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Experimental Study on Variations of C
O
2Concentration in the Presence of
Indoor Plants and Respiration of Experimental Animals
Geun Sug Oh, Gun Joo Jung*, Min Ho Seo, and Young Bin Im
Division of Architecture, College of Engineering, Pukyong National University, Busan 608-739, Korea
*Corresponding author: envjung@pknu.ac.kr
Received August 12, 2010 / Accepted March 15, 2011
GKorean Society for Horticultural Science and Springer 2011
Abstract. This study aims to suggest an improved experimental method to reveal the ability of indoor plants to reduce CO2
concentrations, as well as to display the individual CO2 reduction characteristics of various indoor plants in accordance with
this improved method. In previous studies, experiments were conducted under the condition in which the CO2 concentration in
the experimental chamber is set only once to a high initial level of 1,000 ppm. However, in real conditions, CO2 concentration
gradually increases in a room after the occupants enter. Hence, the existing experimental method can be improved in view of
“light saturation and CO2 compensation”. Accordingly, in this study, the CO2 reduction characteristics of indoor plants under 2
conditions used in the existing method of measurement (Case 1) and the condition in the new method, which considers that
CO2 concentration gradually increases through the respiration of experimental animals (Case 2)-were measured and compared
against each other. For all plant samples, the level of CO2 reduction was higher in Case 2 than in Case 1, and the rate of CO2
reduction increases with time. The inflection point of CO2 concentration appeared at leaf areas of 9,000 cm2 in peace lily and
areca palm, and 6,000 cm2 in weeping fig.
Additional key words: CO2 reduction quantity, compensation point, indoor air quality, respiration of experimental animals
Hort. Environ. Biotechnol. 52(3):321-329. 2011.
DOI 10.1007/s13580-011-0169-6
Research Report
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Previous studies have reported that urban residents spend
more than 80% of their whole day indoors despite their busy
and diversified lifestyles (Jenkins et al., 1992; Mølhave et
al., 2003; Orwell et al., 2004; US EPA, 2010; Wolkoff, 2003).
Particularly for office workers, who stay for a long time in a
single space, the quality of indoor air has a large influence
on the condition of the human body. However, as most
indoor air quality (IAQ) improvement methods suggested
thus far (US EPA, 2011) require large energy consumption
(Godish et al., 1989), their application ultimately and inev-
itably damage the balance among the components that provide
comfort to humans. For this reason, interest in utilizing plants
for passive methods such as cross-ventilating or mechanical
air conditioning of the interior of buildings is increasing, as
well as the number of researches reporting its efficacy.
From large trees that compose luxuriant forest to small
plants that grow under the shade of these trees (which humans
use as indoor plants under conditions of less-bright illumin-
ation), all plants create a microclimate environment around
their leaves and roots for survival. Plants undergo photo-
synthesis from which nutritive elements are generated, using
light, CO2, and H2O as raw materials (Wolverton, 1996).
During photosynthesis, air pollutants are eliminated as the
stomas of plants absorb the air pollutants together with CO2
(Kondo et al., 1992). Most of the absorbed pollutants are used
for plant growth (Chang et al., 2007; Winner, 1994). Moreover,
plants eliminate air pollutants absorbed into the soil through
the microorganisms in the rhizosphere (Orwell et al., 2004;
Wood et al., 2002). Therefore, the natural survival activity of
plants allows humans to live in a comfortable environment
with fresh air.
Previous studies have suggested that plants eliminate volatile
organic compounds such as benzene (Cornejo et al., 1999;
NASA, 1989; Orwell, 2004; Yoo et al., 2006), ethylbenzene
(Darlington, 2001), toluene (Cornejo et al., 1999; Darlington,
2001; Yoo et al., 2006), xylene (Darlington, 2001), and tri-
chloroethylene (Cornejo et al., 1999; NASA, 1989), even for-
maldehyde (Giese et al., 1994; NASA, 1989). Plants have also
been suggested to eliminate gas pollutants such as NOx (Fujii
et al., 2005; Henrik, 1986; Wolverton, 1985), O3 (Park et al.,
1998), CO (Wolverton, 1985), and CO2 (Fujii et al., 2005;
Han et al., 1996; Lee, 2004; Oh et al., 2009), as well as
Geun Sug Oh, Gun Joo Jung, Min Ho Seo, and Young Bin Im
322
particulate matter (Lohr, 1996).
However, plants do not consistently maintain equivalent
photosynthesis quantity [net photosynthesis quantity (NP)],
and the apparent gross photosynthesis quantity (GP) varies
depending on the environmental conditions. The formula for
GP is given below.
Gross photosynthesis (GP) = Net Photosynthesis (NP) +
Respiration (R)
If GP equals R, NP becomes zero. Relevant plants are
considered to be at the compensation point, which is an in-
dicator of the adaptability of a plant to the surrounding con-
ditions. Although NP increases in proportion to the quantity
of light, when the introduced light is small in quantity, in
cases where variable levels of light are used, the reaction
curve becomes horizontal. This means that another factor
(normally CO2 supply) besides the quantity of light limits
photosynthesis. This condition is referred to as light saturation
or CO2 limitation (Ridge, 2008).
Photosynthesis generally varies in accordance with the
balance of light quantity and CO2 concentration in plant cells
(Zeiger et al., 1982). That is, stronger light induces a higher
photosynthetic rate and a lower concentration of CO2 in the
cells (Jarvis et al., 1981). If the CO2 concentration is limited,
the rates of stomatal conductance and photosynthesis also
become limited (Farquhar et al., 1980; Hall et al., 1980;
Tenhunen et al., 1984).
Oh et al. (2009) reported that plants adapted to low-light
conditions showed no large difference in the quantity of CO2
purification, even in the lower illumination intensity indoors
(1,000 lx), which is below the compensation point (3,000 lx),
if all other conditions are the same. Kil et al. (2008) also
suggests that the quantity of photosynthesis and formalde-
hyde elimination is influenced by the presence or absence of
light. However, the intensity of illumination almost has no
influence on these factors. In reality, light strength inside
offices is regulated by criteria on the illumination intensity
on the work surface in many countries [i.e., 300 - 1,000 lx; in
Korea, provision 3011 of KSA (Korean Standards Association
1991) is being implemented]. For indoor plants that are
already adapted to low and homogeneous illumination con-
ditions, CO2 concentration becomes the major limitation factor
for photosynthesis.
CO2, which is generated by individuals occupying a space
and by combustion apparatus, is an essential as well as limiting
factor, as described previously. CO2 is known to be an indoor
air pollutant and an indicator of IAQ (Wargocki et al., 2000).
Although CO2 does not inflict harm on the human body by
itself, an increase in its concentration sometimes indicates
the deterioration of the normal thermal condition or the in-
crease in different pollutant elements. High CO2 concen-
trations can also cause sick building syndrome (SBS) by
inducing metabolic disorders in individuals.
Regardless of the area, period, and time of inspection;
air-conditioning systems; presence or absence of occupants;
and nature of the office environment, the results of actual
measurements of CO2 inside offices conducted in many
countries after the year 2002 show that CO2 quantity is within
international regulation criteria (1,000 ppm by ASHRAE) or
the regulation criteria of the respective country (e.g., in Korea,
the criteria for IAQ in facilities of mass use, as set by the
Rule on Industrial Health Criteria, is 1,000 ppm) (Gupta et
al., 2007; Hong et al., 2008; Jeong et al., 2006, 2007; Mui et
al., 2008; Oh et al., 2010; Park et al., 2000; Sekhar et al.,
2002, 2003; Wargocki et al., 2002). However, Kim et al.
(1993) reported that more than half of the respondents of
their questionnaire experienced SBS symptoms. Moreover,
Lee et al. (1995) and Gupta et al. (2007) reported many cases
in which the measured quantity of CO2 satisfied the stipulated
regulation criteria but not the occupants staying in the indoor
space. In this regard, it seems that continuous reduction in
CO2 concentration is needed even for environments having a
CO2 concentration lower than the regulated values. Besides
playing a role in the elimination of indoor air pollutants, in-
door plants also contribute to conserving energy, providing
a positive psychological effect on individuals, and improving
the overall quality of indoor air.
However, since the initial study by NASA (1980), most
of the studies on the role of indoor plants in the improvement
of IAQ in many countries were conducted in which the
initial concentration of pollutants was set as the general air-
quality criteria regardless of the actual pattern of pollutant
generation. This existing research method may be appropriate
for comparison of IAQ improvement among various kinds
of plants, or among various kinds of plants in diverse en-
vironmental conditions. However, it is inappropriate for real
office conditions for which the means of maintaining CO2
levels are different.
In the studies by Wood et al. (2006) and Kim et al. (2009),
in which the plants were placed in real office space, the sug-
gested volume of plants needed to maintain appropriate
pollutant concentration was smaller than that suggested by
previous studies in which experiments were performed in a
chamber. This result was confirmed by a previous study (Oh
et al., 2010) that used peace lily as the experimental sample.
That is, CO2 reduction seems higher when the CO2 concen-
tration is continuously increasing, such as in real space,
compared to experiments in which the CO2 concentration is
set only once initially to a high extent, when the other
environment conditions related to photosynthesis of plants
are the same. Moreover, this difference in the reduction of
CO2 quantity increased with measurement time.
Accordingly, this study aims to reveal the air-purification
Hort. Environ. Biotechnol. 52(3):321-329. 2011. 323
ED
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Fig. 1. Experimental chamber system. (A) Glazing chamber 0.93
m × 0.6 m × 0.9 m, (B) Measurement and data logging system,
(C) Tube connection and air dryer, (D) Fan for air flow, (E) Seal
nut of the chamber frame, (F) Hamsters in the chamber.
characteristic according to plant kind. We added 2 more plants
to that used by Oh et al. (2010). This follow-up study suggests
a more substantial impact of indoor plants on indoor air im-
provement than was previously suggested.
The outcome of this study and the expected effect according
to this outcome are described below.
First, this study reveals that the air-pollutant-lowering effect
of indoor plants in real space is higher than the value reported
by the existing studies. Second, the experiments revealed the
largest value of leaf surface area that should be considered
in relation to the size of chamber. By applying this value into
real space, the optimal leaf surface area needed in relation to
the volume of the space can be determined. This value can be
utilized as a reference data for future studies on this subject.
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The experiment location is a room in the east side of the
4th floor of the Architecture Department Building at Pukyong
National University in Busan metropolitan area, Korea. A
chamber system for the experiment was installed at the
center of the room.
The chamber was small, on the scale of about 0.5 m3
volume (0.93 m × 0.6 m × 0.9 m); made of glass, the top
part of which can be opened and closed; and was designed
in such a way that air leakage can be reduced by the op-
pressing pressure exerted by the weight of the upper opening
part. Two units of silicon hose for gas exchange were installed
and fixed at a height of 1.2 m from the bottom, at 1 side of
the chamber. These hoses were connected to the gas inlet of
the CO2 measurement apparatus [IAQ Analyzer(ISR400) by
NDIR method] so that air can be constantly circulated between
the chamber and measurement apparatus. The sensor part
for the measurement of temperature, humidity, air velocity,
and illumination intensity, was installed on the side of the
chamber (at a height of 1.2 m from the bottom of laboratory),
and was connected to the measurement apparatus outside the
chamber. The schematic of the installation of the chamber
system is shown in Fig. 1.
The condition inside the chamber, except for CO2 concen-
tration, complies with the required environmental condition
for the comfort of occupants (ASHRAE Standard 55, 2004),
the values of which are similar to those set by existing studies.
The details of the experimental condition are as follows.
Temperature and humidity were set in the range of 21-25G
and 30-60%, respectively, by using an electric heat pump
(EHP)/cooler as well as a heater in the laboratory, and by
using a newly manufactured silica gel canister through which
vapor flow passage can be adjusted for setting the humidity.
The simple canister was installed in the part where air returns
to the chamber from the CO2 measurement apparatus. Air
current was set at 0.3-0.5 ms-1 by using a small ball-bearing
fan. The fan was installed about 5 cm from the glass surface
of the chamber. The air was directed toward the wall to reduce
the direct influence of air current on the plants. Light intensity
was set at 16 ± 5 ȝmolm-2s-1 (approximately 1,000 lx) by
controlling the volume of incoming daylight (skylight window
3.4 m × 1.6 m, pair glass) and using a fixed quantity of
artificial light (fluorescent lamp 32 W, 2 sets; daylight color
fluorescent lamp 45 W, 2 sets). This light intensity was con-
sistently maintained during experiment.
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To set the CO2 concentration similar to that in real offices,
experimental animals (i.e., hamsters) were placed inside the
chamber.
Considering that the volume of the chamber is much
smaller compared to real office space, male junglian hamsters,
a kind of dwarf hamsters, were used. The breeding environment
was maintained such that the hamsters can sustain psycho-
logical and physiological stability as the experiment progresses.
To comply with the standards of Korea Good Laboratory
Practice (KGLP) (Korea Food and Drug Administration, KFDA,
2005) and ASHRAE (2004), the following conditions were
set: 23 ± 2Gtemperature, 40 ± 10% relative humidity, less
than 0.2 m/s air current, 150-300 lx illumination density, 12
h of illumination time, less than 60 dB noise, and less than
Geun Sug Oh, Gun Joo Jung, Min Ho Seo, and Young Bin Im
324
D
E
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Fig. 2. Plants used in the small-chamber test. (A) Peace lily:
Spathiphyllum clevelandii, (B) Weeping fig: Ficus benjamina, (C)
Areca palm: Chrysalidocarpus lutescens.
20 ppm ammonia, which produces an offensive smell. To
avoid problems due to territoriality among the experimental
animals, the hamsters were bred separately.
A ball-type automatic water supply apparatus was installed.
Feed and water were supplied once a day in the evening. To
keep the chamber clean, the bedding was replaced twice a
week.
Oh et al. (2010) reported that CO2 concentrations vary
according to the age of experimental animals (6-9 weeks).
The hamsters showed an increasing pattern in terms of CO2
concentration as time passes (more than 0.99 R2 value), and
the quantity of CO2 generation from respiration seems to be
similar among the experimental animals. In the experiment,
3 individual hamsters were grouped to maintain a continuous
CO2 generation pattern in accordance with time, like in the
real office condition. CO2 generation was measured every 1
min for 90 min. The resulting average linear tilt was 71.17
ppm/10 min (more than 0.99 R2 value), which is in the range
of 45-83 ppm/10 min, the linear tilt of CO2 generated by
occupants in a real office (Oh et al., 2010). The CO2 con-
centration after 90 min appeared to be similar to that of the
initial 1-time setting used in existing studies (1,000 ppm).
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Three species of indoor plants were used in the experiment:
peace lily (P, Spathiphyllum clevelandii), weeping fig (W,
Ficus benjamina), and areca palm (A, Chrysalidocarpus
lutescens). These plants have higher CO2 reduction effects
even at conditions of indoor illumination compared with
other foliage plants known to purify indoor air (Oh et al.,
2009). The sample plants are shown and described in Fig. 2.
Although peace lily shows higher CO2 reduction levels in
daylight than in artificial light, it can grow and survive in a
semi-sunny, semi-shaded, or completely shaded place.
As it has a high adaptability to shade-it can survive in
conditions in which more than 90% of sunshine is blocked,
in conditions under the criteria of a sunny summer day in
Korea (Korea National Institute of Horticultural & Herbal
Science; NIHHS, 2011)-peace lily exerts a high CO2 reduction
effect even under conditions of low illumination intensity,
such as at indoor places where the regulated illumination
intensity on the work surface, as required by law, is 300-
1,000 lx. As weeping fig grows and develops in regions with
distinct dry and rainy seasons, it sheds its leaves during the
dry season for survival, which shows its sensitivity to stress
induced by water shortage, as well as its adaptability to diverse
light intensities. Additionally, for rubber trees like weeping
figs, the light compensation point and required light intensity
for survival are low as they produce special proteins under
light-deprived conditions (NIHHS, 2011). Lastly, although
areca palm demands a somewhat higher light intensity (1,600-
4,300 lx) compared to the other experimental plants, it shows
a high CO2 reduction effect when both daylight and artificial
light are simultaneously considered, even when illumination
intensity is low (i.e., lower than 1,000 lx) (Oh et al., 2009).
In most plants, chlorophyll production and CO2 fixation
decrease in the absence or in low quantities of water. To
prevent this problem in the experimental plants, water was
supplied twice a week and culture medium was injected once
a week. To reduce the influence of variations in environmental
condition on the CO2 reduction effect, purification of experi-
mental plants was performed more than 4 weeks in advance,
under laboratory conditions, including temperature, humidity,
and illumination intensity, similar to those used in the ex-
periment.
The leaf areas of the plant samples were calculated such
that an average-sized leaf was primarily selected and scanned
Hort. Environ. Biotechnol. 52(3):321-329. 2011. 325
for computerization. The ratio of leaf vein length to leaf area
was calculated by the CAD program. Subsequently, the lengths
of veins of all leaves were measured, and the previously cal-
culated ratio was applied to these measured leaf vein lengths.
The leaf area per pot for all experimental plants was adjusted
to 3,000 cm2 by pruning. Moreover, to keep the CO2 reduction
effect uniform in the rhizosphere of each experimental plant,
the respective plants were individually planted in identical
soils (peat moss 50% + bark 20% + wood by-product 20% +
sand 10%) in black plastic pots(15 cm dia. × 20 cm ht.).
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For the experimental period, a number of sunny days, in
accordance with the criteria provided by the Korea Meteo-
rological Administration, were selected in the period of
December 2009-April 2010. Measurements were done every
1 min for 90 min between 10:00 and 16:00, when the natural
light was comparatively homogeneous. The experiment was
halted in cases when the measured light intensity is to below
16 ± 5 ȝmolm-2s-1 (approximately 1,000 lx).
The experiment was conducted by separating Case 1 (in
which the CO2 concentration was set only once at an initial
value of 1,000 ppm, which is higher than in real space and is
similar to that used in the existing studies) and Case 2 (in
which the CO2 concentration was set at 350-450 ppm, which
was initially similar to that in the natural condition, but
increased to G1,000 ppm through the respiration of hamsters)
according to the criteria of CO2 concentration variation pattern.
Moreover, the leaf surface area varied from 3,000 cm2 to
15,000 cm2, with an interval of 3,000 cm2, for all the ex-
perimental plants. The measurements were conducted 5 times
per leaf-area classification, and results in which 3 out 5 meas-
urements showed a stable variance pattern of CO2 concentration
were used for analysis. To determine the net CO2 reduction
quantity among all plants, the variation of CO2 concentration
from an initial value of 1,000 ppm was measured in an empty
chamber 3 times for 24 h. The resulting variation was lower
than 10 ppm, which is within the range of measurement error.
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In view of ‘light saturation and CO2 compensation’, the
photosynthesis rates of plants vary in accordance with the
variation of CO2 concentration in a space having indoor
illumination. As the existing studies were conducted at a
higher CO2 concentration (set once at an initial concentration
of 1,000 ppm) than that in real office conditions, the rate of
photosynthesis is limited by CO2 concentration. The effect
of plants on the improvement of indoor-air quality may be
underestimated because the photosynthesis rate of the experi-
mental plants is estimated to be lower than that in reality.
This study investigated the CO2 reduction characteristics of
experimental plants in a chamber in which the CO2 concen-
tration gradually increases due to the respiration of experimental
animals (i.e., hamsters). This condition is similar to the CO2
variation pattern in real offices. The experiment was conducted
according to leaf-area classification, after dividing Case 1 and
Case 2 according to the CO2 generation pattern in the chamber.
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The experimental value of the variation in CO2 concentration
is suggested by the average value in an interval of 10 min.
The variation patterns of CO2 concentration for the above-
described experiments are shown in Fig. 3-5.
The pattern of CO2 concentration variation was similar
among all experimental plants with the passage of time. The
R2 values were significant at 0.97-0.99. While in Case 1, the
variation tilt of CO2 concentration (negative value) became
steeper as the leaf area of the plants increased, in Case 2, the
variation tilt of CO2 concentration (positive value) became
gentler as the leaf area of the plants increased. This means
that the difference in the tilt of CO2 concentration increased
as the respiration of hamsters increased. That is, for both Case
1 and Case 2, the larger the leaf area, the higher the CO2
reduction effect of indoor plants.
As Oh et al. (2010) suggested, for all experimental plants,
the CO2 reduction effect seemed to be higher in the later part
of the measurement period for Case 1, while it is higher in
the earlier part of the measurement period for Case 2. Hence,
it is believed that in real space, the improvement effect of
indoor plants on IAQ will continuously increase. However,
CO2 reduction quantities differed according to the increase
in leaf area among the experimental plants. This issue will
be examined in the next section.
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The resulting values of CO2 reduction quantities (ppm/1.5
h) by the experimental plants were converted to CO2 reduction
quantities (1.5 h) in proportion to the volume of the chamber
(about 500 L), as shown in Fig. 6.
For all experimental plants, the values of Case 2 were
higher than those of Case 1, and the differences were 0.005-
0.04 L for peace lily, 0.01-0.06 L for weeping fig, and
0.025-0.06 L for areca palm. Therefore, the CO2 reduction
effect of the plants is believed to be higher in real offices,
where the CO2 concentration increases continuously due to
occupants, than the values in the existing studies, in which
the CO2 concentration was set at a higher value than what
exists in real space. From the experimental results, it seems
that areca palm has the highest CO2 reduction effect: The
CO2 reduction values (L/1.5 h) were 0.07-0.13 for peace lily,
0.04-0.13 for weeping fig, and 0.11-0.17 for areca palm.
This implies that areca palm can be preferentially selected
as an indoor plant for the purpose of reducing indoor CO2
Geun Sug Oh, Gun Joo Jung, Min Ho Seo, and Young Bin Im
326
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Fig. 3. CO2 variation patterns of peace lily. Leaf Area (cm2). (A)
3,000, (B) 6,000, (C) 9,000, (D) 12,000, and (E) 15,000.
Fig. 4. CO2 variation patterns of weeping fig. Leaf Area (cm2). (A)
3,000, (B) 6,000, (C) 9,000, (D) 12,000, and (E) 15,000.
Hort. Environ. Biotechnol. 52(3):321-329. 2011. 327
F
E
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Fig. 5. CO2 variation patterns of areca palm. Leaf Area (cm2). (A)
3,000, (B) 6,000, (C) 9,000, (D) 12,000, and (E) 15,000.
F
E
D
Fig. 6. Volume of CO2 removal as influenced by leaf area at 1.5
hrs after the initi ation of the experiment. (A) Peace lily: Spathiphyllum
clevelandii, (B) Weeping fig: Ficus benjamina, (C) Areca palm:
Chrysalidocarpus lutescens.
concentrations in regions with a temperate climate.
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By varying the leaf area up to 15,000 cm2 for all 3 ex-
perimental plants, the amount of CO2 reduction was found
to vary only minutely despite the increase in leaf area. It is
assumed that the leaf areas considered were the maximum
plant quantities that the chamber can accommodate.
For peace lily and areca palm, the inflection point appeared
at the leaf area of 9,000 cm2. If these experimental plants
had more than 9,000 cm2 of leaf area for a 500-L chamber
volume, the ratio of leaves in which photosynthesis can be
actively generated would be lowered due to the high-density
arrangement, as these plants have characteristically large
leaf area and grow with boughs extended and spread toward
Geun Sug Oh, Gun Joo Jung, Min Ho Seo, and Young Bin Im
328
all directions. Hence, it becomes difficult for the plants to
further increase the apparent gross photosynthesis quantities,
and the variation in the quantities of CO2 reduction becomes
minute. Unlike the other 2 experimental plants, the inflection
point of CO2 reduction quantity appeared at 6,000 cm2 of
leaf area in weeping fig. As weeping fig has a characteristically
small leaf area and grows tall, even in the case of a high
input volume of this plant into the chamber, the ratio of sun
leaves versus shade leaves was almost unchanged. Despite
these characteristics, it is believed that the inflection point
appears at 6,000 cm2 of leaf area because this size offers
adaptability to diverse environmental conditions as the plant
grows naturally in places with distinct rainy season and dry
season. As the weeping fig sheds all its leaves to survive
during the dry season in nature, it is believed to adjust its
photosynthesis quantity by itself in conditions beyond the
requirement for adequate survival and growth, such as very
high plant density; in conditions of lower illumination density;
or some other conditions.
Finally, the maximum density was converted to leaf area
per 1 m3 of plant volume to allow application in real space
which gave values of 18,000 cm2m-3 for peace lily and areca
palm, and 12,000 cm2m-3 for weeping fig.
Ac know ledg ement: This research was supported by Basic
Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Edu-
cation, Science and Technology (2010-009430).
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