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Effect of different photosynthetic photon flux densities (0, 500, 1000, 1500 and 2000 μmol m(-2)s(-1)), temperatures (20, 25, 30, 35 and 40 °C) and CO2 concentrations (250, 350, 450, 550, 650 and 750 μmol mol(-1)) on gas and water vapour exchange characteristics of Cannabis sativa L. were studied to determine the suitable and efficient environmental conditions for its indoor mass cultivation for pharmaceutical uses. The rate of photosynthesis (PN) and water use efficiency (WUE) of Cannabis sativa increased with photosynthetic photon flux densities (PPFD) at the lower temperatures (20-25 °C). At 30 °C, PN and WUE increased only up to 1500 μmol m(-2)s(-1) PPFD and decreased at higher light levels. The maximum rate of photosynthesis (PN max) was observed at 30 °C and under 1500 μmol m(-2)s(-1) PPFD. The rate of transpiration (E) responded positively to increased PPFD and temperature up to the highest levels tested (2000 μmol m(-2)s(-1) and 40 °C). Similar to E, leaf stomatal conductance (gs) also increased with PPFD irrespective of temperature. However, gs increased with temperature up to 30 °C only. Temperature above 30 °C had an adverse effect on gs in this species. Overall, high temperature and high PPFD showed an adverse effect on PN and WUE. A continuous decrease in intercellular CO2 concentration (Ci) and therefore, in the ratio of intercellular CO2 to ambient CO2 concentration (Ci/Ca) was observed with the increase in temperature and PPFD. However, the decrease was less pronounced at light intensities above 1500 μmol m(-2)s(-1). In view of these results, temperature and light optima for photosynthesis was concluded to be at 25-30 °C and ∼1500 μmol m(-2)s(-1) respectively. Furthermore, plants were also exposed to different concentrations of CO2 (250, 350, 450, 550, 650 and 750 μmol mol(-1)) under optimum PPFD and temperature conditions to assess their photosynthetic response. Rate of photosynthesis, WUE and Ci decreased by 50 %, 53 % and 10 % respectively, and Ci/Ca, E and gs increased by 25 %, 7 % and 3 % respectively when measurements were made at 250 μmol mol-1 as compared to ambient CO2 (350 μmol mol(-1)) level. Elevated CO2 concentration (750 μmol mol(-1)) suppressed E and gs ∼ 29% and 42% respectively, and stimulated PN, WUE and Ci by 50 %, 111 % and 115 % respectively as compared to ambient CO2 concentration. The study reveals that this species can be efficiently cultivated in the range of 25 to 30 °C and ∼1500 μmol m(-2)s(-1) PPFD. Furthermore, higher PN, WUE and nearly constant Ci/Ca ratio under elevated CO2 concentrations in C. sativa, reflects its potential for better survival, growth and productivity in drier and CO2 rich environment.
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Physiol. Mol. Biol. Plants, 14(4)–October, 2008
299Photosynthetic response of Cannabis sativa
Correspondence and Reprint requests : Suman Chandra
Photosynthetic response of Cannabis sativa L. to variations in
photosynthetic photon flux densities, temperature and CO2 conditions
Suman Chandra1, Hemant Lata1, Ikhlas A. Khan1,2 and Mahmoud A. Elsohly1,3
1National Center for Natural Product Research, School of Pharmacy, University of Mississippi, MS-38677, USA.
2Department of Pharmacognosy, University of Mississippi, MS-38677, USA.
3Department of Pharmaceutics, School of Pharmacy, University of Mississippi, University, MS 38677, USA.
Effect of different photosynthetic photon flux densities (0, 500, 1000, 1500 and 2000 μmol m-2s-1), temperatures (20, 25,
30, 35 and 40 oC) and CO2 concentrations (250, 350, 450, 550, 650 and 750 μmol mol-1) on gas and water vapour exchange
characteristics of Cannabis sativa L. were studied to determine the suitable and efficient environmental conditions for its
indoor mass cultivation for pharmaceutical uses. The rate of photosynthesis (PN) and water use efficiency (WUE) of Cannabis
sativa increased with photosynthetic photon flux densities (PPFD) at the lower temperatures (20-25 oC). At 30 oC, PN and
WUE increased only up to 1500 μmol m-2s-1 PPFD and decreased at higher light levels. The maximum rate of photosynthesis
(PN max) was observed at 30 oC and under 1500 μmol m-2s-1 PPFD. The rate of transpiration (E) responded positively to
increased PPFD and temperature up to the highest levels tested (2000 μmol m-2s-1 and 40 0C). Similar to E, leaf stomatal
conductance (gs) also increased with PPFD irrespective of temperature. However, gs increased with temperature up to 30 oC
only. Temperature above 30 oC had an adverse effect on gs in this species. Overall, high temperature and high PPFD showed
an adverse effect on PN and WUE. A continuous decrease in intercellular CO2 concentration (Ci) and therefore, in the ratio
of intercellular CO2 to ambient CO2 concentration (Ci/Ca) was observed with the increase in temperature and PPFD. However,
the decrease was less pronounced at light intensities above 1500 μmol m-2s-1. In view of these results, temperature and light
optima for photosynthesis was concluded to be at 25-30 oC and ~1500 μmol m-2s-1 respectively. Furthermore, plants were
also exposed to different concentrations of CO2 (250, 350, 450, 550, 650 and 750 μmol mol-1) under optimum PPFD and
temperature conditions to assess their photosynthetic response. Rate of photosynthesis, WUE and Ci decreased by 50 %,
53 % and 10 % respectively, and Ci/Ca, E and gs increased by 25 %, 7 % and 3 % respectively when measurements were
made at 250 μmol mol-1 as compared to ambient CO2 (350 μmol mol-1) level. Elevated CO2 concentration (750 μmol mol-
1) suppressed E and gs ~ 29% and 42% respectively, and stimulated PN, WUE and Ci by 50 %, 111 % and 115 % respectively
as compared to ambient CO2 concentration. The study reveals that this species can be efficiently cultivated in the range of
25 to 30 oC and ~1500 μmol m-2s-1 PPFD. Furthermore, higher PN, WUE and nearly constant Ci/Ca ratio under elevated CO2
concentrations in C. sativa, reflects its potential for better survival, growth and productivity in drier and CO2 rich environment.
[Physiol. Mol. Biol. Plants 2008; 14(4) : 299-306] E-mail :
Key words : Cannabis sativa, Photosynthesis, Transpiration, Water use efficiency
Abbreviations : PPFD - Photosynthetic photon flux density, PN - Photosynthesis, Rd – Dark respiration, PN max - Maximum
rate of photosynthesis, E - Transpiration, gs - Leaf stomatal conductance, Ci - Leaf internal CO2 concentration, Ci/Ca - Internal
to ambient CO2 concentration, WUE - Water use efficiency
Research Article
The ability of a species to acclimate and adapt to
environmental variations is directly/indirectly associated
with its ability to modulate photosynthesis and water
vapour exchange (Pearcy, 1977; Berry and Downtown,
1982; Stoutjesdijk and Barkman, 1992; Ayuko et al., 2008;
Dieleman and Meinen, 2008; Kruse et al., 2008), which
in turn affects biochemical and physiological processes
in the leaf and, consequently the physiology and
productivity of whole plant. Studies on gas exchange
characteristics may provide valuable information on
functioning of plants in variable environment.
Photosynthesis, being the primary source of carbon
and energy, plays a prominent role in the logistics of
plant growth. There is a close correlation between
Physiol. Mol. Biol. Plants, 14(4)–October, 2008
300 Chandra et al.
productivity and yield of the plants with their
photosynthetic rate, in the given environment, as more
than 90% of dry matter of live plants is derived from
photosynthetic CO2 assimilation (Zelitch, 1975).
Therefore, photosynthesis is a valuable physiological
tool to evaluate the response of plants to environmental
stresses and for the rapid selection of plants for a
particular environmental condition (Joshi and Palni, 2005;
Monclus et al., 2006) or selection of suitable
environmental conditions for a particular plant species.
Furthermore, elevated CO2 may increase
photosynthetic carbon assimilation and may accelerate
plant growth and potentially improve productivity.
Indeed, a doubling in CO2 concentration increases crop
yield by 30% or more, in experiments conducted under
close environmental conditions such as green houses
and growth chambers (Kimball, 1983a, b; 1986; Cure,
1985; Poorter, 1993; Idso and Idso, 1994). Therefore, in
the present study, C. sativa plants were exposed to a
range of CO2 concentration to understand their response
in term of their photosynthetic capacity to the range of
elevated CO2 labels.
Cannabis sativa L. is widely distributed around the
world. Originally indigenous to temperate regions of
Asia, it now grows in a variety of habitats ranging from
sea level in tropical areas to alpine foot hills of
Himalayas. Cannabis has a long history of the medicinal
use in Middle East and Asia, with references as far back
as the 6th century B.C. This species was introduced in
the Western Europe medicine in the early 19th century
A.C. to treat epilepsy, tetanus, rheumatism, migraine,
asthma, trigeminal neuralgia, fatigue, and insomnia
(Doyle and Spence, 1995; Zuardi, 2006). C. sativa
contains cannabinoids, a unique class of terpenophenolic
compounds, which accumulates mainly in glandular
trichomes of the plant (Hammond and Mahlberg, 1977).
Over 70 cannabinoids have been isolated from Cannabis
sativa, the major biologically active compound being
Δ9- tetrahydrocannabinol, commonly referred as THC
(Mechoulam and Ben-Shabat, 1999). Besides its
psychoactivity, THC possesses analgesic, anti-
inflammatory, appetite stimulant and anti-emetic
properties making this cannabinol a very promising
therapeutic drug, especially for cancer and AIDS patients
(Sirikantaramas et al., 2005). The pharmacologic and
therapeutic potency of preparations of Cannabis sativa
L. and its main active constituent Δ9-
tetrahydrocannabinol (THC) has been extensively
reviewed by researchers (Mechoulam, 1986; Formukong
et al., 1989; Grinspoon and Bakalar, 1993; Mattes et al.,
1993; 1994; Brenneisen et al., 1996).
THC has a tremendous commercial value in the
pharmaceutical market. Since C. sativa is a natural and
inexpensive source of THC (as compared to producing
it synthetically), efforts to select Cannabis varieties
with high THC content are underway. However, due to
the allogamous (cross fertilization) nature of the species,
it is very difficult to maintain the chemical profile of
selected high THC-producing genotypes under field
conditions. Since this plant is also used as an illicit
drug, its cultivation in open field must be done in
secured areas and is highly regulated in the USA and
some other parts of the world. Considering these
limitations, indoor cultivation of a selected high yielding
genotype/clone under controlled environmental
conditions is the most suitable way to maintain its
potency and efficacy while circumventing the regulatory
problems. The objective of this study was to determine
the effect of light intensity, temperature and CO2
conditions on gas and water vapour exchange
characteristics of C. sativa L. to establish suitable and
efficient environmental conditions for its indoor
To study the photosynthetic response of C. sativa under
different PPFD and temperature levels, leaves of twenty
vegetatively propagated, four month old plants from a
single mother plant of high yielding Mexican variety
were exposed to a range of PPFD (0, 500, 1000, 1500 and
2000 μmol m-2s-1) and temperature conditions (20, 25, 30,
35 and 40 oC) under controlled humidity (55 ± 5 %) and
CO2 (350 ± 5 μmol mol-1) concentration to determine
suitable environmental conditions for it’s optimum
photosynthetic assimilation. Thereafter, leaves were
acclimated under optimum light and temperature
conditions and exposed to different CO2 concentrations
(250, 350, 450, 550, 650 and 750 μmol mol-1) to study the
effect of CO2 on photosynthetic and water vapour
characteristics of this species. All the measurements
were carried out on five upper undamaged, fully
expanded and healthy leaves of each plant with the
help of a closed portable photosynthesis system (Model
LI-6400; LI-COR, Lincoln, Nebraska, USA) equipped with
light, temperature, humidity and CO2 controls. Different
PPFD were provided with the help of an artificial light
source (Model LI-6400-02; light emitting silicon diode;
LI-COR), fixed on the top of the leaf chamber and were
recorded with the help of quantum sensor kept in range
of 660-675 nm, mounted at the leaf level. The rate of
dark respiration was measured by maintaining the leaf
cuvette at zero irradiance. To avoid any radiation from
Physiol. Mol. Biol. Plants, 14(4)–October, 2008
301Photosynthetic response of Cannabis sativa
outside the leaf chamber was covered with a black cloth
through the respiratory measurements. Temperature of
the cuvette was controlled by the integrated Peltier
coolers, which is controlled by the microprocessor.
Different concentrations of CO2 were supplied to the
cuvette of climatic unit (LI-6400-01, LI-COR Inc., USA)
by mixing pure CO2 with CO2 free air and were measured
by infrared gas analyzer. All the measurements for gas
and water vapour exchange were first recorded at lowest
PPFD and temperature condition and then subsequently
to the increasing levels of these parameters. Similarly,
leaves under optimum PPFD and temperature conditions
were first exposed to the lowest level of CO2
concentration followed by elevated levels. Air flow rate
(500 mmol s-1) and relative humidity (55 ± 5%) were kept
nearly constant throughout the experiment. Since steady
state photosynthesis is reached within 30–45 min, the
leaves were kept for about 45–60 min under each set of
light conditions before the observations were recorded.
Four gas exchange parameters viz., photosynthetic rate
(PN), transpirational water loss (E), stomatal conductance
for CO2 (gs) and intercellular CO2 concentration (Ci)
were measured simultaneously at steady state condition
under various permutations and combinations of light
and temperature. Water use efficiency (WUE) was
calculated as a ratio of the rate of photosynthesis and
transpiration. A correlation and multiple regression
analysis of data was performed on the basis of multiple
linear hypothesis PN, E, gs, Ci, Ci/Ca and WUE as a
dependent variable on PPFD, temperature and different
CO2 concentrations using SYSTAT-11 (Systat Software
Inc. San Jose, CA, USA) statistical software.
Both photosynthetic assimilation and biomass
production are temperature- and light-dependent
processes. The potential for photosynthetic acclimation
to growth temperature is quite variable between species.
Generally, variations in PN reflect adjustment to the
respective growth environment and also to the resistance
to climate rigors. Although plants can exhibit a high
degree of plasticity with respect to temperature response
of photosynthesis, there is a general consensus that
the optimum temperature for photosynthesis for an
individual plant species reflects the environmental
temperature range for which the species is genetically
and physiologically adapted (Berry and Bjorkman, 1980).
On other hand, response of photosynthesis to PPFD
has been a long standing interest. At the leaf surface,
low PPFD might be a limiting factor and high PPFD may
be a threat to the plant metabolism if the irradiance
exceeds the demand of photosynthesis (Osmond, 1994;
Aguirre-von Wobeser et al., 2000). Therefore,
determination of the conditions for optimum gas and
water vapour exchange processes is a prerequisite for
growing any species indoor. According to our data on
C. sativa, temperature optima for PN was observed at 30
oC. In general, temperature higher than 30 oC had an
adverse effect on PN (Fig. 1A). At 25 oC, rate of
photosynthesis increased with increasing PPFD, but this
trend peaked with 1500 μmol m-2s-1 PPFD at 30 oC, and
decreased at higher light intensities. Similar effect of
Fig. 1. A. Variations in net photosynthesis in C. sativa with
varying photosynthetic photon flux densities (PPFD) and
temperature conditions. B. The temperature dependence of
Dark respiration in Cannabis sativa.
Photon Flux Density (μmolm
0 500 1000 1500 2000
Net Photosynthesis
( m
(ol m
Temperature (
20 25 30 35 40
Dark Respiration
μmol m
Physiol. Mol. Biol. Plants, 14(4)–October, 2008
302 Chandra et al.
PPFD was observed at temperatures higher than 30 oC.
Maximum rate of photosynthesis (PN max) was 24.60 μmol
m-2s-1 at 30 oC and under 1500 μmol m-2s-1 PPFD. The
interaction of PPFD and temperature demonstrates that
high PPFD and higher temperature together (PPFD ×
temperature) had an adverse effect on PN. In general,
effect of PPFD (r = 87) was more prominent in regulating
PN in Cannabis sativa as compared to temperature (r =
An increase in Rd (μmol m-2s-1 PPFD) was observed
with increasing temperature up to 30 oC and decreased
at higher temperature (Fig. 1B). Working on two different
populations of Podophyllum hexandrum, Singh and
Purohit (1997) reported a linear increase in Rd with
temperature (up to 40 oC) in alpine population whereas;
in temperate population, Rd increased with temperature
up to 30 oC and decreased at higher levels. 2 to 10 fold
increase in Rd was reported by Joshi and Palni (1998)
in different tea leaves with increase in temperature from
20 to 40 oC; higher temperature however, was associated
with clones having higher photosynthetic rates. In C.
sativa, decrease in Rd followed a trend similar to PN,
with varying temperatures. Reduced PN, and increased
Rd are reported to limit the productivity in some plant
species at higher temperatures (Alexander et al., 1995;
Thornton et al., 1995).
Stomatal conductance was commensurate to PPFD
levels, irrespective of temperature (Fig. 2). A positive
correlation (r = 56) was observed between PPFD and gs
in C. sativa. On other hand, gs increased with increasing
temperature up to a maximum value at 30 oC and
decreased at higher temperatures under all the PPFD
labels. Maximum value of gs was recorded at 30 oC and
highest level of PPFD (2000 μmol m-2s-1).
In contrast to gs, E increased in response to both
higher temperature and high PPFD. Lowest value of E
(2.38 ± 0.28 mmol m-2s-1) was observed at 20 oC under
0 μmol m-2s-1 PPFD, whereas highest value (7.60 ± 0.33
mmol m-2s-1) was recorded at 40 oC under 2000 μmol
m-2 s-1 (Fig. 3). Transpiration rate is known to depend
on gs (Alexander et al., 1995), and it seems to be major
factor driving E in the present study. An increase in E
and decrease in gs is reported in many plant studies
(Rawson et al., 1977; Schulze et al., 1972).
Intercellular CO2 concentration (Ci) decreased with
increase in PPFD and temperatures up to highest level
tested (PPFD up to 2000 μmol m-2s-1 and temperature up
to 40 oC (Fig. 4). Highest Ci (367 ml L-1) was observed
at lowest PPFD and temperature conditions i.e. 20 oC
and 0 μmol m-2s-1 PPFD and, thereafter lowest Ci (149 ml
L-1) was recorded at highest PPFD and temperature
conditions. However, the decrease was less pronounced
at light intensities above 1500 μmol m-2s-1. Effect of
temperature on depression of Ci was more prominent
above 30 oC. Higher temperature and higher light
together had a significant adverse effect on Ci of this
species. Photosynthetic data particularly on Ci and gs,
Fig. 2. Variations in stomatal conductance in C. sativa with
varying photosynthetic photon flux densities (PPFD) and
temperature conditions.
Fig. 3. Variations in rate of transpiration in C. sativa with
varying photosynthetic photon flux densities (PPFD) and
temperature conditions.
Photon Flux Density (
μmol m
mmol m
0 500 1000 1500 2000
Stomatal Conductance
Photon Flux Density (μmol m
0 500 1000 1500 2000
(mm ol m
Rate of Transpiration
Physiol. Mol. Biol. Plants, 14(4)–October, 2008
303Photosynthetic response of Cannabis sativa
indicates that both stomatal and mesophyll factors seems
to be involved in the mechanism of control of
photosynthesis by temperature and light in C. sativa.
Similar to Ci, a gradual decrease in Ci/Ca ratio was
also observed with increasing PPFD and temperature
conditions (Table 1). About 32 %, 41 %, 44 %, 50 % and
57 % decrease in Ci/Ca ratio was observed at 20, 25, 30,
35 and 40 oC respectively when plants were exposed
from 0 to 2000 μmol m-2s-1 PPFD. Similarly, about 3 %,
17 %, 29 %, 37 % and 39 % depression was observed
under 0, 500, 1000, 1500 and 2000 μmol m-2s-1 PPFD
when plants were exposed to 40 oC as compared to 25
oC. Although essentially a biochemical process,
photosynthesis is often regarded as a diffusive process.
The rate of diffusion of CO2 is largely controlled by two
factors, gs and CO2 concentration gradient between
carboxylation site and ambient air (Ca). This CO2
concentration gradient at given gs and Ca is established
predominantly by Ci, which is a result of mesophyll
efficiency. Therefore, the diffusive entry of CO2 into
leaf is a reflection of intrinsic mesophyll capacity.
Sheshshayee et al. (1996) have reported Ci/gs ratio as
an indicator of mesophyll efficiency and a representation
of mesophyll control on PN. Our data also represent
highest mesophyll efficiency (i.e. lowest Ci/gs ratio)
around 30 oC and 1500 μmol m-2s-1 PPFD. Values of Ci/
gs ratio increased with temperature higher than 30 oC,
which further confirms that a combination of 30 oC
temperature and 1500 μmol m-2s-1 PPFD may be best
suitable for the indoor cultivation of C. sativa.
Fig. 4. Variations in intercellular CO2 concentration in C.
sativa with varying photosynthetic photon flux densities
(PPFD) and temperature conditions.
Fig. 5. Variations in water use efficiency in C. sativa with
varying photosynthetic photon flux densities (PPFD) and
temperature conditions.
Table 1. Effect of different photosynthetic photon flux density and temperature conditions on Ci/Ca ratio in the
leaves of Cannabis sativa.
Light Intensities Temperature (0C)
(μmol m-2s-1)
20 25 30 35 40
000 1.04 ± 0.12 1.04 ± 0.14 1.02 ± 0.11 1.01 ± 0.09 1.01 ± 0.07
500 0.82 ± 0.05 0.79 ± 0.06 0.74 ± 0.06 0.71 ± 0.06 0.68 ± 0.05
1000 0.80 ± 0.06 0.75 ± 0.04 0.66 ± 0.06 0.59 ± 0.04 0.57 ± 0.06
1500 0.71 ± 0.04 0.62 ± 0.06 0.58 ± 0.05 0.51 ± 0.05 0.45 ± 0.04
2000 0.70 ± 0.06 0.61 ± 0.05 0.57 ± 0.05 0.50 ± 0.04 0.43 ± 0.03
Photon Flux Density (μ
mol m
0 500 1000 1500 2000
Intercellular CO
Photon Flux Density (μ
mol m-2
0 500 1000 1500 2000
Water Use Efficiencyx 100
Physiol. Mol. Biol. Plants, 14(4)–October, 2008
304 Chandra et al.
At 20 and 25 oC, WUE increased with increase in
PPFD up to 2000 μmol m-2s-1 (Fig. 5). On the other
hand, WUE increased only up to 1500 μmol m-2s-1 PPFD
at 30 oC and decreased thereafter at higher light levels.
Temperature higher than 30 oC had an adverse effect on
WUE of this species. The maximum WUE was observed
at 30 oC and under 1500 μmol m-2s-1 PPFD.
Photosynthesis appears to have a greater influence than
E over regulating water use efficiency in C. sativa. A
highly significant positive correlation was observed
between WUE and PN (r = 0.92). Together, high
temperature and high PPFD had an adverse effect on
the WUE in C. sativa.
Increasing atmospheric CO2 is a global environmental
concern. Atmospheric CO2 has risen from pre- industrial
value of ~ 280 μmol mol-1 to present concentration of ~
372 μmol mol-1 and is expected to exceed 700 μmol mol-
1 by the end of century (Prentice et al., 2001; Long et
al., 2004). Since ambient CO2 concentration as a
substrate is still a limiting factor for photosynthesis in
C3 plants, attempts are being made to study how changes
in atmospheric CO2 concentration will affect crops
(Bowes, 1993; Drake et al., 1997; Long et al., 2004).
This study on Cannabis sativa shows that PN, WUE
and Ci decreased by 50 %, 53 % and 10 % respectively,
and Ci/Ca, E and gs increased by 25 %, 7 % and 3 %,
respectively, when measurements were made at 250 μmol
mol-1 as compared to ambient CO2 (~350 μmol mol-1)
level (Table 2). An average of 30 to 33 % increase in PN
and productivity of C3 plants with doubling atmospheric
CO2 concentration has been already reported by Kimball
1983a, b; 1986; Idso and Idso 1994; Bazzaz and Gabutt,
1988; Cure and Acock, 1986. In C. sativa, a doubling of
CO2 concentration (750 μmol mol-1) suppressed E and gs
~29 % and 42 % respectively, and stimulated PN, WUE
and Ci by 50%, 111 % and 115 % respectively as
compared to ambient CO2 concentration. Doubling CO2
level had a significant effect on all these parameters.
Suppression in gs and consequently in E (Emaus et al.,
1993; Thomas et al., 1994) and improvement in PN and
WUE and Ci (Kimball 1983a, b; 1986; Idso and Idso
1994, Morison, 1993) under elevated CO2 concentration
is reported in many other plant species. Higher WUE
under elevated CO2, primarily because of decreased gs
and E, may enable this species to survive under drought
conditions. This species maintained nearly constant
values of Ci/Ca with increasing CO2 concentration
despite the increase in PN and WUE, and decrease in gs
and E, represents a close coordination between stomatal
and mesophyll functions (Morison, 1993) and reported
to improve growth and productivity of plant (Jones,
In view of our results, it is concluded that C. sativa
can utilize a fairly high level of PPFD and temperature
for its gas and water exchange processes, and can
perform much better if grown at ~ 1500 μmol m-2 s-1
PPFD and around 25 to 30 oC temperature conditions.
Furthermore, higher PN, WUE and nearly constant Ci/Ca
ratio under elevated CO2 concentration, reflects its
potential for improved growth and productivity in drier
and CO2 rich environment.
This research was supported by National Institute of
Drug Abuse (NIDA), USA, Contract # NO1DA-0-7707.
Table 2. Effect of different levels of CO2 on net photosynthesis (PN), transpiration (E), stomatal conductance (gs),
internal CO2 concentration (Ci), Ratio of internal to external CO2 concentration (Ci/Ca) and water use
efficiency (WUE) on the leaves of Cannabis sativa.
CO2 levels PNEg
sCi Ci/Ca WUE ×
(μmol mol-1)(μmol CO2(mmol H2O (mmol CO2(μl L-1) ratio 100
250 12.48 ± 1.76 5.69 ± 0.47 202.76 ± 19.78 138.00 ± 11.42 0.55 2.19
350 24.64 ± 2.24 5.31 ± 0.35 195.99 ± 18.40 202.00 ± 14.00 0.47 4.64
450 24.76 ± 1.89 5.76 ± 0.44 189.78 ± 16.97 260.00 ± 19.34 0.58 4.30
550 26.54 ± 2.12 4.87 ± 0.38 148.37 ± 13.99 330.00 ± 22.47 0.60 5.46
650 30.48 ± 2.76 4.65 ± 0.76 136.08 ± 12.36 385.00 ± 33.24 0.61 6.56
750 36.80 ± 3.18 3.75 ± 0.33 112.76 ± 10.32 435.00 ± 37.23 0.58 9.81
Physiol. Mol. Biol. Plants, 14(4)–October, 2008
305Photosynthetic response of Cannabis sativa
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... The criteria for including the articles in the review was based on the chemical analysis of phytocannabinoids 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 phytocannabinoids were cited because the results obtained could contribute to the development of future researchers. These studies were not counted in the 78 selected articles. ...
... 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 µmol 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). ...
... 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 CO 2 concentration [133]. ...
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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.
... However, hemp plants can reach a considerable photosynthetic rate even in medium radiation ( Figure No. 1). The above agrees with Chandra et al. (2008), who indicated that photosynthesis in hemp plants is highly influenced by the intensity and quality of the plants' light. Our results are like those obtained by Chandra et al. (2015) and Tang et al. (2017), where the maximum values of A in hemp plants were observed between 1,300 and 1,500 µmol of photons m -2 s -1 at a temperature of 30°C, while lower PPDF reduced CO2 assimilation capacity and therefore yield, while higher values, did not have a significant effect on this capacity. ...
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Los métodos de reversión sexual se utilizan comúnmente en los programas de fitomejoramiento, permitiendo la formación de flores masculinas a partir de plantas femeninas y viceversa. Este trabajo tuvo como objetivo evaluar métodos de reversión sexual en plantas femeninas de Cannabis y su efecto sobre el intercambio de gases. Plantas tratadas con 1-metil-ciclopropano (1-MCP) y aminoetoxivinilglicina (AVG) mostraron diferencias en fotosíntesis neta (A) y conductancia estomática (gs) entre los periodos antes y después de los tratamientos de reversión sexual. El rendimiento cuántico (Qy), la tasa de transporte de electrones (ETR) y la disipación no fotoquímica (NPQ) no mostraron relación con los tratamientos, se observó un incremento en Qy y ETR y una reducción en NPQ después de la aplicación de los tratamientos. 1-MCP, AVG y STS (tiosulfato de plata) fueron efectivos en la reversión sexual, mientras que los cambios en el fotoperiodo no indujeron la formación de flores masculinas. La inducción de la reversión sexual en plantas de Cannabis no generó variaciones en los mecanismos que disipan la energía a través de los fotosistemas.
... Therefore, one may also expect that UV radiation induces secondary metabolite biosynthesis in cannabis. Increased irradiance leads to an elevated total THC concentration in the plants [40] along with an increased photosynthetic rate and water use efficiency [41]. Specifically, it has been reported that the amount of THC and CBD dramatically increased in cannabis leaves following treatment with 100 mM GA3 [42]. ...
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Cannabis sativa L. is an annual dioecious plant that belongs to the Cannabaceae family and is essential for different pharmaceutical and nutritional properties. The most important and prevalent cannabinoids in cannabis are cannabidiol and delta-9-tetrahydrocannabinol. The application of elicitors is an effective method to improve secondary metabolite production, leading to a whole spectrum of molecular, genetic, and physiological modifications. Therefore, the expression changes of four key genes (THCAS, CBDAS, PT, and OLS) of the cannabinoids pathway along with the delta-9-tetrahydrocannabinol and cannabidiol metabolites fluctuation were surveyed following the application of ascorbic acid as an elicitor. Cannabis was sprayed immediately before flowering with ascorbic acid. Treated and untreated (control) plants were sampled in different time courses for real-time PCR and HPLC experiments. Results showed significant increases in THCAS, CBDAS, PT, and OLS expression after ascorbic acid treatments. The results of metabolite quantification also indicated that secondary metabolites, especially delta-9-tetrahydrocannabinol and cannabidiol, increased after the ascorbic acid application. This study contributes to the growing body of knowledge of the functions of key genes in the cannabinoids pathway to the engineering of cannabis for improving the production of delta-9-tetrahydrocannabinol and cannabidiol metabolites in this plant.
... The actual time of flowering depends on weather, environmental conditions, and management operations, and the variety of photoperiods in cannabis genotypes depends on different species and environmental conditions (Sengloung, 2009). The highest photosynthetic efficiency in this plant is observed at less than 1500 photosynthetic photon flux density (PPFD) and the approximate temperature recorded for the best efficiency is about 25 CÀ30 C (Chandra et al., 2008). ...
Cannabis which is a medicinal and industrial plant is native to Central Asia. It has been used as a source of food, fuel, fiber, medicine, and drugs for centuries. Cannabis has valuable agronomic traits, such as being easy to cultivate and creating diversity in organic farming. From an agricultural point of view, it is a high-yielding crop and does not require much pesticide, herbicide, and fertilizer compared to other kinds of crops, and therefore will have a less negative impact on the environment. Cannabis seeds are rich in protein and oil and have long been used by humans. Cannabis sativa seed oil contains good amounts of unsaturated fatty acids, including linoleic acid and linolenic acid, which are good for human nutrition and health and reduce cholesterol and high blood pressure. Cannabis can also be considered an excellent option for the production of two important biofuels, namely, biodiesel and bioethanol. In terms of phytochemicals, this plant is very complex and it contains more than 480 different chemical compounds. Some of these compounds belong to the primary metabolites such as amino acids, fatty acids, and steroids, while compounds such as cannabinoids, flavonoids, acetylbenoids, terpenoids, lignans (phenolic amides and lignamides), and alkaloids are among the secondary metabolites produced by the valuable cannabis plant. Therefore cannabis is similar to a very strong factory in terms of phytochemicals, which has a high value in the pharmaceutical industry. Today, the positive effect of cannabis pharmaceutical compounds has been reported in studies done on the effect of cannabis on various diseases, such as cancer, multipple sclerosis (MS), and acquired immune deficiency syndrome (AIDS), which further increases the value of this plant. As a result, due to the high importance of cannabis in pharmacy, medicine, and industry, agricultural science must study the environmental factors affecting the cultivation of this valuable plant to increase its yield and efficiency of the plant. Today, light-emitting diodes are considered an influential factor in the production of agricultural products by many researchers. Over the past years, several studies have been conducted on the effect of light spectra on plant growth and development, which have well demonstrated the importance of the blue and red spectra. Consequently, finding the right combination of light spectra for better growth and development of plants, including cannabis, is under investigation to get the best performance from the plant at the most appropriate time.
... Cannabis has been found to benefit from extremely high light intensities (Chandra et al., 2008;Potter and Duncombe, 2012;Eaves et al., 2020;Rodriguez-Morrison et al., 2021). Inflorescence yield was found to increase linearly with light intensity up to photosynthetic photon flux densities of 1,800 µmol·m −2 ·s −1 (Rodriguez-Morrison et al., 2021). ...
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Current biological control strategies in cannabis (Cannabis sativa) cultivation have resulted in poor efficacy for managing certain insect pests. The cannabis industry has grown at a rapid pace, surpassing our ability to develop knowledge on the production systems for this crop. Currently, the research focus is on optimizing agronomic and environmental factors to maximize the yield and quality of cannabis. However, cannabis growers are increasingly challenged by severe insect pest pressure, with few effective options. Decades of research have optimized biological control strategies in other crops. The implementation of effective biological control strategies in cannabis is hindered by a variety of morphological, biochemical, and agronomic factors unique to this crop. Here, we review the rather limited literature relevant to insect pest management in indoor cannabis production. Further, we have identified three factors that we believe are primarily responsible for the ineffectiveness of biological control in cannabis: Plant morphology including trichome density and floral resources, effects of plant biochemistry on prey suitability, and finally the effects of supplemental lighting including photoperiod, intensity, and spectrum. We highlight the importance of prioritizing the evaluation of these factors to improve our understanding of the tritrophic interactions governing the success of biological control in cannabis cultivation. As intensive research efforts are underway to optimize agronomic practices for cannabis, it is also important to consider their relevance to biological control.
... Bilodeau, Wu, Sen, Rufyikiri, MacPherson, and Lefsrud (2019) point out that although we have not identified a superior lighting value for Cannabis cultivation, some studies have given good results with values between 1500 and 2000 µmoles/m 2 /s. One of these studies is Chandra et al. (2008), where temperature, CO 2 concentration, humidity, and light intensity are related to producing a Mexican Cannabis variety of high productivity. Its conclusions indicate that Cannabis grows better in a PPFD (photon flux per second) of 1500 µmoles at a temperature of 25 to 30 degrees Celsius and with increases in CO 2 levels in the cultivation area. ...
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The medical Cannabis industry has experienced significant growth in recent years thanks to changes in the legislation of many countries. Latin America has been no stranger to these dynamics. Therefore, it is interesting to study the progress made in this industry, and investigation of the impact may have on the competitiveness in the production and processing of this plant. This article aims to identify and analyze the salient aspects of the use and development of technologies for the medical cannabis industry. There is a particular emphasis on the horticulture of the plant. To this end, the authors carried out a technological surveillance exercise. Technological surveillance is a systematic analysis of scientific and technical information to identify research and technological development trends. As a result, it was possible to focus on three areas: improving growing conditions, products related to cultivation, and improving genetics. These results contribute to describing the global technological panorama of medicinal Cannabis cultivation. Additionally, they are the basis for decision making in orienting the use of technologies of interest internationally and in considering the possibilities of diversification in this emerging industry in countries such as Colombia.
... In addition to the genetic variety, many environmental factors affect the composition of the secondary metabolites in the Cannabis plant (Tang et al., 2016). These include growth conditions such as humidity, light quality and intensity, CO 2 concentration and mineral nutrition (Chandra et al., 2008;Chandra et al., 2017;Bernstein et al., 2019a). The tissue type is also an important factor as within the plant there is a locationand organ-specific distribution of the active secondary metabolites (Happyana et al., 2013;Bernstein et al., 2019a;Bernstein et al., 2019b). ...
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Medical Cannabis and its major cannabinoids (−)-trans-Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) are gaining momentum for various medical purposes as their therapeutic qualities are becoming better established. However, studies regarding their efficacy are oftentimes inconclusive. This is chiefly because Cannabis is a versatile plant rather than a single drug and its effects do not depend only on the amount of THC and CBD. Hundreds of Cannabis cultivars and hybrids exist worldwide, each with a unique and distinct chemical profile. Most studies focus on THC and CBD, but these are just two of over 140 phytocannabinoids found in the plant in addition to a milieu of terpenoids, flavonoids and other compounds with potential therapeutic activities. Different plants contain a very different array of these metabolites in varying relative ratios, and it is the interplay between these molecules from the plant and the endocannabinoid system in the body that determines the ultimate therapeutic response and associated adverse effects. Here, we discuss how phytocannabinoid profiles differ between plants depending on the chemovar types, review the major factors that affect secondary metabolite accumulation in the plant including the genotype, growth conditions, processing, storage and the delivery route; and highlight how these factors make Cannabis treatment highly complex.
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Cannabis (Cannabis sativa) flourishes under high light intensities (LI); making it an expensive commodity to grow in controlled environments, despite its high market value. It is commonly believed that cannabis secondary metabolite levels may be enhanced both by increasing LI and exposure to ultraviolet radiation (UV). However, the sparse scientific evidence is insufficient to guide cultivators for optimizing their lighting protocols. We explored the effects of LI and UV exposure on yield and secondary metabolite composition of a high Δ9-tetrahydrocannabinol (THC) cannabis cultivar ‘Meridian’. Plants were grown under short day conditions for 45 days under average canopy photosynthetic photon flux densities (PPFD, 400–700 nm) of 600, 800, and 1,000 μmol m–2 s–1, provided by light emitting diodes (LEDs). Plants exposed to UV had PPFD of 600 μmol m–2 s–1 plus either (1) UVA; 50 μmol m–2 s–1 of UVA (315–400 nm) from 385 nm peak LEDs from 06:30 to 18:30 HR for 45 days or (2) UVA + UVB; a photon flux ratio of ≈1:1 of UVA and UVB (280–315 nm) from a fluorescent source at a photon flux density of 3.0 μmol m–2 s–1, provided daily from 13:30 to 18:30 HR during the last 20 days of the trial. All aboveground biomass metrics were 1.3–1.5 times higher in the highest vs. lowest PPFD treatments, except inflorescence dry weight – the most economically relevant parameter – which was 1.6 times higher. Plants in the highest vs. lowest PPFD treatment also allocated relatively more biomass to inflorescence tissues with a 7% higher harvest index. There were no UV treatment effects on aboveground biomass metrics. There were also no intensity or UV treatment effects on inflorescence cannabinoid concentrations. Sugar leaves (i.e., small leaves associated with inflorescences) of plants in the UVA + UVB treatment had ≈30% higher THC concentrations; however, UV did not have any effect on the total THC in thesefoliar tissues. Overall, high PPFD levels can substantially increase cannabis yield, but we found no commercially relevant benefits of adding UV to indoor cannabis production.
Cannabis sativa L. has raised a lot of interest in recent years, due to the different utilities of the plant, being useful in different types of industries, as well as the discovery of possible therapeutic utilities of different secondary metabolites of the plant. This chapter presents the effect of the different environmental factors on the different vital phases of the plant, emphasizing its effects on its secondary metabolism. Secondly, we will review different agronomic techniques related to irrigation, the behavior of the plant in water scarcity scenarios, mineral nutrition and the use of different phytohormones and chemical supplements, focusing on their influence on the secondary metabolism of C. sativa L. Finally, the use of the novel biostimulants and biocontrols in this crop and their future prospects are discussed.
Although the vegetative stage of indoor cannabis (Cannabis sativa) production can be relatively short in duration, there is a high energy demand due to higher light intensities (LI) than the clonal propagation stage and longer photoperiods than the flowering stage (i.e., ≥ 16 vs. 12 h). While electric lighting is a major component of both energy consumption and overall production costs, there is a lack of scientific information to guide cultivators in selecting a LI that corresponds to their vegetative stage production strategies. To determine the vegetative plant responses to LI, clonal plants of ‘Gelato’ (indica-dominant hybrid genotype) were grown for 21 days with canopy-level photosynthetic photon flux densities (PPFD) ranging between 135 and 1430 µmol·m⁻²·s⁻¹ with a 16-h photoperiod (i.e., daily light integrals of 7.8–82.4 mol·m⁻²·d⁻¹). Plant height and growth index (i.e., a canopy volume metric) responded quadratically; the number of nodes, stem thickness, and aboveground dry weight increased asymptotically; and internode length and water content of aboveground tissues decreased linearly with increasing LI. Foliar attributes had varying responses to LI. Chlorophyll content index (i.e., SPAD value) increased asymptotically, leaf size decreased linearly and specific leaf weight increased linearly with increasing LI. Generally, PPFD levels of ≈ 900 µmol·m⁻²·s⁻¹ produced compact, robust plants while PPFD levels of ≈ 600 µmol·m⁻²·s⁻¹ promoted more open plant architecture (i.e., taller plants with longer internodes), which can increase intra-canopy airflow and may reduce development of potential foliar pests in compact (e.g., indica-dominant) genotypes.
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This rigorous yet accessible text introduces the key physical and biochemical processes involved in plant interactions with the aerial environment. It is designed to make the more numerical aspects of the subject accessible to plant and environmental science students, and will also provide a valuable reference source to practitioners and researchers in the field. The third edition of this widely recognised text has been completely revised and updated to take account of key developments in the field. Approximately half of the references are new to this edition and relevant online resources are also incorporated for the first time. The recent proliferation of molecular and genetic research on plants is related to whole plant responses, showing how these new approaches can advance our understanding of the biophysical interactions between plants and the atmosphere. Remote sensing technologies and their applications in the study of plant function are also covered in greater detail.
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Carbon assimilation rates (A) have been shown to be substrate limited associated with the stomatal conductance (Gs). However, a significant species as well as genotypic variation in intercellular CO2 concentration (Ci) in the present study implies that mesophyll factors also play an important role in regulating A through its effect in determining the CO2 concentration gradient. Efficiency of the mesophyll factors is often estimated by measuring the initial slope of CO2 response curve (dA/dCi). In this paper we provide experimental evidences to show that Ci at a given Gs is also a reflection of mesophyll efficiency. Steady state Ci levels are maintained by both Gs and the efficiency of the mesophyll to utilize the substrate CO2. Therefore, at a given stomatal conductance, lower Ci should indicate a better mesophyll efficiency. The Ci/GS ratio also showed an inverse relationship with dA/dCi. Therefore, Ci/GS ratio can be considered as a novel approach to estimate the mesophyll efficiency. We also noticed that the Ci/Gs ratios showed a significant inverse relationship with assimilation rate, suggesting the role of mesophyll factors in the regulation of photosynthesis. The implications of the Ci/GS ratio are discussed in this paper.
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Seeds of Maranthes corymbosa Blume, a monsoon rain forest species of northern Australia, were sown under ambient or elevated CO2 concentrations in tropical Australia. Seedlings were grown under conditions of photon flux density, temperature and atmospheric vapour pressure deficit which followed ambient variations as closely as possible. Specific leaf area, chlorophyll, stomatal density, stomatal conductance and assimilation responses to photon flux density were measured after 30 weeks growth. Gas exchange characteristics were divided into morning and afternoon data sets and analysed separately. Stomatal density decreased and leaf area:dry weight ratio decreased in response to elevated CO2. In contrast there was no effect of elevated CO2 upon chlorophyll (total or ratio of a:b). Apparent quantum yield and rates of light saturated assimilation (Amax) increased in response to elevated CO2. There was a significant decline in apparent quantum yield for both treatments between morning and afternoon. Stomatal conductance (gs) declined in response to elevated CO2. There was no significant difference in gs between morning and afternoon for ambient grown trees, but gs declined significantly between morning and afternoon for elevated CO2 grown trees. Instantaneous transpiration efficiency (ITE) was higher for elevated CO2 grown trees compared with control trees. There was a significant increase in ITE between morning and afternoon data for ambient grown trees; in contrast a significant decline in ITE was observed for elevated CO2 grown trees between morning anf afternoon data sets. The slope of the regression between assimilation rate and stomatal conductance increased for plants grown under elevated CO2. These data are discussed and compared with the responses of plants adapting to different photon flux densities.
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Ultraviolet-B (UV-B) radiation and nitrogen are expected to increase simultaneously with future changes in global climate. In this study, growth and photosynthetic responses of Picea asperata seedlings to enhanced UV-B and to nitrogen supply were studied. The experimental design included two levels of UV-B treatments (ambient UV-B, 11.02 kJ m-2 d-1; enhanced UV-B, 14.33 kJ m-2 d-1) and two nitrogen levels (0; 20 g N m-2) to determine whether nitrogen can alleviate the negative impacts of enhanced UV-B on seedling growth and photosynthesis. Enhanced UV-B significantly inhibited plant growth and impaired net photosynthetic rate, stomatal conductance, transpiration rate, the light-saturated assimilation rate, assimilation capacity, light compensation point, dark respiration rate, apparent quantum yield, photosynthetic pigments and maximum quantum yield of photosynthesis of P. asperata seedlings, whereas minimal fluorescence and intercellular CO2 concentration increased by enhanced UV-B. On the other hand, nitrogen supply improved the photosynthetic performance and plant growth, but only under ambient UV-B. In fact, nitrogen supply could not alleviate the photosynthetic impairments in P. asperata seedlings exposed to enhanced UV-B radiation.
The glandular secretory system in Cannabis sativa L. (marihuana) consists of three types of capitate glandular hairs (termed bulbous, capitate-sessile, and capitate-stalked) distinguishable by their morphology, development, and physiology. These gland types occur together in greatest abundance and developmental complexity on the abaxial surface of bracts which ensheath the developing ovary. Bulbous and capitate-sessile glands are initiated on very young bract primordia and attain maturity during early stages of bract growth. Capitate-stalked glands are initiated later in bract growth and undergo development and maturation on medium, to full sized bracts. Glands are epidermal in origin and derived, with one exception, from a single epidermal initial. The capitate-stalked gland is the exception and is of special interest because it possesses a multicellular stalk secondarily derived from surrounding epidermal and subepidermal cells. Glands differentiate early in development into an upper secretory portion and a subtending auxiliary portion. The secretory portion, depending on gland type, may range from a few cells to a large, flattened multicellular disc of secretory cells. The secretory portion produces a membrane-bound resinous product which caps the secretory cells. Capitate-stalked glands are considered to be of particular evolutionary significance because they may represent a gland type secondarily derived from existing capitate-sessile glands.
Four members of an annual community were used to investigate the effects of changing neighborhood complexity and increased CO"2 concentration on competitive outcome. Plants were grown in monoculture and in all possible combinations of two, three, and four species in CO"2-controlled growth chambers at CO"2 concentrations of 350, 500, and 700 @mL/L with ample moisture and high light. Species responded differently to enhanced CO"2 level. Some species (e.g., Abutilon theophrasti) had increased biomass with increasing CO"2, while others (e.g., Amaranthus retroflexus) had decreased biomass with increasing CO"2 concentration. In mixtures, species tended to interact strongly, and, in some cases, the interaction canceled out the effects of CO"2. Furthermore, there were cleared differences in species behavior in different competitive mixtures as assessed by total biomass and seed biomass, and by an index of response to neighbors. In general, competitive arrays that had C"3 species depressed the response of C"4 species, especially Amaranthus. Ambrosia artemisiifolia was the strongest competitor in this assemblage. Strong statistical interactions between CO"2 and the identity of the competing species in mixtures were found to be primarily due to the as yet unexplained response of plants with CO"2 at 500 @mL/L. The potential effects of CO"2 on community structure could be profound, particularly at the intermediate levels of CO"2 that are predicted to be reached during the first half of the next century.
The stomata of plants growing in the Negev Desert, namely the stomata of the mesomorphic leaves of Prunus armeniaca, the xeromorphic stems of Hammada scoparia, and the succulent leaves of Zygophyllum dumosum, respond to changes in air humidity. Under dry air conditions diffusion resistance increases. Under moist air conditions diffusion resistance decreases. When the stomata close at low air humidity the water content of the apricot leaves increases. The stomata open at high air humidity in spite of a decrease in leaf water content. This excludes a reaction via the water potential in the leaf tissue and proves that the stomatal aperture has a direct response to the evaporative conditions in the atmosphere. In all species the response to air humidity is maintained over a period of many hours also when the soil is considerably dry. The response is higher in plants with poor water supply then in well watered plants. Thus for field conditions and for morphologically different types of photosynthesizing organs the results confirm former experiments carried out with isolated epidermal strips.
Carbon dioxide and water vapor exchange at different levels of photosynthetically active radiation and temperature were measured in alpine and temperate populations of Podophyllum hexandrum Royle. The photosynthesis rate and stomatal conductance were high in alpine population irrespective of the temperature and the level of photosynthetically active radiation. Maximum and minimum rates of photosynthesis were recorded at 20° and 40°C, respectively, in both populations. The water use efficiency decreased with increasing temperatures, but was higher in the temperate population. Light compensation values were also higher in temperate plants at all temperatures. Cultivation of alpine ecotypes would appear to be easier at lower elevations as compared with temperate types.