Increasing Inflorescence Dry Weight and Cannabinoid Content in Medical Cannabis Using Controlled Drought Stress

Abstract

Controlled application of drought can increase secondary metabolite concentrations in some essential oil-producing crops. To evaluate the effects of drought on cannabis ( Cannabis sativa L.) inflorescence dry weight and cannabinoid content, drought stress was applied to container-grown cannabis plants through gradual growing substrate drying under controlled environment. Fertigation was withheld during week 7 in the flowering stage until midday plant water potential (WP) was approximately −1.5 MPa (drought stress threshold). This occurred after 11 days without fertigation. A well-irrigated control was used for comparison. Leaf net photosynthetic rate (P n ), plant WP, wilting (leaf angle), and volumetric moisture content (VMC) were monitored throughout the drying period until the day after the drought group was fertigated. At the drought stress threshold, P n was 42% lower and plant WP was 50% lower in the drought group than the control. Upon harvest, drought-stressed plants had increased concentrations of major cannabinoids tetrahydrocannabinol acid (THCA) and cannabidiolic acid (CBDA) by 12% and 13%, respectively, compared with the control. Further, yield per unit growing area of THCA was 43% higher than the control, CBDA yield was 47% higher, ∆ ⁹ -tetrahydrocannabinol (THC) yield was 50% higher, and cannabidiol (CBD) yield was 67% higher. Controlled drought stress may therefore be an effective horticultural management technique to maximize both inflorescence dry weight and cannabinoid yield in cannabis, although results may differ by cannabis cultivar or chemotype.

Full text

HORTSCIENCE 54(5):964–969. 2019. https://doi.org/10.21273/HORTSCI13510-18
Increasing Inflorescence Dry Weight
and Cannabinoid Content in Medical
Cannabis Using Controlled
Drought Stress
Deron Caplan
1
, Mike Dixon, and Youbin Zheng
2
School of Environmental Sciences, University of Guelph, Guelph, Ontario,
N1G 2W1, Canada
Additional index words. Cannabis sativa, marijuana, deficit irrigation, plant water potential,
medicinal crops, volumetric soil moisture content
Abstract. Controlled application of drought can increase secondary metabolite con-
centrations in some essential oil-producing crops. To evaluate the effects of drought on
cannabis (Cannabis sativa L.) inflorescence dry weight and cannabinoid content, drought
stress was applied to container-grown cannabis plants through gradual growing
substrate drying under controlled environment. Fertigation was withheld during week
7 in the flowering stage until midday plant water potential (WP) was approximately
L1.5 MPa (drought stress threshold). This occurred after 11 days without fertigation.
A well-irrigated control was used for comparison. Leaf net photosynthetic rate (P
n
),
plant WP, wilting (leaf angle), and volumetric moisture content (VMC) were monitored
throughout the drying period until the day after the drought group was fertigated. At
the drought stress threshold, P
n
was 42% lower and plant WP was 50% lower in the
drought group than the control. Upon harvest, drought-stressed plants had increased
concentrations of major cannabinoids tetrahydrocannabinol acid (THCA) and canna-
bidiolic acid (CBDA) by 12% and 13%, respectively, compared with the control.
Further, yield per unit growing area of THCA was 43% higher than the control, CBDA
yield was 47% higher, Δ
9
-tetrahydrocannabinol (THC) yield was 50% higher, and
cannabidiol (CBD) yield was 67% higher. Controlled drought stress may therefore be an
effective horticultural management technique to maximize both inflorescence dry weight
and cannabinoid yield in cannabis, although results may differ by cannabis cultivar or
chemotype.
The historic prohibition of cannabis (Can-
nabis sativa L.) has stunted scientific re-
search on its production, leaving growers to
rely on guides and online resources based
heavily on anecdotal information. In the past
decade, the regulations surrounding cannabis
production and use, especially for medicinal
purposes, have become increasingly liberal-
ized in North America and in some parts of
Europe (Chandra et al., 2017), allowing re-
search in this field.
The essential oil of female cannabis
inflorescences gives the crop its value as a
medicinal and recreational product; these
oils are concentrated mostly in glandular
trichomes and contain a diverse array of
secondary metabolites, including a class
of meroterpenoid compounds known as
phytocannabinoids (cannabinoids; Chandra
et al., 2017; Potter, 2014). Some cannabi-
noids, including THC and CBD, have been
widely studied for their psychoactive and
medicinal properties (Elzinga et al., 2015;
Mechoulam et al., 1970; Vemuri and
Makriyannis, 2015), but the medicinal prop-
erties of other cannabinoids and cannabi-
noid interactions are still mostly unknown
(McPartland and Russo, 2001; Russo, 2011).
In live plants, cannabinoids exist largely as
carboxylic acids such as THCA and CBDA
(Muntendam et al., 2012). These acids de-
carboxylate during storage (Ross and ElSohly,
1997; Taschwer and Schmid, 2015) and upon
heating (Kimura and Okamoto, 1970) to
become neutral cannabinoids, such as THC
and CBD.
The inflorescence dry weight and second-
ary metabolite content in cannabis is largely
controlled through breeding and phenotype
selection (Muntendam et al., 2012); however,
horticultural management techniques such as
fertilization (B
ocsa et al., 1997; Caplan et al.,
2017a; 2017b), choice of growing substrate
(Caplan et al., 2017a, 2017b), air temper-
ature in the growing environment (Chandra
et al., 2011; Latta and Eaton, 1975), horti-
cultural lighting intensity and quality (Lydon
et al., 1987; Potter and Duncombe, 2012),
and photoperiod (Potter, 2009) also have a
substantial impact. Further, controlled expo-
sure to stress may be an effective method to
increase the production of some secondary
metabolites in cannabis. For example, treat-
ment with ultraviolet B radiation, which is
not used in photosynthesis, may increase
THC concentration in cannabis inflorescences
under controlled environment conditions
(Lydon et al., 1987).
Drought stress is a major stimulator of
secondary metabolites in plants. This is
exemplified in herbs and spices cultivated
in semiarid regions such as the Mediterra-
nean. Intermittent drought and high levels
of solar radiation in these areas have been
attributed to aromatic herbs and spices with
abundant essential oil (Kleinw€
achter and
Selmar, 2015). In the literature, there are
no reports on the effects of drought stress
on cannabis secondary metabolism; how-
ever, secondary metabolite accumulation
due to drought stress has been documented
in a number of other herbaceous species
(Baher et al., 2002; Bettaieb et al., 2009;
Kleinw€
achter and Selmar, 2015). In sum-
mer savory (Satureja hortensis), plants that
were highly drought stressed during the
flowering stage had 31% higher essential
oil concentration than a well-watered con-
trol (Baher et al., 2002). Likewise, drought
stress increased essential oil concentration
in lemon balm (Melissa officinalis L.) and
lemon catmint (Nepeta cataria L. f.citrio-
dora) compared with a well-watered con-
trol but did not for sage (Salvia officinalis L.).
Although concentrations were higher, essen-
tial oil yield (per unit growing area) of lemon
catmint and lemon balm was lower in the
drought-stressed plants because of reduced
growth and harvestable plant material.
In contrast, both Bettaieb et al. (2009)
and Nowak et al. (2010) have documented
not just increased essential oil concentra-
tion in sage by up to four times, but also
higher essential oil yield in drought-
stressed plants compared with a nonstressed
control. Other than the notable exceptions
in sage, increased essential oil yield per unit
gro wing area is rarely cited (Kleinw€
achter and
Selmar, 2015), possibly because drought
stress has well-documented negative impacts
on plant growth and can reduce harvest-
able plant material. Drought reduces rates of
carbon assimilation as a result of both stoma-
tal and metabolic limitations (Chaves, 1991;
Flexas et al., 2002; Tezara et al., 1999). To
maximize essential oil or secondary me-
tabolite yield, the level and timing of the
drought stress should be such that dry
weight losses are minimized (Nakawuka
et al., 2014).
In applying drought stress over extended
periods, researchers generally aim to main-
tain constant levels of root zone WP, either
through use of a solute-infused substrate
(Charles et al., 1990; Van Der Weele et al.,
2000) or by regulating soil/growing-substrate
moisture content (Baher et al., 2002; Blanch
et al., 2009; Manukyan, 2011; Nowak et al.,
2010). This allows for long-term assessment
of the drought stress response; however,
Received for publication 21 Aug. 2019. Accepted
for publication 11 Mar. 2019.
We are grateful to VIVO Cannabis Inc (formerly
ABcann Medicinals Inc.) for providing funding,
as well as materials, expertise, and ground-level
support. We also thank EZ-GRO Inc. for supplying
materials. We thank Newton Tran, Jared Stoochn-
off, and Jonathan Stemeroff for their vital technical
support.
1
Current address: The Flowr Group (Okanagan) Inc.,
Kelowna, British Columbia, V4V 1S5, Canada.
2
Corresponding author. E-mail: yzheng@uoguelph.ca.
964 HORTSCIENCE VOL. 54(5) MAY 2019

these methods involve a sustained level of
drought rather than mimicking natural sub-
strate saturation and drying cycles. Allow-
ing the growing substrate to dry before
irrigation increases the level of root zone
oxygen, which can improve nutrient uptake
and root growth and prevent root-borne
disease (Caplan et al., 2017a; Jackson and
Colmer, 2005; Zheng et al., 2007). Substrate-
drying techniques that incorporate a wet-
ting and drying cycle are preferred to
observe both the immediate effects of the
stressor as well as subsequent acclimation.
This technique requires the use of a grow-
ing substrate that can effectively re-saturate
after an extended dry period. Peat-based
substrates without incorporated wetting agents,
for example, may not be effective (Fields
et al., 2014).
Drought stress timing is also essential to
minimize dry weight losses and maximize
essential oil yield and the concentration of
secondary metabolites; differences in growth
stage and natural timing of phytochemical
accumulation must be considered by species
(Petropoulos et al., 2004). The cannabis life
cycle includes two growth stages, vegetative
and flowering, which are controlled by pho-
toperiod. A short-day photoperiod (12 h)
triggers flowering that may last 7to12
weeks depending on cultivar and growing
conditions (Potter, 2014). Cannabinoids ac-
cumulate mostly during the flowering stage,
but the timing of peak cannabinoid concen-
tration varies by chemotype and cultivar.
Drug-type varieties of chemotype I have a
high THCA:CBDA ratio (>1.0), whereas
varieties of chemotype II have an interme-
diate ratio (generally 0.5–2.0) (Pacifico
et al., 2008). For chemotype I, peak THCA
concentration is approximately week 9 of
the flowering stage, and for chemotype II,
the peak is approximately week 7. Peak
CBDA in chemotype I is approximately
week 11 of the flowering stage; in chemo-
type II, it varies minimally from week 8
onward (Aizpurua-Olaizola et al., 2016;
Muntendam et al., 2012).
In the present study, drought stress was
applied to a chemovar II cultivar during week
7 of the flowering stage. It was hypothesized
that controlled drought stress may be a valu-
able tool for growers to improve the quality
of their cannabis crops. The objective was to
evaluate the effects of drought stress on
inflorescence dry weight and cannabinoid
content and yield in cannabis.
Materials and Methods
Plant culture
Fourteen-day-old vegetatively propa-
gated rooted cuttings (10 cm high with
6 leaves) of Cannabis sativa L. ‘NC:Med
(Nebula)’ were transplanted into round
blow-molded black pots (102 mm diame-
ter ·89 mm height) containing a custom-
blended organic growing substrate [40% to
45% (vol/vol) sphagnum peatmoss, 20%
to 25% chunk coconut coir, 20% to 25%
horticultural grade perlite and 5% to 10%
worm casings; Premier Tech Horticulture,
Rivi
ere-du-Loup, QC, Canada] with one
plant per pot. Cuttings were taken from
the same stock plant and were therefore genet-
ically identical. Pots were placed in a walk-in
growth chamber (15 m
2
) at a density of 97
plants/m
2
. Growth chamber environmental pa-
rameters are presented in Table 1.
Plants were hand-fertigated, as per Caplan
et al. (2017b), using Nutri Plus Organic
Grow liquid organic fertilizer (4.0N–1.3P–
1.7K; EZ-GRO Inc., Kingston, ON, Can-
ada) at a rate that supplied 389 mg
N/L amended with 2 mL·L
–1
of calcium-
magnesium supplement (0.0N–0.0P–0.0K–
3.0Ca–1.6Mg; EZ-GRO Inc.), diluted with
reverse osmosis (RO) water and with a 20%
leaching fraction. Other nutrient element
concentrations of Nutri Plus Organic Grow
were (in mg·L
–1
): 14.5 Zn, 12.0 B, 2.6 Mo,
2.1Cu,and8.5Fe.Fertigationwasadmin-
istered when mean substrate moisture was
30%, measured using a WET-2 soil mois-
ture sensor (Delta-T Devices Ltd., Cam-
bridge, UK).
At 15 d after transplant (DAT), 8 plants
with similar height and canopy size were
selected and transferred into a larger walk-
in growth chamber (130 m
2
)fortheflower-
ing stage. This was considered the first day
of the flowering stage (DFS). Plants were
transplanted into 11-L blow-molded black
pots (279 mm diameter ·241 mm height)
containing Pro-Mix HP Mycorrhizae (Premier
Tech Horticulture) and spaced on growing
tables at a density of 6.4 plants/m
2
.Trial
plants were bordered on all sides by canna-
bis plants of the same age and of similar
size.
During the first 10 DFS, plants were hand-
fertigated at a rate that supplied 389 mg N/L
of Nutri Plus Organic Grow, as per Caplan
et al. (2017b), whenever substrate moisture
content reached 20%. From then on, plants
were fertigated as per Caplan et al. (2017a),
using Nutri Plus Organic Bloom (2.00N–
0.87P–3.32K; EZ-GRO Inc.) at a rate that
supplied 170 mg N/L, diluted with RO
water. Other nutrient element concentra-
tions in Nutri Plus Organic Bloom were (in
mg·L
–1
): 100 Mg, 10.0 Zn, 12.8 B, 0.1 Mo,
2.3 Cu, and 6.8 Fe. Flowering-stage fertiga-
tion solutions were also amended with 5
mL·L
–1
of calcium-magnesium supplement
(0.0N–0.0P–0.0K–3.0Ca–1.6Mg; EZ-GRO
Inc.) and with Organa ADD micronutrient
supplement, at a rate that supplied 22.9 mg
N/L (2.0N–0.0P–0.0K; EZ-GRO Inc.).
Other nutrient element concentrations in
Organa ADD were (in mg·L
–1
): 100.0 Ca,
29851 Zn, 4892 Mn, 1239 B, 12.7 Mo, 2419
Cu, and 2917 Fe. Fertigation solution pH
was adjusted to maintain substrate pH be-
tween5.5and6.3,measuredusingasoilpH
probe (Hanna HI 99121; Hanna Instruments,
Woonsocket, RI).
Treatments
At 39 DFS, plants were randomly
assigned to drought or control treatment
groups,with4plantsineachgroup.Each
potted plant was an experimental unit. The
control was irrigated as previously de-
scribed for the flowering stage, with a
fertigation event triggered when the sub-
strate moisture content of an individual
plant reached 20%. Fertigation was with-
held from drought treatment until plant WP
reached between –1.4 and –1.5 MPa.
Drought stress indicators
Plant water potential. Stem psychrom-
eters and data loggers (PSY1; ICT Interna-
tional Pty Ltd., Armidale, NSW, Australia)
were installed on each plant, and plant WP
measurements were taken every 15-min-
utes. The procedures outlined by Tran et al.
(2015) were followed to install and main-
tain the psychrometers. Plant WP was
noted immediately before fertigating the
drought group and daily, at midday up
until 1 d after the fertigation. Psychrometer
reinstallations were necessary if plant WP
readings suddenly dropped to zero or were
positive while lights were on. These cir-
cumstances usually indicated that the vapor
seal between the psychrometer and the
stem was broken, condensation had accu-
mulated inside the chamber, or the thermo-
couple was damaged (Stoochnoff et al.,
2018).
Substrate moisture content. Capacitance-
type substrate moisture sensors (ECH
2
O-TE;
Decagon Devices Inc., Pullman, WA) were
inserted vertically into the substrate sur-
face of each pot and connected to two five-
port data loggers (EM50; Decagon Devices
Inc.). The moisture sensors measured di-
electric permittivity every 15 min, which
was converted to volumetric moisture con-
tent (VMC) using a substrate-specific cal-
ibration. To ensure that the substrate in the
drought treatment adequately rehydrated
after fertigation, VMC at midday the day
after fertigation of the drought group was
compared with that of the control, mea-
suredatanequalinterval after the control
plants were last irrigated.
Leaf net photosynthetic rate. Leaf net
photosynthetic rate (P
n
) was measured each
daybetween8and9hintothelightcycle
beginning at 39 DFS as well as immedi-
ately before fertigating the drought group.
Measurements were made using a portable
photosynthesis measurement system (LI-
6400XT; LI-COR Biosciences, Lincoln,
NE) on the youngest fully expanded leaf
(center leaflet >10 cm). Light was supplied
by 6400-02B red-blue light-emitting di-
odes (LI-COR) with photosynthetically ac-
tive radiation set to around chamber canopy
level (450 mmol·m
–2
·s
–1
). CO
2
concentra-
tion in the leaf cuvette was maintained at
800 mmol·mol
–1
, and block temperature
was maintained at 20 C.
Relative leaf angle. Initial leaf angle was
measured at midday at treatment initiation
using a handheld pivoting angle-finder and a
protractor. Subsequent leaf angle measure-
ments were taken when wilting was first
evident then, three to four times a day after
that until fertigation of the drought group.
HORTSCIENCE VOL. 54(5) MAY 2019 965
MISCELLANEOUS

New, fully expanded leaves on a side-branch
from the first internode were selected for
measurement, and petioles were marked
with colored tape for future measurement
(Fig. 1). The angle between the center of the
middle leaflet and the stem from which it
originates was measured. The leaflet tips
were not used as reference points because
‘‘tip curl’’ is common in cannabis, some-
times related to a nutrient disorder. As
leaves wilted, increasing leaf angle relative
to the initial angle was noted.
Inflorescence dry weight and cannabinoid
measurements. Plants were harvested at 54
DFS. Stems were cut at substrate level; large
leaves were removed from stems, and plants
were hung to dry at 18 C(
SD ± 0.1 C) and
45% RH (SD ± 1.9%) for 2 d then cured at
18 C(
SD ± 0.1 C) and 57% RH (SD ± 4.3%)
for 12 d. Inflorescences were then cut from
branches (both shoot apex and axillary
branches), and leaves that were protruding
from the inflorescences were trimmed using a
Twister T4 mechanical trimming machine
(Keirton Inc., Surrey, BC, Canada) before
inflorescence dry weight measurement.
The dried, cured apical inflorescences of
three plants from each group was stored
under dark and cool conditions according
to United Nations Office on Drugs and
Crime (2009) before being analyzed by an
independent laboratory (RPC Science and
Engineering, Fredericton, NB, Canada).
Analysis of the neutral cannabinoids THC,
CBD, cannabinol (CBN), cannabichromene
(CBC), and cannabigerol (CBG), as well
as acid forms THCA, CBDA, and cannabi-
gerolic acid (CBGA), were conducted by
high-performance liquid chromatography
as described in section 5.4.8 of United Na-
tions Office on Drugs and Crime (2009).
Moisture content of the dry inflorescence
was determined using the methods described
in the U.S. Pharmacopeial Convention, sec-
tion 921, method 3 (U.S. Pharmacopeia and
National Formulary, 2017) and cannabinoid
concentration was corrected to zero percent
moisture content. Cannabinoid yield was cal-
culated as cannabinoid concentration multi-
plied by inflorescence dry weight, expressed
per unit area (g·m
–2
) and corrected to 0%
moisture content.
Statistical analysis
Data were analyzed using JMP Statistical
Discovery Version 13.0 (SAS Institute Inc.,
Cary, NC) at a type 1 error rate of #0.05.
Differences among means were tested us-
ing Student’s ttest. If cannabinoid concen-
trations were below the detection limit
(<0.05%), the values were excluded from
the analysis. The residuals of the preceding
analyses were tested for normality and equal-
ity of variance using The Shapiro-Wilk’s and
Bartlett’s tests, respectively.
Results
Drought stress indicators
During the 54-day flowering period, there
were no symptoms of nutrient disorder and
no observable differences in plant appear-
ance between control and drought groups
until the drought treatment was without
fertigation for 9 d. From 9 d without
fertigation to harvest, plants under drought
treatment showed signs of veinal chlorosis
on older leaves and, to a lesser extent,
newly formed leaves on the entire plant.
Wilting was observed after 11 d without
fertigation when leaf angle in the drought
treatment was 52% ± 0.7 higher than the
initially measured angles.
Up until 11 d without fertigation in the
drought treatment, plant WP did not differ
from the control groups (P=0.78;n=4for
day 10). Immediately before fertigating the
drought group, on the 11th day without
fertigation, plant WP in the drought treat-
ment was 50% lower than in the control
(Table 2). The day after fertigating plants
in the drought treatment, their mean mid-
day plant WP recovered to the same level as
the control.
There were also notable differences in
net photosynthetic rate (P
n
) and substrate
Table 1. Growth chamber environmental parameters during the trial.
Days after transplant
0–2 3–4 5–9 10–15
Vegetative stage (18-h photoperiod)
PAR
z
(mmol·m
–2
·s
–1
) 100 ± 1.3
y
200 ± 1.9 300 ± 2.6 400 ± 4.1
Air temperature (C) ------------------------------------------------------- 24 ± 0.1/23 ± 0.9 -------------------------------------------------------
Relative humidity (%) ------------------------------------------------------- 73 ± 5.1/73 ± 4.4 -------------------------------------------------------
CO
2
concentration (ppm) ---------------------------------------------------- 691 ± 99.1/601 ± 31.9 ----------------------------------------------------
Days in the flowering stage
0–5 6–9 10–48 49–54
Flowering stage (12-h photoperiod)
PAR
x
(mmol·m
–2
·s
–1
) ---------------------- 262 ± 40.7 ---------------------- ---------------------- 427 ± 70.5----------------------
Air temperature (C) ------------------ 22 ± 0.2/22 ± 0.3 ------------------ -------------------20 ± 0.4/18 ± 0.7 -----------------
Relative humidity (%) 70 ± 0.4/70 ± 0.5 65 ± 0.7/65 ± 0.3 60 ± 0.9/61 ± 1.2 55 ± 1.4/60 ± 1.4
CO
2
concentration (ppm) -------------------------------------------------- 731 ± 190.8/666 ± 151.3 --------------------------------------------------
z
Photosynthetically active radiation (PAR) was maintained using fluorescent lighting (Philips Lighting, Markham, ON, Canada) and measured the at canopy level.
y
Values are mean ± SD during light/dark periods.
x
PAR was maintained using 315-W Green Power Master Elite Agro ceramic metal halide lamps (Philips Lighting) and measured at the canopy-level.
Fig. 1. Location for leaf angle measurement to indicate the degree of wilting in cannabis.
966 HORTSCIENCE VOL. 54(5) MAY 2019

volumetric moisture content (VMC) be-
tween drought and control treatments
around the time of fertigating the drought
group (Table 2). Imm ediately b efore fe rtiga-
tion, P
n
in the drought-stressed plants was
42% lower than the control, and VMC was
84% lower than the control. On the day after
fertigation of the drought group at midday, P
n
partially recovered in the drought-stressed
plants but was still 32% lower than the control.
Further, VMC in the drought group on the day
after fertigation did not differ from that of the
control as measured the day after it was last
irrigated during this period.
Inflorescence dry weight and
cannabinoids
Inflorescence dry weight in the control
was 178 ± 9.4 g·m
–2
and was 232 ± 18.5 g·m
–2
in the drought treatment, but inflorescence
dry weight did not differ statistically between
the two treatments (P= 0.06; n = 3). The
moisture content of the dried and cured
inflorescences was 8 ± 0.1% in the control,
11% lower than that in the drought treatment,
at 9 ± 0.1% (P= 0.01; n = 3). Henceforward,
the inflorescence dry weight and cannabinoid
contents are corrected to 0% moisture.
Of the analyzed cannabinoids, all were
detected in at least one sample, these in-
cluded THC, THCA, CBD, CBDA, CBG,
CBGA, and CBN. In the drought treatment,
only one sample had a detectable concentra-
tion of CBG and CBN, and in the control,
there were no samples with detectable CBN;
therefore, comparisons could not be made for
these cannabinoids, and the means for CBN
were not presented.
The drought treatment elicited a 12% in-
crease in THCA concentration and a 13%
increase in CBDA concentration but had no
effect on the concentrations of the other detected
cannabinoids (Table 3, top). Drought had sub-
stantial effects on cannabinoid yield, expressed
as grams of cannabinoid from inflorescences
per unit growing area (g·m
–2
). In the drought
treatment, THC yield was 50% higher, THCA
yield was 43% higher, CBD yield was 67%
higher, and CBDA yield was 47% higher than
in the control (Table 3, bottom).
Discussion
The controlled drought treatment substan-
tially increased the concentrations of both
major cannabinoids, THCA and CBDA, as
well as yield of THCA, CBDA, THC, and
CBD compared with the control. These re-
sults suggested that the level of drought stress
applied was adequate to stimulate cannabi-
noid production without reducing inflores-
cence dry weight for this cultivar.
PlantWPprovedtobeaneffective
indicator of drought stress; at wilting
point, there was a significant difference in
plant WP between drought and control
groups. The stem psychrometer is a useful
tool for nondestructive assessment of
plant–environment interactions that may
vary by species and between individual
plants (Dixon and Tyree, 1984). Traits
such as crown architecture, root structure,
and leaf morphology all affect water trans-
port (Ali, 2010) and therefore drought re-
sponses. The combined effect of these and
environmental parameters can be quanti-
fied through plant WP measurements
(Dixon and Tyree, 1984; Stoochnoff
et al., 2018). The use of stem psychrome-
ters for irrigation scheduling is, however,
not commercially viable. The sensors are
costly and require significant technical
training. Substrate VMC or leaf wilting
are easier to measure and can be useful
indicators of drought if correlated to plant
WP data. Leaf angle measurements can be
made in seconds using a protractor and/or
angle finder; substrate VMC measurement
generally requires several substrate mois-
ture sensors, but data can be collected
remotely (Bogena et al., 2007).
In the present study, leaf wilting was an
effective indicator of plant stress. At the
irrigation threshold for the drought treat-
ment, plants were visibly wilted, and the
indicator leaf angle increased by 50%
from the turgid leaf angle. Using wilting
as a drought-stress indicator may therefore
be an effective method in cannabis pro-
duction, particularly because it is easily
measured. In potato (Solanum tuberosum L.),
for example, leaf wilting may be the most
obvious visual indicator of drought stress
(Banik et al., 2016). Notably, wilting re-
sponse to drought may vary by species (Xu
et al., 2010) and can depend on the degree
to which an individual plant is acclimated
to drought stress (Banik et al., 2016; Flexas
et al., 2009); therefore, using a 50% increase
in leaf angle wilting threshold as a drought
stress indicator may be most effective if used
with similar varieties of cannabis (chemovar
II) and under similar environmental condi-
tions (Table 1) to the present study.
To our knowledge, this was the first
evaluation of the effects of controlled drought
stress on cannabis; although, as previously
described, drought can increase essential oil
yield in some herbaceous crops. In drought-
stressed sage, essential oil and monoterpene
yield can increase up to 281% (Bettaieb et al.,
2009) and 20% (Nowak et al., 2010), respec-
tively, over a well-watered control. Likewise, in
Table 2. Plant water potential, leaf net photosynthetic rate, and substrate moisture of cannabis under drought conditions and after subsequent fertigation at 7 weeks
in the flowering stage.
Treatment Plant water potential (MPa)
Net photosynthetic rate
(mmol·m
–2
·s
–1
)
Volumetric substrate moisture
content (%)
Immediately before fertigation
(wilting point)
Control –1.0 ± 0.05
z
13.2 ± 1.14 33.3 ± 2.89
Drought –1.5 ± 0.12 7.7 ± 0.80 5.3 ± 1.23
Significance
y
** ** ***
Midday after fertigation Control –0.9 ± 0.09 13.9 ± 1.01 43.2 ± 1.36
x
Drought –0.6 ± 0.10 9.4 ± 0.65 39.4 ± 4.02
Significance NS ** NS
z
Data are means ± SEM; n = 3 for volumetric moisture content of the control and n = 4 for all other means.
y
NS, *, **, ***Nonsignificant or significant at P< 0.05, 0.01, or 0.0001, respectively.
x
Measured the day after the control was last irrigated during this period.
Table 3. Cannabinoid concentration and yield in the inflorescences of cannabis exposed to drought stress at week 7 in the flowering stage.
Treatment Yield THC THCA CBD CBDA CBG CBGA
Concn (%)
z
Control — 0.3 ± 0.02 4.7 ± 0.03 0.2 ± 0.01 9.1 ± 0.05 0.06 ± 0.004 0.45 ± 0.012
Drought — 0.3 ± 0.01 5.3 ± 0.09 0.2 ± 0.01 10.3 ± 0.09 0.08
w
0.49 ± 0.028
Significance
x
—NS ** NS ** ND
y
NS
Cannabinoid Yield (g·m
–2
)
z
Control 164 ± 8.5 0.4 ± 0.03 7.7 ± 0.40 0.3 ± 0.02 15 ± 0.7 0.1 ± 0.01 0.7 ± 0.03
Drought 211 ± 16.5 0.6 ± 0.07 11 ± 0.9 0.5 ± 0.04 22 ± 1.7 0.1
w
1.0 ± 0.12
Significance NS *** * ND NS
z
Data are means ±SEM and are corrected to zero percent moisture content; n = 3 unless otherwise indicated.
y
ND, no data or insufficient data to compare means.
x
NS, *, **, ***Nonsignificant or significant at P< 0.05, 0.01, or 0.0001, respectively.
w
n=1.
HORTSCIENCE VOL. 54(5) MAY 2019 967

curly-leafed parsley (Petroselinum crispum ssp.
crispum L. cv. curly-leafed) grown under
drought stressed conditions, plants were smaller
but had higher oil concentrations than the well-
watered control. The density of the plants could
therefore be increased to accommodate the
decreased size and essential oil yield per unit
area would be higher (Petropoulos et al., 2008).
Increases in secondary metabolite concen-
tration due to drought stress usually coincides
with reduced growth; however, this was not the
case in the present study, at least in terms of
inflorescence growth because there was no
difference in inflorescence dry weight between
drought and control groups. Secondary metab-
olites are formed from photosynthetic carbon
(Pe~
nuelas and Llusi
a, 2002), and drought can
reduce P
n
, as exemplified in the present study.
In fact, it is common for P
n
of plants exposed to
drought stress to recover only to 40% to 60% of
their predrought levels on the day after irriga-
tion, and P
n
may never fully recover (Delfine
et al., 2005; Kirschbaum, 1987; Sofo et al.,
2005). Nonetheless, in the present study, the
yield of some cannabinoids increased irrespec-
tive of reduced carbon assimilation. Protective
mechanisms help plants tolerate drought until
some cumulative physiological threshold is
exceeded, and only at this point is growth
impeded (Ali, 2010; Xu et al., 2010). A period
of drought insufficient to impede growth, such
as in the present study, may be crucial to
increasing secondary metabolite yield.
An understanding of the biochemical
origins of cannabinoids and how they relate
to other secondary metabolites may be useful to
speculate how drought stress increased canna-
binoid yield in the present study. For example,
cannabinoids are closely related to some terpe-
noids that protect plants under stress. Cannabi-
noids and terpenoids share a similar biochemical
pathway. Isopentenyl pyrophosphate is the basic
building block of all terpenoids and is pro-
duced either in the cytosol and mitochondria
through the Mevalonate pathway (Banthorpe
et al., 1972) or in the plastids through the
Mevalonate-independent (DXP) pathway
(Eisenreich et al., 1998). The DXP pathway is
the source of all mono, di-, and tetraterpenes,
which include many essential oil components
(Gershenzon et al., 2000). In cannabinoid
synthesis, geranyl pyrophosphate (GPP) fr om
the DXP pathway is combined with olive-
tolic acid (OA), a product of the polyketide
pathway (Flores-Sanchez and Verpoorte,
2008; Hanu
s et al., 2016), to produce can-
nabigerolic acid (CBGA) (Fellermeier et al.,
2001). CBGA is then converted to more
commonly known cannabinoids, such as
THCA and CBDA, through various syn-
thases (Taura et al., 1996).
Carotenoids and xanthophylls, which
are also produced through the DXP pathway,
are involved in mitigating photo-oxidative
damage caused by environmental stress
(Demmig-Adams, 1990). Stressors such as
drought elicit the formation of reactive
oxygen species in the chloroplasts, espe-
cially under high solar radiation (Penuelas
et al., 2004). Production of antioxidant
compounds in the chloroplasts such as
carotenoids and the xanthophylls is upre-
gulated in stressed plants to prevent cell
damage (Eskling et al., 1997; Munn
e-
Bosch and Alegre, 2000). This may also
be the case for other terpenes with antiox-
idant properties (Delfine et al., 2005; Llusi
a
and Pe~
nuelas, 1998; Munn
e-Bosch and
Alegre, 2000). Environmental stressors that
normally up-regulate terpenoid synthesis and
accumulation may do the same for some
cannabinoids because of their related bio-
chemical origins.
Both water and fertilizer were applied
together through fertigation, and thus there
were likely differences in the amount of
fertilizer applied to the treatment groups.
The concentration of fertilizer in the irriga-
tion water remained constant, and because
the drought stress group was irrigated less
than the control, it was also fertilized less.
Nutrients are largely taken up through the
roots along with water by mass transport, so
it can be difficult to sustain adequate min-
eral nutrition uptake in dry substrates
(Silber et al., 2003). This is a limitation in
any method for long-term drought applica-
tion. Nonetheless, research has shown that
flowering-stage cannabis performs similarly
under a range of organic fertigation rates
(Caplan et al., 2017a), and in the present trial,
thedrought-stressedgrouphadinflorescence
dry weight similar to the control, which would
not be expected if nutrition were lacking
(Caplan et al., 2017a).
Repetition of drought stress and subse-
quent acclimation can influence the way in
which plants respond to the stressor (Banik
et al., 2016). The present study evaluated the
effects of drought at a single point during the
flowering stage, but timing of drought stress
and drought-stress frequency could also af-
fect secondary metabolism in cannabis. Some
higher plants have the ability to acclimate to
drought stress; stress resistance may increase
after exposure to a low level of stress (Banik
et al., 2016; Flexas et al., 2009). The accli-
mation responses from repeated drought
stress could therefore further stimulate sec-
ondary metabolites in cannabis, although a
longer drought event may be required to elicit
the response in acclimated plants. More re-
search is needed to evaluate the effects of
drought-stress timing and acclimation on
cannabis inflorescence dry weight and sec-
ondary metabolism. Lastly, because rates of
cannabinoid accumulation vary by chemovar
(Aizpurua-Olaizola et al., 2016; Muntendam
et al., 2012), the effect of drought on other
chemovars should be explored.
Conclusions
This study suggested that controlled
drought stress can increase the concentration
of the major cannabinoids THCA and CBDA
and the yield of THCA, CBDA, THC, and
CBD in chemovar II cannabis without re-
ducing inflorescence dry weight and irre-
spective of decreased P
n
. These results were
achieved by gradually drying the substrate
over 11 d until plant WP reached approxi-
mately –1.5 MPa during week 7 in the
flowering stage. Comparable results can
be expected using leaf wilting as a
drought-stress indicator with fertigation
triggered at a leaf angle 50% higher than
in its turgid state. This method for admin-
istering drought stress and the results of
this study should be applicable for similar
varieties of chemovar II cannabis; how-
ever, other chemovars or varieties may
respond differently.
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