultivation (cover pruning, defoliation and
branch removal) with an approach to the benefits and possible effects on productivity.
Figure 3.
Main pruning techniques applied in cannabis cultivation (cover pruning, defoliation and
branch removal) with an approach to the benefits and possible effects on productivity.
In cannabis management, it is common to perform a cover pruning (known as topping)
or removal of the apical meristem (known as fimming) to increase the inflorescence yield
per plant [
74
]. In these processes, the upper apex of the stem is removed or damaged to
break the apical dominance [
75
]. The difference between the two techniques is the length
of the pruned branch [
74
]. The removal of the apical meristem changes hormone balances
(e.g., auxin and cytokinin) in the plant, stimulating the development of side shoots by the
Agronomy 2022,12, 1492 9 of 30
relieving of apical dominance [
72
]. The removal of all the leaves and secondary branches
from the bottom third of the plant is a pruning technique widely applied by growers and
is known as ’lollipopping’. This technique aims at directing the plant resources to the top
inflorescences and improving the air circulation as an aid to pest management [46,76].
Folina et al. (2020) [
75
] observed in two European hemp cultivars (‘Fedora 23’ and ‘Fu-
tura 75’) an increase in CBD (22.7% and 18.1%, respectively) and inflorescence yields (24.5%
and 12.9%, respectively) in plants that were pruned by topping. It was also observed in
other research that topped plants produced significantly more inflorescence biomass (13.5%)
than unpruned plants, but the total CBD concentration was not significantly affected [
72
].
Another study showed that pruning (topping) did not influence the productivity of flowers
and phytocannabinoids [
77
]; however, this response may have been due to the high density
of plants cultivated per area, which makes the branching of pruned plants difficult.
Danziger and Bernstein (2021) [
73
] investigated the effects of eight different plant
architecture modulations in a ‘Topaz’ cultivar containing high CBD (8–16%) and low THC
(<1%) levels. According to the results, among the eight treatments, defoliation (performed
three weeks before harvest) and the removal of the branches and leaves from one-third
of the plant (at the transition to short-day) promoted an increase in the CBDA, THCA,
CBDVA, and THCVA concentrations in relation to the controls. However, double pruning
(pruning the plants twice in the vegetative stage) was the treatment that promoted the
greatest increase in inflorescence yields and in phytocannabinoid concentrations (CBDA,
CBDVA, THCA, THCVA, and CBCA were about 59, 61, 50, 53, and 59%, respectively) [
73
].
The higher light incidence inside the canopy may have contributed to the increase in yield,
as the authors observed that the double pruning technique promoted an increase in the
light intensity at different heights along the plants. The technique known as ’lollipopping’
did not influence the productivity of flowers or phytocannabinoids [73].
In another study, Danziger and Bernstein (2021) [
46
] evaluated how eight plant archi-
tecture modulation treatments involving leaf removal (defoliation) or pruning (topping
or branch removals) affected the phytocannabinoid profiles and spatial standardization
in plants of two cannabis cultivars, ‘Fuji’ and ‘Himalaya’. Both cultivars were high THC
(10–16%) and low CBD (<0.1%) producers. The responses in relation to the technique
applied and the phytocannabinoid productivity varied according to the variety and the
evaluated phytocannabinoid. For example, the production of CBDVA increased with the
application of single pruning in the ‘Fugi’ variety, while it decreased in the ‘Himalaya’
variety [46]. However, the changes in phytocannabinoid concentrations and inflorescence
yields were small (except for the ‘1
º
branch removal’ treatment, which greatly reduced both
parameters). When compared to the other treatments, the ‘1
º
branch removal’ treatment
induced a decrease in the productivity of CBDVA, CBDA, THCA, CBGA, THCVA, THC,
and CBC in both varieties. The ‘lollipopping’ technique promoted a slight increase in the
production of CBDVA in the ‘Fugi’ variety and CBCA in the ‘Himalaya’ variety. The prun-
ing techniques evaluated in this study were not recommended to promote increased flower
productivity, as both varieties showed high flower productivity in the control treatment
(unpruned plants) [46].
The choice of pruning technique should consider whether the objective is to increase yield,
modify the microclimate, or promote greater uniformity in the production of phytocannabinoids
in the plant. Thus, it is important to assess whether the operational cost (such as labor, tools,
and materials) increases yield at a level that brings financial return or promotes greater final
quality due to the increased uniformity in the concentration of the phytocannabinoids.
3.2. Effects of Light
Both in general and in the horticultural industry, growers use different radiation spec-
tra and intensities to manipulate plant morphology, flowering, and secondary metabolite
production [
78
]. Therefore, studies evaluating the effects of radiation on different genotypes
are essential to provide a suitable environment for growth, ensuring the best genotype
Agronomy 2022,12, 1492 10 of 30
performance and avoiding the superfluous use of electricity for artificial lighting while still
obtaining high yields.
The modification of the light spectrum during the development of cannabis is a
widespread technique among growers that has been shown as an alternative to manipulate
the production of phytocannabinoids [
79
81
]. Table 1summarizes the most relevant studies
in relation to the effects, advantages, and disadvantages of different sources of light and
radiation for phytocannabinoid production.
Table 1.
The most relevant studies in relation to the effects, advantages, and disadvantages of different
sources of light and radiation for phytocannabinoid production (LI: light intensity).
Reference Number Objective Advantage Disadvantages
[82]
Evaluate daily doses of 0, 6.7
and 13.4 kJ m2UV-B
radiation
THC content increased
linearly with UV-B dose
radiation
Not reported
[83]
Evaluate supplemental UV
radiation ranging from 0.01 to
0.8 µmol m2s1.
Not reported
Decreased inflorescence yield
with increasing UV exposure
level; no induced
enhancements to THC, CBD,
and CBG
[84]
UVA+UVB radiation and LI
ranging from 350 to
1400 µmol m2s1
Inflorescence yield increased
linearly from 19.4 to
57.4 g plant1as LI increased
from 350 to
1400 µmol m2s1.
No effect in phytocannabinoid
concentration and
inflorescence yield with
supplemental UV. UV
radiation decreased harvest
index. LI had no effect on
cannabinoid content
[85]Evaluate supplemental UV
radiation Increased THC concentration Decreased inflorescence yield
[80]Evaluate LEDs and HPS
lamps
LED lamps (with highest
proportion of blue light)
provided high flower
productivity and increased
CBGA accumulation (up to
400%)
Decreased concentrations of
other phytocannabinoids
(such as THCA, CDBA or
CBCA) by up to 40% under
LED lamps.
[79]Evaluate LEDs and HPS
lamps
HPS lamps induced higher
flower yield. LEDs provided
highest CBG concentration
(increasing by about 200%
with a higher incidence of
blue light).
HPS lamps induced lower
THC and CBD production
(about 30%) than when
employing LED lamps.
[86]Evaluate LI ranging from 120
to 1800 µmol·m2·s1.
Inflorescence yields increased
linearly from 116 to 519 g m
2
(i.e., 4.5 times higher the
apical inflorescence density
increased linearly as the LI
increased
No effects on the production
of any of the
phytocannabinoids measured
[87]
Evaluate the influence of
photoperiod duration (10 and
12 h)
An increase of 2 h a day of the
photoperiod doubled the
average amount of THC.
An increase of 2 h a day of the
photoperiod decreased
cannabichromene content.
Research has shown that photosynthetically active radiation (PAR) can be a key modu-
lator of biomass and phytocannabinoid production, suggesting a responsive photochemical
machinery [
88
]. However, there is little information concerning the mechanisms involved.
Khajuria et al. (2020) [
88
] observed through chlorophyll fluorescence that genotypes with
higher CBD than THC contents provided better protection to the photosynthetic machin-
Agronomy 2022,12, 1492 11 of 30
ery with the potential to tolerate light stress conditions. High THC content had a strong
negative correlation with the photochemical efficiency of PSII and reduced the values
of nonphotochemical quenching (NPQ) dependent on zeaxanthin. NPQ is a protective
mechanism responsible for dissipating excess energy from the photochemical machinery,
protecting it from photoinhibition. According to the authors, chlorophyll fluorescence
could be used as a quick tool for high-throughput cannabis-screening assays based on
phytocannabinoid content [88].
For the cultivation of cannabis under protected or indoor environments, it is common
to use artificial lighting to supply or supplement the light energy demand. Several lamp
types are used, and the most commonly used ones are high-pressure sodium (HPS) lamps,
light-emitting diodes (LEDs) [86], and fluorescent lamps [69,79].
HPS lamps are predominantly within the yellow spectrum, with low emissions in the
blue and UV regions and low red/far-red ratios, allowing an excessive lengthening of the
stem (etiolation) and, thus, reducing THC production [
78
]. HPS lamps emit high levels of
radiation in the green region, negatively affecting THC production [
78
,
79
,
89
]. About 90%
of cannabis growers in Canada for cannabinoid production use HPS lamps [
90
] because
they have a low upfront cost and high photon output [91].
LED usage in horticulture has expanded in the last decade due to the advantages they
offer over conventional light sources, such as greater energy efficiency, lower heat emission,
and longer lifetimes [
92
,
93
]. An advantage of LEDs is their high efficacy and low electrical
(operating) costs compared to HPS lamps [
91
,
93
]. LED lamps allow the supply of specific
wavelengths, facilitating research on the impact of wavelength on secondary metabolite
production [
94
], but there is relatively little information available about different radiation
intensity and quality effects on cannabis secondary metabolite composition [
92
], and some
of the results are contradictory (for example, the influence of blue light on productivity).
Westmoreland et al. (2021) [
91
] investigated the effect of the blue photon fraction
using different ranges (the lowest fraction of blue photons was 4% from HPS lamps, and it
increased to 9.8, 10.4, 16, and 20% for LEDs). The treatments did not influence the THC
and CBD yields. According to the authors, as the percent of blue light increased from 4 to
20%, the flower yield decreased by 12%. This showed that flower yield increased by 0.77%
per 1% decrease in blue photons, but the potential that changed in the fraction of other
wavelengths could have contributed to the results [91].
Namdar et al. (2019) [
92
] evaluated the development and characteristics of two
cannabis varieties, ‘CS12’ and ‘CS14’ (both strains producing high amounts of THCA),
grown under fluorescent lamps in the vegetative phase and HPS lamps in the reproduc-
tive phase (standard growing conditions) compared to production when using LED lamps
(blue:red ratio equal to 4:1). According to the authors, the plants that bloomed under LEDs
accumulated much higher amounts of CBGA (a precursor of other phytocannabinoids), with
a ratio of CBGA:THCA of 1:2. At the same time, those cultivated using HPS lamps showed a
CBGA:THCA ratio of 1:16. The authors observed pronounced inhibition of flowering under
LED light, with a decline of 40% in the total flower mass per plant as compared with HPS
lamps [
92
]. However, the variation in these results may have been due to the difference in
PAR among the treatments, which was 90
µ
mol m
2
s
1
for the LED lamps and 500
µ
mol
m
2
s
1
for the HPS lamps, which explains the higher concentration of phytocannabinoids
due to the lower production of flowers in the LED treatment (lower intensity).
Similar results were obtained by Danziger and Bernstein (2021) [80] regarding the in-
crease in CBGA production. The authors also observed an increase in CBGA accumulation
(up to 400%) in treatments with LED lamps (with the highest proportions of
blue
=47%
)
compared to HPS lamps (with lower proportions of blue
=
4.5%). In contrast, the concen-
trations of other phytocannabinoids (such as THCA, CDBA, and CBCA) decreased by up
to 40% under LED culturing, exhibiting a specific metabolite response to light spectra and a
potential for the inhibition of enzymatic activity under LED light [
80
]. Another study found
a 2.5% increase in THC content in plants supplemented with LED light with a blue and
Agronomy 2022,12, 1492 12 of 30
redder spectrum [
85
]. However, this variation in THC production may have been caused
by the higher light intensity compared to the control treatment.
Light supplementation at the bottom of the plant canopy with red–blue (R–B) and
red–green–blue (RGB) spectra impacted the phytocannabinoid, terpene, and inflorescence
yields [
64
]. Both treatments promoted changes in the metabolomic profiles of the plants, and
higher flower yields than the control treatment (without additional light) were observed.
Plants submitted to R–B supplementation showed greater homogeneity of phytocannabi-
noids in the lower and upper parts of the canopy, and plants under the RGB treatment
showed greater changes in the terpene profiles. Future studies should be carried out to
quantify the pools of phytocannabinoid precursors (isopentenyl diphosphate (IPP) and
dimethylallyl diphosphate (DMAPP)) to better understand the influence of spectral quality
on terpene and phytocannabinoid biosynthesis [64].
Magagnini et al. (2018) [
79
] evaluated the effects of three light sources (one HPS and
two types of LED) with different radiation spectra and the same PAR (
450 µmol m2s1
)
on the morphology and the phytocannabinoid content of the ‘G-170’ cannabis variety
(high THC content). According to the authors, the wavelengths in the blue and UV-A
regions could positively affect THC and CBG synthesis [
79
]. Magagnini et al. (2018) [
79
]
and Namdar et al. (2019) [
92
] both observed lower flower yields under LED lights than
under HPS lamps. Namdar et al. (2019) [
92
] found a significant difference in the total
phytocannabinoid content in plants grown with LEDs (>60% compared to HPS). Neverthe-
less, Magagnini et al. (2018) [
79
] did not observe variation in the total phytocannabinoid
concentration. The results obtained in the two studies may have been related to LEDs of
different spectra, different light intensities, or different plant genotypes.
Wei and colleagues [
95
] observed higher flower and CBD yields when using two types
of LEDs than when using HPS lamps, but the LED light intensity was higher (540 and
552 µmol m2s1
) than the HPS intensity (191
µ
mol m
2
s
1
). In another study, higher
THC production was observed in plants grown under LED light than HPS lamps [
96
].
One of the difficulties in evaluating the influence of light quality is the need to standard-
ize the light intensity and spectra (only varying the spectrum that is evaluated). This
standardization is essential to evaluate the effect of the specific spectrum.
Islam et al. (2021) [
81
] observed variations in phytocannabinoid concentrations in a
cannabis genotype grown under different light spectra. The THC concentration was higher
in all the treatments than under natural light, while the CBD concentrations showed higher
values under different spectra. The low PAR intensity of the treatments (300
µ
mol m
2
s
1
,
except for the treatment with natural light, which was not informed) may have influenced
the synthesis of phytocannabinoids and the responses to different light spectra. According
to the authors, green light played a significant role in the synthesis of CBD and CBDA, as
the far-red and UV-A spectrum (along with green) played positive and negative roles in
this process, respectively [
81
]. It is important to note that, in this study, the analyses of the
phytocannabinoids were performed on the leaves during the vegetative phase.
There are some indications that UV-B radiation influences phytocannabinoid con-
centration [
97
]. According to Pate (1983) [
97
], plants tended to accumulate more THC at
high-altitude locations due to higher UV-B radiation. Lydon et al. (1987) [
82
] evaluated the
influence of UV-B radiation on net CO
2
assimilation rate, growth, and phytocannabinoid
production in two cannabis varieties with different chemical profiles. The authors observed
that the production of THC was significantly higher in the variety with high THC. In con-
trast, for the variety that predominantly produced CBD, no modifications in the CBD levels
were observed. Thus, a positive correlation between UV-B radiation and THC production
was observed [82].
Recent works have evaluated the influence of UV light supplementation and have
presented contradictory results regarding the production of phytocannabinoids. Rodriguez-
Morrison and colleagues (2021) [
83
] evaluated the responses of two cannabis varieties, ‘Low
Tide’ (LT) and ‘Breaking Wave’ (BW), grown under supplemental UV radiation. The authors
observed that the total dry inflorescence yield derived from apical tissues decreased in
Agronomy 2022,12, 1492 13 of 30
both cultivars with increasing UV exposure levels, but the total dry inflorescence yield only
decreased in LT. The apical inflorescence dry weight decreased linearly by 78% and 69%
in LT and BW, respectively, from the lowest to highest UV levels. In another study, it was
also observed that there was no effect in phytocannabinoid concentration and inflorescence
yield in the genotype ‘Meridian’ (high THC content) with supplemental UV-A spectra [
84
].
However, Jenkins (2021) [
85
] observed a 4.1% increase in the THC concentration in two
genotypes (‘Larry OG’ and ‘Super White’) and no changes in another genotype (‘Pootie
Tang’). The UV supplementation decreased the inflorescence yield of ‘Larry OG’ [
85
].
Further research with different genotypes is required to determine if there is a combination
of UV spectrum, intensity, and time of application that would bring beneficial commercial
effects for cannabis production [83,84].
The effects of PAR levels ranging from 120 to 1800
µ
mol m
2
s
1
were evaluated
by Rodriguez-Morrison and colleagues (2021) [
86
]. The authors observed that the dry
inflorescence yield, the density of the apical inflorescence, and the harvest index increased
linearly with increasing canopy-level PPFD of up to 1800
µ
mol m
2
s
1
, while the leaf-
level net CO
2
assimilation rate saturated well below 1800
µ
mol m
2
s
1
. Moreover, the
inflorescence yield and apical inflorescence density increased linearly as the PAR levels
increased from 120 to 1800
µ
mol m
2
s
1
[
86
]. These researchers did not observe effects on
the production of any of the phytocannabinoids measured. Even under ambient CO
2
, the
linear increases in yield and the net CO
2
assimilation rate indicated that the availability
of PAR was still limited with whole-canopy net CO
2
assimilation rates at levels as high as
1800 µmol m2s1[78].
Other works have shown that the increase in PAR intensity is positively correlated
with the net CO
2
assimilation rate [
98
] and yield [
84
,
90
,
99
]. Llewellyn and colleagues
(2021) [
84
] evaluated the effects of PAR intensities ranging from 350 to 1400
µ
mol m
2
s
1
on yield and potency in the ‘Meridian’ genotype (high THC content). The authors observed
that the inflorescence yield increased linearly as the PAR intensity increased (Table 1).
Eaves et al. (2020) [
90
] observed a direct linear correlation between increased yield and
PAR up to 1500
µ
mol m
2
s
1
. The correlation between the increase in PAR intensity and
yield can be explained by the corresponding increase in the net CO2assimilation rate due
to the high PAR saturation point of cannabis, which corresponds to the PAR intensity at
which the net CO
2
assimilation rate begins to decrease. As observed by Chandra et al.
(2015) [
100
], the net CO
2
assimilation rate increased from 44% to 140% when exposed to
400–2000 µmol m2s1
according to the evaluated genotype, confirming the high light
saturation point of the plant.
Potter and Duncombe (2012) [
99
] observed that the average flower yields in the total
radiation intensity regime of 600 W m
2
, 400 W m
2
, and 270 W m
2
were 0.9 g W
1
,
1.2 g W
1
, and 1.6 g W
1
, respectively, with a higher yield at the lowest total radiation
intensity. However, in the seven varieties evaluated, the total flower yield in the lowest
total radiation intensity regime ranged from 280 to 470 g m
2
(average 422 g m
2
). The
ranges were 350–630 g m
2
(average 497 g m
2
) and 470–630 g m
2
(average 544 g m
2
)
in greater intensity regimes [
99
]. With this study, it is possible to observe the relevance
of the genotype as a response to management. Potter and Duncombe (2012) [
99
] also
observed that a higher total radiation intensity caused an increase in the total THC content.
However, it was due to the higher production of female flowers, which contain higher THC
concentrations than the leaves.
In a study developed by Bouchard (2009) [
101
], an average of 0.5 g of flowers per
watt was observed. However, this metric (installed wattage) has the advantage of negating
the effects of different fixture efficacies (
µ
mol J
1
). Since photosynthesis is considered
a quantum phenomenon, crop yield may be more appropriately related to incident or
absorbed photons and integrated over the entire production cycle (i.e., light integral;
mol m2
) in a yield metric that is analogous to the maximum quantum yield (QY: g
mol1) [86].
Agronomy 2022,12, 1492 14 of 30
The effects of radiation quality on phytocannabinoid production were evaluated by
Mahlberg and Hemphill (1983) [
89
] in a Mexican variety using colored filters to alter the
radiation spectrum. The authors concluded that the THC content in the young leaves of
plants grown in a shaded environment under solar radiation, both under red light and
under blue light, did not differ significantly from the THC content of the control plants
(grown under sunlight). However, the leaves of plants grown under green light contained
significantly lower THC levels than those grown under full solar radiation [89].
Finally, in addition to light quality and intensity, photoperiod can influence flower
and phytocannabinoid yields (Table 1) [
67
,
87
]. The number of days a plant remains under
a long photoperiod influences the size and number of flowers, as well as phytocannabinoid
yield [
41
,
102
,
103
]. There is a lack of scientific information to determine the number of
days a plant should spend in long-day lighting to maximize yield [
102
]. According to
a meta-analysis carried out by Dang et al. (2022) [
102
], the increases in floral biomass
and phytocannabinoid productivity responded differently according to the duration of
the vegetative phase (long photoperiod). Floral productivity and the duration of the veg-
etative phase showed a negative linear relationship, suggesting that shorter vegetative
growth periods are preferable to increase flower productivity. In contrast, THC and CBD
showed a negative quadratic relationship with long day length durations, suggesting
that intermediate periods of vegetative growth maximize phytocannabinoid concentra-
tions [
102
]. This information is important because it can be used to define management
methods, such as ScroG and SoG techniques, depending on the purpose of the crop (flower
or phytocannabinoid).
3.3. Plant Density
Plant density can affect the final yields of different crops. When cannabis cultiva-
tion is for phytocannabinoid production, it is desirable to lower densities, easing the
plants’ lateral branch development and increasing the number of leaves and lateral flow-
ering [
55
]. Toonen et al. (2006) [
62
] evaluated 77 indoor plantations (intended for inflo-
rescence production) in the Netherlands and observed an average of 15 plants m
2
. The
plant density used for inflorescence production is highly variable. Studies have reported
an average of 10 to
20 plants m2
[
104
], although it is also common to cultivate only one or
two
plants m2
[
105
]. It is important to consider that small and nondense plants support
homogenous light distribution and less microclimatic variations within the crop and, con-
sequently, allow greater stability in the concentration of phytocannabinoids in the plant
(less variation between the apical and basal parts of the plant) [80].
Vanhove and colleagues (2021) [
63
] evaluated density (16 or 20 plants m
2
) and light
intensity (400 W m
2
or 600 W m
2
) on the yields of four cannabis genotypes and observed
that the yield per plant of all the genotypes was higher at a density of 16 plants m
2
.
However, there was no significant difference in yields between the two densities when this
yield was expressed m
2
[
63
]. This finding indicated that light interception is a determining
factor of yield. The lower density of plants provided greater light absorption throughout
the canopy, promoting greater net CO
2
assimilation rates and yields. However, only certain
genotypes (those with a plant structure based on pronounced lateral shoot development)
showed increased flower yield when subjected to higher light intensity [63].
Vanhove et al. (2012) [
106
] also found no significant difference in flower yield per area
(g m
2
) at different densities. However, they observed higher flower yield per plant at a
lower density (12 plants m
2
) compared to a higher density (16 plants m
2
), corroborating
the results obtained by Vanhove et al. (2011) [
63
] and Benevenute et al. (2021) [
107
]. Accord-
ing to the results of Vanhove et al. (2012) [
106
], the yield m
2
did not differ significantly
between different plant densities, but significant differences did occur between different
varieties. Benevenute et al. (2021) [
107
] observed that the phytocannabinoid concentration
in flowers was not affected by plant density, and increasing the plant density decreased the
flower yield per plant but increased the total flower (per area) and CBD yields.
Agronomy 2022,12, 1492 15 of 30
García-Tejero and colleagues (2019) [
108
] evaluated the effect of plant density (33,333,
16,667, and 11,111 plants ha
1
) on the yields of two hemp varieties (‘Ermes’ and ‘Carma’).
The highest density (33,333 plants ha
1
) promoted greater yields of CBG and CBD in
the ‘Carma’ variety. In comparison, the ‘Ermes’ variety showed greater production of
CBG with intermediate spacing (16,667 plants ha
1
) and CBD in both the denser- and
the higher-spaced regimes [
108
]. Additionally, García-Tejero et al. (2020) [
61
] evaluated
both the phytocannabinoid and dry matter yields of three cannabis varieties with CBD
chemotypes (‘Sara’, ‘Pilar’, and ‘Theresa’) and two with CBG chemotypes (‘Aida’ and
‘Juani’) grown at three densities (PD1, 9.777; PD2, 7.333; and PD3, 5.866 plants ha
1
). These
authors concluded the importance of the genotypes, as well as the advantages of PD1, to
obtain the highest total phytocannabinoid yield due to the larger number of plants per
hectare and the capacity of the plants to produce side branches under these conditions [
61
].
Backer et al. (2019) [
104
] carried out a meta-analysis study of factors determining cannabis
yield. The authors observed that increasing plant density reduced yield per watt and THC
per square meter. According to the authors, plant density was not an effective predictor of
yield per square meter [104].
Taking into consideration the diversity of the information, more research is needed to
evaluate the influences of plant density and plant architecture on the cultivation of inflores-
cence and phytocannabinoid production. The development of research to determine the
impact of plant density on the productivity of phytocannabinoids and flowers is important,
but it is also necessary to evaluate the influence on the duration of the production cycle
of different varieties to know the costs involved in maintaining plants with longer cycles
(generally less densely cultivated) because the increase in productivity must compensate
for the costs involved in production.
3.4. Ontogeny
Ontogeny is the entire sequence of events involved in the development of an organ-
ism. It starts from the seed and goes through different developmental stages, such as
the seedling stage, vegetative stage, and reproductive stage, and ends in the senescence
stage [
74
]. Chemical and physiological plant variability are key factors in regulating sec-
ondary metabolite production related to medicinal product standardization that have not
been studied as of yet. This variation can be regulated both in the organs and during
development at the ontological level [
71
]. The concentration of phytocannabinoids changes
with the development of the plant [
28
,
30
,
53
,
109
113
]. Table 2summarizes the most relevant
studies in relation to ontogeny cannabis research and the results about the influence of
phytocannabinoid production.
Agronomy 2022,12, 1492 16 of 30
Table 2.
Nonexhaustive summary about the most relevant studies in relation to the effects of ontogeny
on phytocannabinoid production.
Reference Number Objective Results
[53]Evaluate the evolution of major
phytocannabinoids during cannabis growth
Maximum concentrations of THCA and CBDA in
flowers were attained at the end of flowering, during
senescence, but varied according to chemotype.
[72]Investigate the impact of harvest time on
phytocannabinoid yield.
Dry weight of inflorescence plant1increased from
7.7 g at fifth week of flowering to a maximum of 25.1
g at eleventh week of flowering; the CBDA yield
increased from 415 mg plant1at the fifth week of
flowering, reaching a maximum (an average of
1300
·
mg plant
1
) at the ninth and eleventh weeks of
flowering
[71]
Evaluate the differences in the concentration of
phytocannabinoids between the apical and basal
parts of the plant
THC, CBD, CBG, CBC, and THCV showed a
significant increase in the parts of the plant close to
the apex; only CBT had an inverse correlation with
plant height (highest concentration was in the lower
organs); the levels of CBC and CBG were similar or
higher in the inflorescence leaves than in flowers.
[114]
Evaluate the potential of nutritional
supplementations (humic acids and inorganic N, P,
and K) to affect the cannabinoid profile throughout
the plant.
Higher concentrations of THC, CBD, CBG, CBC, and
THCV in the flowers closest to the apex, while
higher CBT and CBN concentrations were found in
the flowers located lower; nutritional supplements
influenced cannabinoid profile and may reduce
cannabinoid variability throughout the plant,
[115]
Evaluate the hypothesis that P uptake, distribution,
and availability in the plant affect the biosynthesis
of phytocannabinoids.
The apical flowers showed higher concentrations of
CBDA and THCA under low P availability (5 mg
L1); under high P availability (90 mg L1), the
concentrations of these compounds did not differ in
these two locations.
[61]
Evaluate the differences in the concentration of
phytocannabinoids in different plant organ
positions in the plant.
Variations in phytocannabinoid biosynthesis
according to the position of the organs of the plants
so that the upper and median parts showed higher
phytocannabinoid and flower yields when
compared to the basal plant parts, probably due to
higher flower yield.
In 1971, two research projects were developed to evaluate the concentrations of phyto-
cannabinoids in different plant organs [
25
,
26
]. According Fetterman and collaborators, all
the hemp plant parts contained phytocannabinoids, and they showed that the decrease in
THC content in different parts of the plants followed the order of bracts, flowers, leaves,
smaller stems, larger stems, roots, and seeds [
26
], with similar results in other works [
27
,
113
].
Another study confirmed that bracts were the organ with the highest concentration of phy-
tocannabinoids [
52
]. Ohlsson and collaborators reported that phytocannabinoids were
most abundant in flowering tops and the young, small leaves surrounding the flowers [
25
].
Other research reported a lack of detectable levels of phytocannabinoids in the seeds
(intact or crushed) and in pollen grains [
116
]. According to Jin et al. (2020) [
117
], the
phytocannabinoid content decreased from inflorescences to leaves, stem bark, and roots.
Guiroud et al. [
34
] investigated the evolution of the phytocannabinoid content dur-
ing the growth of seven varieties of different chemotypes (based on the CBDA/THCA
ratio): three of each from chemotype I (THCA/CBDA ratio: >1.0) and chemotype III
(THCA/CBDA ratio: 0.5–2.0), and one from chemotype II (THCA/CBDA ratio: <1.0). The
authors observed that the plants from chemotypes II and III needed more time to reach
peak production of THCA and CBDA than chemotype I (ninth flowering week). Chemotype
III plants synthetized CBDA continuously until the end of the experiment with eleven weeks
Agronomy 2022,12, 1492 17 of 30
of flowering, while CBGA reached a maximum concentration around five weeks of flowering
and decreased afterwards [
53
]. Similarly, Massuela et al. (2022) [
72
] also observed an increase
in CBDA at a later harvest time in a chemotype III genotype. The harvest time was statistically
significant for the biomass of inflorescence and the total CBDA concentration [
72
]. According
to these authors, the ideal harvest time for the evaluated genotype (high CBD content) was
around nine weeks of flowering; however, after this period (eleventh week), a slight increase
in CBD concentration was observed. Thus, it is important to carry out more studies that
evaluate the production of phytocannabinoids throughout the development of the plant until
the period of decline in phytocannabinoid production.
Stack et al. (2021) [
110
] observed the rapid accumulation of phytocannabinoids in
different varieties about 3.5 weeks after the onset of flowering. Hammami et al. (2021) [
59
]
evaluated the phytocannabinoid production of 12 varieties (intended for fiber or grain
production) at different stages of the reproductive phase and observed that the synthesis of
these compounds varied among the genotype and during cannabis ontogenic development.
The phytocannabinoid content in glandular trichomes varies with trichome age, trichome
type, and trichome location on the plant [118,119].
Indeed, the concentration of phytocannabinoids varies during the reproductive phase
[37,58]
,
and there may be a relationship between the chemotype and the sensitivity of the genotype to the
photoperiod [
120
]. In agreement, Yang et al. (2020) [
120
] found that the maximum concentration
of CBG in three photoperiod-sensitive genotypes was highest in the fourth week of flowering and
subsequently decreased. In the photoperiod-neutral genotypes, the maximum concentrationof
CBG was in the second and third weeks of flowering, and after this production peak, a significant
drop in CBG production was observed, but in the twelfth week, the contraction returned to
the maximum value. These authors also observed that, in the sixth week, all the genotypes
showed maximum concentrations of CBDA and THCA, but the photoperiod-sensitive genotypes
showed higher total phytocannabinoid concentrations in the second week of flowering, while
the maximum total phytocannabinoid concentration in the photoperiod-neutral varieties was in
the eighth and ninth weeks of flowering.
Although the reproductive phase is characterized by the highest production of phyto-
cannabinoids, these compounds are also produced in lower concentrations in leaves during
the vegetative phase [
60
,
81
] and in seedlings [
121
]. It is important to consider that each
phytocannabinoid presents a dynamic according to the development of the plant due to
the biosynthesis mechanism involved in the production and the speed of degradation of
each compound. Although the concentration of phytocannabinoids varies during plant
development, the CBD/THC ratio (related to the chemotype of the plant) remains stable,
given that it is a characteristic of greater genetic control [
122
]. However, although there are
reports that the CBD/THC ratio changes during ontogeny, this change does not alter the
chemotype of the plant. [120].
Since the 1970s, many studies have been published that show the difference in the con-
centration of phytocannabinoids between the apical and basal parts of the plant so that the
apical part tends to accumulate higher concentrations of certain phytocannabinoids, such as
THC [
71
,
119
,
123
], with recent studies confirming this assumption
[30,61,71,111,115,124,125]
.
Bernstein et al. (2019) [
71
] observed that the THC content in the flowers closest to the apex
was 12% compared to only 6% in the flowers located lower (Table 2). For CBD, CBG, and
CBC, the increase was three times higher from the base to the top of the plant. The THC and
CBD concentration levels in leaves close to inflorescences were about half of those found
in flowers from the median and basal parts, while the THC, CBD, THCV, CBG, and CBN
concentrations in fully developed leaves represented about 10% of those found in the flowers.
The light interception differences of the leaves are partly attributable to the intercanopy
attenuation of PAR from self-shading, which may have influenced the variations in phy-
tocannabinoid concentrations observed by Bernstein et al. (2019) [
71
]. The leaves located
at the bottom of the plant receive lower radiation intensities than the upper leaves, and
this difference in PAR interception influences the net CO
2
assimilation rate. This parameter
tends to diminish on the lower leaves compared to the upper ones (more exposed to light),
Agronomy 2022,12, 1492 18 of 30
as observed by Danziger and Bernstein (2021) [
46
] in a medical variety and Bauerle et al.
(2020) [
126
] in a hemp variety. Hawley et al. (2014) [
64
] supported this hypothesis since
they observed greater homogeneity in the phytocannabinoid and terpene profiles in the
upper and lower canopy parts of plants subjected to light supplementation. Danziger
and Bernstein (2021) [
46
] also observed that defoliation provided greater uniformity in
phytocannabinoid content in the upper and lower canopy parts of plants, which could be
the result of increased light uniformity due to penetration to the bottom parts of the plants.
In addition, different factors can influence the concentrations of phytocannabinoids in
different organs of the plant. For example, Shiponi and Bernstein (2021) [
115
] observed that
the availability of phosphorus (P) influenced the concentration of phytocannabinoids in
apical and basal inflorescences. Bernstein et al. (2019) [
114
] also observed that nutritional
supplementation influenced the concentrations of phytocannabinoids in different plant
organs and locations along the plant height. Danziger and Bernstein (2021) [
80
] observed
that different light spectra influenced the biomass of the plant organs and phytocannabi-
noid concentrations in top inflorescences and bottom inflorescences when observing three
medical cannabis varieties (‘CS10’, ‘CS12’, and ‘CS14’). However, similar phytocannabinoid
concentrations were seen in the two different parts. These studies have demonstrated
the potential of management techniques (such as nutritional availability and light quality)
for regulating the concentrations of specific secondary metabolites in defined parts in the
cannabis plant.
3.5. Temperature, Altitude and CO2Concentration
The optimum temperature for cannabis development ranges between 25–30
C, de-
pending on the plant’s genotype [
21
]. One study evaluated wild plants collected from two
regions in northern India with different altitudes and temperatures [
127
]. It concluded that
plants grown in drier regions (higher altitude and temperature) presented an increased
density and augmented trichomes. However, the author stated that these variations may
also be related to genotype differences [117].
Environmental conditions influenced by altitude have been proved to be important
factors inducing variations in the secondary metabolite composition of cannabis inflores-
cences. Giupponi et al. (2020) [
128
] evaluated the influence of altitude on the productivity
of secondary metabolites in the inflorescences of four cannabis genotypes. The authors
noted that, in plants cultivated at high altitudes, the production of terpenes was signifi-
cantly higher, and increases of 46, 36, and 39% in the concentrations of CBDA, CBCA, and
CBGA, respectively, were observed. In contrast, Bruci et al. (2012) [
30
] observed that plants
grown in lower-altitude regions in Albania showed higher THC production.
According to Paris et al. (1975) [
129
], temperature and relative air humidity signifi-
cantly influenced THC content. The authors observed that the THC content was 30 times
higher with increased temperature (22
C and 24
C) and reduced relative humidity (50%
compared to 80%). In this experiment, the humidity variation was considerably higher
than the temperature variation, leading to a greater influence of relative humidity on the
THC content.
The few studies relating the influence of temperature to phytocannabinoid produc-
tion are conflicting. Whilst one study showed that higher temperatures (32
C vs. 22
C)
increased phytocannabinoid content [
54
,
130
], other works have shown that phytocannabi-
noid content decreases with increasing temperature (32
C vs. 23
C) [
54
,
130
,
131
]. Indeed,
temperature can influence the production of phytocannabinoids, and lower temperatures
can delay the time to flowering, but the response varies depending on genetics and other
environmental conditions, such as photoperiod [67].
Bazzaz and colleagues (1975) [
131
] evaluated phytocannabinoid production in varieties
from temperate and tropical climates grown at low (23
C day and 16
C night) and high
(32
C day and 23
C night) temperatures. The authors observed higher THC yields under
low-temperature conditions. However, the same varieties grown in both environments
showed wide qualitative variation (CBD and THC) in the phytocannabinoid composition.
Agronomy 2022,12, 1492 19 of 30
This variation may be due to using seeds collected for the study, probably with no genetic
stability, so it would have been more appropriate to employ clones obtained through
vegetative propagation. Therefore, this type of stress response is likely to be more complex,
involving several factors and varying according to the genetics of the plants.
Chandra et al. [
132
] evaluated gas exchange in seven cannabis varieties (different
chemotypes) subjected to different temperatures. The maximum net CO
2
assimilation rate
for each variety ranged from 25
C to 35
C, where the varieties with the highest net CO
2
assimilation rates had superior chlorophyll content. It was also found that the relative THC
content was related to the carotenoid content so that the varieties with high THC contents
presented higher carotenoid contents than those varieties with a low capacity for THC
synthesis. High temperature conditions above 35
C could cause a decrease in the net CO
2
assimilation rate [9,132] due to reduced stomatal conductance.
Although some authors reported that the net CO
2
assimilation rate of cannabis was
limited at temperatures above 35
C [
132
], research comparing plant development under
varying temperatures can shed light on how different genetic characteristics result in
physiologically different responses to these conditions and provide concrete data on the
flower and the influence on phytocannabinoid yield.
Despite the lack of research evaluating the influence of CO
2
supplementation on
cannabinoid productivity, this is a relevant issue considering that yield and plant growth
always reflect an interplay between different factors, such as light, temperature, nutrients,
and CO
2
concentration [
64
]. A high CO
2
concentration increases net CO
2
assimilation and
accelerates plant growth, with the potential to increase yield [
98
]. Plants grown under
ideal conditions and at higher CO
2
concentrations may show a 50% increase in net CO
2
assimilation rate compared to plants grown under normal CO2concentration [133].
In this species, an increase in the CO2concentration in a specific area around the leaf
can cause gains in net CO
2
assimilation rate [
98
]. Chandra et al. (2008) [
98
] evaluated
the photosynthetic response of a specific leaf area of a cannabis variety under different
temperatures, PAR intensities, and CO
2
concentrations through a portable photosynthe-
sis system. The highest net CO
2
assimilation rate was verified at 30
C and a PAR of
1500 µmol m2s1
. In general, the effect of light intensity on photosynthetic carbon assimi-
lation was more prominent than temperature [
134
]. The net CO
2
assimilation rate decreased
by about 50% when the evaluations were carried out at 250
µ
mol mol
1
compared with the
higher CO
2
environmental concentration (350
µ
mol mol
1
) [
134
]. A 50% increase in the net
CO
2
assimilation rate at a CO
2
concentration of 750
µ
mol mol
1
was also observed. The
best performance was observed under conditions of high PAR for temperatures between
25 to 30 C
, with high potential for increased growth and yield in environments enriched
with CO
2
[
98
]. In this study, only one variety was evaluated, being relevant to evaluate
other genotypes under these conditions and the impact of these variables on the flowers
and the phytocannabinoid yield.
CO
2
supplementation should be performed under conditions of high light intensity
in which the plant reaches the point of light saturation. In this agreement, cannabis
has a high light saturation point (
=
1500 mol
µ
mol m
2
s
1
) [
86
,
98
], that is, a high light
intensity is necessary to reach the maximum CO
2
assimilation rate. However, a higher
CO
2
assimilation rate does not always reflect an increase in yield because the source–sink
balance determines whether this type of management causes an increase in yield, as the
photoassimilates can be directed to the production of roots, stems, or leaves instead of to
flowers and phytocannabinoids.
An increase in CO
2
concentration can also influence secondary metabolite production
in plant species such as peppers (Capsicum chinense Jacq.), with increased levels of capsaicin,
or reductions in citric acid in strawberries (Fragaria
×
ananassa Dutch. ‘Camarosa’) and
polyphenols in grape (Vitis vinifera L.’ Red Tempranillo’) [
135
]. Thus, research must be
conducted to test the hypothesis that carbon fertilization can be used to manipulate the
chemical profile of cannabis, promoting greater yields.
Agronomy 2022,12, 1492 20 of 30
3.6. Nutrition and Substrate
An understanding of the nutritional requirements and the physiological and morpho-
logical responses of cultivars to mineral nutrition can improve nutritional management for
adequate nutrient supply [
136
], promoting increased phytocannabinoid and flower yields.
There is some evidence supporting the influence of mineral nutrition on phytocannabinoid
production [
137
]. While some studies report that an increase in macronutrient mineral
content could result in an increase in inflorescence and phytocannabinoid yields [
115
],
others have reported that the increased availability of NPK decreased THC and other phy-
tocannabinoids in inflorescences [
68
,
114
,
115
,
125
] and leaves [
114
,
115
] In fact, nutritional
availability influences phytocannabinoid production with specific effects on organs and
compounds [114,115].
The first study that evaluated the influence of nitrogen on the production of phyto-
cannabinoids presented contradictory results. Coffman and Gentner (1977) [
137
] observed
that N supply in the range of 0–125 mg L
1
N did not affect the THC content in inflo-
rescences. On the other hand, results obtained by Bócsa et al. (1997) [
138
] showed that
the THC content in leaves declined in response to a high-nitrogen regime. However, the
difference in these results may have been related to the availability of other nutrients, plant
genotypes, ontogeny, and environmental conditions. De Prato et al. (2022) [
67
] observed
that plants grown at the lowest (50 N kg ha
1
) and highest (150 N kg ha
1
) N rates pro-
duced higher concentrations of THC than the mid-level N plants (100 N kg ha
1
), but a
significant difference was found between the varieties.
More recently, in another study, de Prato et al. (2022) [
139
] observed differences in
the concentrations of phytocannabinoids in a variety of industrial hemp (‘ECO-GH15’)
cultivated with different doses of K (11, 43, and 129 ppm) with the conventional, fast-
release potassium K of potassium sulphate (K
2
SO
4
) and a slow-release form (
131 ppm of K
)
containing soil microbes (SRK). The authors observed a significant increase in
9
-THC,
8-THC
, and THCA concentrations in the SRK treatment compared with the three conven-
tional K treatments. CBD and CBDV showed an opposite pattern, being smaller in plants
receiving SRK than the conventional K treatments, with the highest level (129 ppm) of
application resulted in the highest concentration of CBDV.
Excessive fertilization during the growing season can result in a decrease in THC
concentration and flower yield [
115
,
140
]. During the vegetative phase and when using two
organic substrates based on coconut fiber, it was suggested that 389 mg of N L
1
could be
considered adequate via the application of liquid organic fertilizer (4.0 N:1.3 P:1.7 K) [
140
].
During the reproductive phase, the ideal rate of this organic fertilizer was around 225 mg of