Effect of Lighting Environment on the CO 2 Concentration Reduction Efﬁciency of Plants by a Model Experiment

: Plants have the potential to reduce CO 2 concentration, but their photosynthesis is directly inﬂuenced by the indoor lighting environment. As a result, the efﬁciency of indoor plants is limited by indoor lighting environment. In order to explore the effect of lighting environments on the reduction of indoor CO 2 concentration by indoor plants, three representative lighting environments were constructed, including a natural lighting environment, a poor lighting environment and an all-day lighting environment, while ﬁve common plants were selected to be planted in ﬁve transparent sealed chambers. Experimental results show that the lighting environment affected the CO 2 concentration largely in transparent sealed chambers. Compared to the transparent sealed chamber without plants, the highest and average CO 2 concentrations were increased by from 47.9% to 160.9% and from 21.6% to 132.4% in the poor lighting environment, respectively, while they decreased by from 60.4% to 84.6% and from 71.4% to 89.7% in the all-day lighting environment. This indicated that plants did not purify the indoor air consistently. Among the selected plants, the most suitable houseplant was Scindapsus aureus , followed by Chlorophytum comosum and Bambusa multiplex .


Introduction
With the accelerated progress of urbanization, high-density buildings have become the theme of urban development due to the large amount of solar radiation absorbed by the reinforced concrete and asphalt that make up the urban surface [1,2], which brings serious environmental problems to cities. Serious environmental pollution and poor air quality are faced by humans globally, and due to the occupants' breath and the building tightness, the pollutant concentration in an indoor environment is 2-4 times higher than that in an outdoor environment [3], especially for CO 2 concentrations. Although CO 2 is commonly considered non-toxic, excessive CO 2 concentrations have been associated with sick building syndrome due to its narcotic effect [4]. Since urban residents spend about 80-90% of their time indoors [5], lowering the CO 2 concentration has become important [6,7]. Studies have proved that elevated CO 2 concentrations in office buildings are associated with the increased disease symptoms of workers [8] and that student academic performance and workplace productivity decrease with increasing CO 2 levels [9,10].
With the continuous improvement of people's living standards, people's demand for life quality has increased, and introducing green plants indoors is one of the most convenient and effective ways to contact the natural environment [11,12]. Green plants have the potential to reduce indoor CO 2 concentration, relieve anxiety, and gain additional benefits [13] for the physical and psychological health of building occupants [14]. High CO 2 concentration and low relative humidity are common in buildings, which can be controlled by mechanical ventilation systems, but this is more costly as well as energy-consuming [14]. Green plants can absorb CO 2 and release O 2 simultaneously through photosynthesis, Buildings 2022, 12, 1848 2 of 12 to reduce the CO 2 concentration and thereby lower indoor air pollution naturally and sustainably [15]. In addition, the photosynthesis of green plants produces negative air ions, which are beneficial to people's health [16,17]. Through a restorative effect, plants can act as a kind of biological filter to purify the air [18,19], so fresh air volume could be reduced appropriately in enclosed rooms with air-conditioning and heating under this conditioning, and therefore, due to the fresh air, the energy consumption can be certainly lowered in the air-conditioning and heating system [3]. Therefore, introducing green plants indoors is one of the most effective, convenient, economical and environmentally friendly methods of indoor air purification [20][21][22].
The CO 2 concentration is a key parameter in determining the amount of fresh air, which indirectly affects the energy consumption of air-conditioning and heating [3]. Some studies have shown that indoor green plants can reduce ventilation requirements and reduce CO 2 concentration [23,24]. Torpy et al. [25] analyzed the CO 2 removal potential of eight common plants, and their results showed that plant types had an impact on photosynthetic capacity due to indoor light level and CO 2 removal rate. Only when indoor light level was higher than a certain value were the higher increases in CO 2 removal detected. Moreover, the absorption capacity of CO 2 could be optimized by appropriately increasing the light. Tudiwer et al. [26] showed that the CO 2 concentration in classrooms with plant systems decreased 3.5% faster than that in classrooms without plants for the same initial concentration of indoor CO 2 . Oh et al. [27] created an ideal room with an initial CO 2 concentration of 1000 ppm and a real room with an initial CO 2 concentration of 35-450 ppm with hamsters to comparatively analyze the CO 2 absorbing ability of plants. Their results showed that the plants with the larger leaf area had a higher CO 2 removal efficiency, and in real spaces, the CO 2 concentration will gradually increase as occupants enter the room, so the CO 2 absorption effect of indoor plants will be better under this condition. Vahdati et al. [28] showed that increasing CO 2 concentration was an effective method to reduce in-vitro water loss during the in-vitro culture of walnuts, because exposing the leaves of invitro-grown plants to increased CO 2 concentration was found to produce more small-sized stomata and fewer large-sized stomata, and the change in stomatal shape can improve the resistance of plants to in-vitro drying and reduce water loss in the first stage. In addition, Mutebi [29] indicated that the photosynthetic carbon sink of mulberry trees was greater than the total carbon emission and has a greenhouse gas mitigation effect.
These studies effectively demonstrated the effect of landscape plants on indoor CO 2 removal, and the light level was a key factor in plant photosynthesis. Most plants had reduced photosynthesis rates in low-light or dark conditions indoors. Plants had different photosynthesis rates under different lighting environments, and within a certain range, the higher the light intensity, the higher the net photosynthesis rate, and therefore the higher the CO 2 uptake rate [30].
Based on the above review, the effect of the lighting environment on indoor CO 2 concentration was analyzed. Three representative lighting environments were built, including a natural lighting environment, a poor lighting environment and an all-day lighting environment, while five common plants were selected to be planted in five transparent sealed chambers, respectively. The CO 2 concentrations in the five transparent sealed chambers with plants were monitored by using one sealed chamber without plants as a reference.

Experimental Plants
To explore the effect of different plant species on indoor CO 2 concentration and, therefore, to find suitable plants for indoor cultivation, five common plants are selected, by taking into account the cultivation status of domestic indoor plants. Five common plants are Scindapsus aureus, Chlorophytum comosum, Bambusa multiplex, Neottopteris nidus and Schlumbergera truncate, which belong to the genera of Araceae, Liliaceae, Poaceae, Aspleniaceae Newman and Cactaceae. The five plants were of similar height and shape, and the substrate was the same. The experiments were carried out from April 2021 to October 2021.

Experimental Chambers and Measurement Methods
In order to facilitate the measurement of CO 2 concentration, six identical transparent sealed chambers were designed in the experiment and their size was 500 mm (length) × 500 mm (width) × 600 mm (height). A transparent, 120 mm-thick solar hollow plate was chosen as the chamber wall. All chambers were sealed with glass glue and silicone rubber, and their tightness was ensured before the experiment. To reduce the interference of air penetration with the experimental data, only the top of the chamber could be temporarily opened and closed. The instrument used for measuring CO 2 concentration in this experiment was iBEM intelligent building indoor environment monitor, which had a CO 2 concentration test range of 400-5000 ppm and test accuracy of ±5%.

Experimental Lighting Environments
This study focused on the effect of plant types on indoor CO 2 concentration under different lighting environments, so three representative lighting environments were built, as shown in Figure 1. 500 mm (width) × 600 mm (height). A transparent, 120 mm-thick solar hollow plate was chosen as the chamber wall. All chambers were sealed with glass glue and silicone rubber, and their tightness was ensured before the experiment. To reduce the interference of air penetration with the experimental data, only the top of the chamber could be temporarily opened and closed. The instrument used for measuring CO2 concentration in this experiment was iBEM intelligent building indoor environment monitor, which had a CO2 concentration test range of 400-5000 ppm and test accuracy of ± 5%.

Experimental Lighting Environments
This study focused on the effect of plant types on indoor CO2 concentration under different lighting environments, so three representative lighting environments were built, as shown in Figure 1. All-day lighting environment: Plants were illuminated indoors with alternating natural lighting and supplemental lighting. When the lighting illumination was lower than 100 Lx, a full-spectrum LED imitation sunlight plant supplemental light was opened to supplement the plants automatically, as shown in Figure 1c. Under this environment, the light illumination was always higher than 100 Lx in a day. The full-spectrum LED mimicking sunlight with a wavelength of 450-800 nm, balancing red and blue light, was employed, while the plant illumination was intelligently controlled using an OKELE rail-light control controller at no less than 100 Lx. The light wavelengths (400-500 nm and 600-700 nm) met the nutritional requirements of plants [27]. All-day lighting environment: Plants were illuminated indoors with alternating natural lighting and supplemental lighting. When the lighting illumination was lower than 100 Lx, a full-spectrum LED imitation sunlight plant supplemental light was opened to supplement the plants automatically, as shown in Figure 1c. Under this environment, the light illumination was always higher than 100 Lx in a day.
The full-spectrum LED mimicking sunlight with a wavelength of 450-800 nm, balancing red and blue light, was employed, while the plant illumination was intelligently controlled using an OKELE rail-light control controller at no less than 100 Lx. The light wavelengths (400-500 nm and 600-700 nm) met the nutritional requirements of plants [27].

Experimental Procedure
In this study, air temperature and relative humidity were controlled within the range of 23 ± 2 • C and 40 ± 10%, respectively. The five plants were put into the chamber, which Buildings 2022, 12, 1848 4 of 12 was sealed at 8 a.m. An unplanted chamber was also used as a reference, as shown in Figure 2, with natural light coming mainly from the south-facing windows.

ments:
• Natural lighting environment: Plants could photosynthesize indoors in the natural lighting environment in the period of 8:00-17:00 (about 9 h/day), and in the other period, all chambers were shaded by the blackout cloth. • Poor lighting environment: Plants could photosynthesize indoors in the natural lighting environment in the period of 9:30-11:30 (about 2 h/day), and in the other period, all chambers were shaded by the blackout cloth. • All-day lighting environment: The natural lighting and full-spectrum LED were both employed to create the all-day illumination higher than 100 Lx.

Experimental Results and Discussion
At the beginning of the experiment, the CO2 concentration in each chamber was fundamentally the same, and all chambers were completely sealed during the experiment. Thus, the variability of CO2 concentration in the chamber could be identified due to photosynthesis and the respiration of the plant.

Effect of Lighting Environments on Plant Growth
A suitable lighting environment is essential for plant growth, and plants use photosynthesis to provide energy for their growth, so photosynthesis is closely related to plant growth. Table 1 shows the side and top photos of the growth states of the five plants before and after the experiment (30 days) in the three lighting environments. It can be seen that, regardless of whether the light duration was 2 h/day or 24 h/day, none of the five plants showed any visually observable difference in growth status before and after the experiment, nor did they show any obvious signs of death. This indicated that three lighting environments selected for this experiment did not negatively affect the growth of indoor plants visible to the naked eye for a short period of time. All-day lighting environment: The natural lighting and full-spectrum LED were both employed to create the all-day illumination higher than 100 Lx.

Experimental Results and Discussion
At the beginning of the experiment, the CO 2 concentration in each chamber was fundamentally the same, and all chambers were completely sealed during the experiment. Thus, the variability of CO 2 concentration in the chamber could be identified due to photosynthesis and the respiration of the plant.

Effect of Lighting Environments on Plant Growth
A suitable lighting environment is essential for plant growth, and plants use photosynthesis to provide energy for their growth, so photosynthesis is closely related to plant growth. Table 1 shows the side and top photos of the growth states of the five plants before and after the experiment (30 days) in the three lighting environments. It can be seen that, regardless of whether the light duration was 2 h/day or 24 h/day, none of the five plants showed any visually observable difference in growth status before and after the experiment, nor did they show any obvious signs of death. This indicated that three lighting environments selected for this experiment did not negatively affect the growth of indoor plants visible to the naked eye for a short period of time.

Effect of Plants on Indoor CO2 Concentration in the Natural Lighting Environment
The natural light time was 9 h/day (8:00-17:00) in the natural lighting environment. Figure 3 shows the variation of CO2 concentration with time in six sealed chambers during the 72 h in the natural lighting environment. It can be seen that the initial CO2 concentration in the sealed chamber was around 750 ppm, while the CO2 concentration in the sealed chamber with plants changed drastically with the light intensity and the CO2 concentration decreased significantly, especially during the light period. The CO2 concentration showed the same sawtooth-type change trend in the chambers planted with Scindapsus aureus, chlorophytum comosum, Bambusa multiplex and Neottopteris nidus.
Natural lighting mainly came from the south-facing windows, and the CO2 concentration in the chamber decreased linearly and rapidly with the increase of indoor light intensity from 8:00 to 12:00. However, during the period from 12:00 to 15:00, the CO2 concentration was basically flat, which may be due to the fact that the plants closed their stomata to reduce water dissipation when solar radiation was too strong. The indoor light intensity gradually decreased from 15:00 to 17:00, and the photosynthetic rate of plants was gradually lower than the respiration rate, so the CO2 concentration in the sealed chamber continued to rise. After that, when the indoor natural light intensity was lower than a certain value, the plants mainly carried out respiration, and the CO2 concentration rose rapidly until 8:00 the next day. Therefore, the highest indoor CO2 concentration in the sealed chamber was in the morning.
Schlumbergera truncate was different from four other types of plants. In the daytime, Schlumbergera truncata closed its stomata to reduce water loss by transpiration and released CO2 by reducing it to sugar metabolism through the Calvin cycle. At night, it absorbed CO2 to form organic acids such as malic acid and opened its stomata to release O2. Therefore, the CO2 concentration increased during the day and decreased during the night in the chamber with Schlumbergera truncata, and the variation range of CO2 concentration was minimal.
In addition, as can be seen in Figure 3, compared to the initial CO2 concentration, CO2 concentration had obviously been reduced. At the end of the third day, the reduction rate of CO2 concentration was up to 47.9% for Schlumbergera truncate, 35.1% for Chlorophytum comosum, 32.9% for Scindapsus aureus, and 16.8% for Neottopteris nidus and for Bambusa multiplex. Therefore, Schlumbergera truncate was the best selection in the natural lighting environment.

Effect of Plants on Indoor CO2 Concentration in the Natural Lighting Environment
The natural light time was 9 h/day (8:00-17:00) in the natural lighting environment. Figure 3 shows the variation of CO2 concentration with time in six sealed chambers during the 72 h in the natural lighting environment. It can be seen that the initial CO2 concentration in the sealed chamber was around 750 ppm, while the CO2 concentration in the sealed chamber with plants changed drastically with the light intensity and the CO2 concentration decreased significantly, especially during the light period. The CO2 concentration showed the same sawtooth-type change trend in the chambers planted with Scindapsus aureus, chlorophytum comosum, Bambusa multiplex and Neottopteris nidus.
Natural lighting mainly came from the south-facing windows, and the CO2 concentration in the chamber decreased linearly and rapidly with the increase of indoor light intensity from 8:00 to 12:00. However, during the period from 12:00 to 15:00, the CO2 concentration was basically flat, which may be due to the fact that the plants closed their stomata to reduce water dissipation when solar radiation was too strong. The indoor light intensity gradually decreased from 15:00 to 17:00, and the photosynthetic rate of plants was gradually lower than the respiration rate, so the CO2 concentration in the sealed chamber continued to rise. After that, when the indoor natural light intensity was lower than a certain value, the plants mainly carried out respiration, and the CO2 concentration rose rapidly until 8:00 the next day. Therefore, the highest indoor CO2 concentration in the sealed chamber was in the morning.
Schlumbergera truncate was different from four other types of plants. In the daytime, Schlumbergera truncata closed its stomata to reduce water loss by transpiration and released CO2 by reducing it to sugar metabolism through the Calvin cycle. At night, it absorbed CO2 to form organic acids such as malic acid and opened its stomata to release O2. Therefore, the CO2 concentration increased during the day and decreased during the night in the chamber with Schlumbergera truncata, and the variation range of CO2 concentration was minimal.
In addition, as can be seen in Figure 3, compared to the initial CO2 concentration, CO2 concentration had obviously been reduced. At the end of the third day, the reduction rate of CO2 concentration was up to 47.9% for Schlumbergera truncate, 35.1% for Chlorophytum comosum, 32.9% for Scindapsus aureus, and 16.8% for Neottopteris nidus and for Bambusa multiplex. Therefore, Schlumbergera truncate was the best selection in the natural lighting environment.

Effect of Plants on Indoor CO2 Concentration in the Natural Lighting Environment
The natural light time was 9 h/day (8:00-17:00) in the natural lighting environment. Figure 3 shows the variation of CO2 concentration with time in six sealed chambers during the 72 h in the natural lighting environment. It can be seen that the initial CO2 concentration in the sealed chamber was around 750 ppm, while the CO2 concentration in the sealed chamber with plants changed drastically with the light intensity and the CO2 concentration decreased significantly, especially during the light period. The CO2 concentration showed the same sawtooth-type change trend in the chambers planted with Scindapsus aureus, chlorophytum comosum, Bambusa multiplex and Neottopteris nidus.
Natural lighting mainly came from the south-facing windows, and the CO2 concentration in the chamber decreased linearly and rapidly with the increase of indoor light intensity from 8:00 to 12:00. However, during the period from 12:00 to 15:00, the CO2 concentration was basically flat, which may be due to the fact that the plants closed their stomata to reduce water dissipation when solar radiation was too strong. The indoor light intensity gradually decreased from 15:00 to 17:00, and the photosynthetic rate of plants was gradually lower than the respiration rate, so the CO2 concentration in the sealed chamber continued to rise. After that, when the indoor natural light intensity was lower than a certain value, the plants mainly carried out respiration, and the CO2 concentration rose rapidly until 8:00 the next day. Therefore, the highest indoor CO2 concentration in the sealed chamber was in the morning.
Schlumbergera truncate was different from four other types of plants. In the daytime, Schlumbergera truncata closed its stomata to reduce water loss by transpiration and released CO2 by reducing it to sugar metabolism through the Calvin cycle. At night, it absorbed CO2 to form organic acids such as malic acid and opened its stomata to release O2. Therefore, the CO2 concentration increased during the day and decreased during the night in the chamber with Schlumbergera truncata, and the variation range of CO2 concentration was minimal.
In addition, as can be seen in Figure 3, compared to the initial CO2 concentration, CO2 concentration had obviously been reduced. At the end of the third day, the reduction rate of CO2 concentration was up to 47.9% for Schlumbergera truncate, 35.1% for Chlorophytum comosum, 32.9% for Scindapsus aureus, and 16.8% for Neottopteris nidus and for Bambusa multiplex. Therefore, Schlumbergera truncate was the best selection in the natural lighting environment.

Effect of Plants on Indoor CO 2 Concentration in the Natural Lighting Environment
The natural light time was 9 h/day (8:00-17:00) in the natural lighting environment. Figure 3 shows the variation of CO 2 concentration with time in six sealed chambers during the 72 h in the natural lighting environment. It can be seen that the initial CO 2 concentration in the sealed chamber was around 750 ppm, while the CO 2 concentration in the sealed chamber with plants changed drastically with the light intensity and the CO 2 concentration decreased significantly, especially during the light period. The CO 2 concentration showed the same sawtooth-type change trend in the chambers planted with Scindapsus aureus, chlorophytum comosum, Bambusa multiplex and Neottopteris nidus.
Natural lighting mainly came from the south-facing windows, and the CO 2 concentration in the chamber decreased linearly and rapidly with the increase of indoor light intensity from 8:00 to 12:00. However, during the period from 12:00 to 15:00, the CO 2 concentration was basically flat, which may be due to the fact that the plants closed their stomata to reduce water dissipation when solar radiation was too strong. The indoor light intensity gradually decreased from 15:00 to 17:00, and the photosynthetic rate of plants was gradually lower than the respiration rate, so the CO 2 concentration in the sealed chamber continued to rise. After that, when the indoor natural light intensity was lower than a certain value, the plants mainly carried out respiration, and the CO 2 concentration rose rapidly until 8:00 the next day. Therefore, the highest indoor CO 2 concentration in the sealed chamber was in the morning.
Schlumbergera truncate was different from four other types of plants. In the daytime, Schlumbergera truncata closed its stomata to reduce water loss by transpiration and released CO 2 by reducing it to sugar metabolism through the Calvin cycle. At night, it absorbed CO 2 to form organic acids such as malic acid and opened its stomata to release O 2 . Therefore, the CO 2 concentration increased during the day and decreased during the night in the chamber with Schlumbergera truncata, and the variation range of CO 2 concentration was minimal.
In addition, as can be seen in Figure 3, compared to the initial CO 2 concentration, CO 2 concentration had obviously been reduced. At the end of the third day, the reduction rate of CO 2 concentration was up to 47.9% for Schlumbergera truncate, 35.1% for Chlorophytum comosum, 32.9% for Scindapsus aureus, and 16.8% for Neottopteris nidus and for Bambusa multiplex. Therefore, Schlumbergera truncate was the best selection in the natural lighting environment. Buildings 2022, 12, x FOR PEER REVIEW 6 of 12  Table 2 compared indoor CO2 concentration and the corresponding reduction rate in the natural lighting environment for each 24 h period among the five plants. To lower the influence from the initial CO2 concentration, the data were analyzed in the second and third 24 h. The highest, lowest and average values of CO2 concentration were from 453 ppm to 642 ppm, from 77 ppm to 309 ppm, and from 253 ppm to 431 ppm, and the corresponding reduction rates were from 16.7% to 41.2%, from 55.7% to 88.9%, and from 41.3% to 64.7% in the second and third 24 h, compared to the CO2 concentration in the sealed chamber without plants. By comparing the ability of five plants to reduce CO2 concentration in the natural lighting environment, it was easily found that Scindapsus aureus had the most significant effect, followed by Chlorophytum comosum, and their reduction rates of average CO2 concentration were from 53.0% to 61.2% and from 60.2% to 64.7%, respectively.  Table 2 compared indoor CO 2 concentration and the corresponding reduction rate in the natural lighting environment for each 24 h period among the five plants. To lower the influence from the initial CO 2 concentration, the data were analyzed in the second and third 24 h. The highest, lowest and average values of CO 2 concentration were from 453 ppm to 642 ppm, from 77 ppm to 309 ppm, and from 253 ppm to 431 ppm, and the corresponding reduction rates were from 16.7% to 41.2%, from 55.7% to 88.9%, and from 41.3% to 64.7% in the second and third 24 h, compared to the CO 2 concentration in the sealed chamber without plants. By comparing the ability of five plants to reduce CO 2 concentration in the natural lighting environment, it was easily found that Scindapsus aureus had the most significant effect, followed by Chlorophytum comosum, and their reduction rates of average CO 2 concentration were from 53.0% to 61.2% and from 60.2% to 64.7%, respectively.

Effect of Plants on Indoor CO 2 Concentration in the Poor Lighting Environment
Plants often suffer from insufficient light when placed in shaded locations or poor lighting environments. Figure 4 shows the variation of CO 2 concentration in six sealed chambers with time during 72 h in the poor lighting environment. It can be seen that the CO 2 concentration was steady at about 620 ppm in the sealed chamber without plants, while the CO 2 concentration increased to different degrees due to the poor light in the sealed chamber with plants.
The plant photosynthesis was obvious during the 2 h with the natural light, while the plant respiration was obvious and CO 2 concentration increased linearly during the 22 h with the poor light. The CO 2 concentrations showed a zigzag trend in the sealed chambers of Scindapsus aureus, Chlorophytum comosum, Bambusa multiplex and Neottopteris nidus. In addition, Schlumbergera truncate was significantly different from the other four species. The CO 2 concentration did not have a significant decrease and showed a continuous increase within 72 h in the sealed chamber with Schlumbergera truncate.
At the end of the third day, the CO 2 concentrations were all above 900 ppm, which was 40% higher than in the sealed chambers with plants. Among five plants, the highest growth rate of CO 2 concentration was 168.2% in the sealed chamber with Neottopteris nidus, followed by Schlumbergera truncate with a 74% growth rate, and by Scindapsus aureus and Chlorophytum comosum with a 51.9% and 48.4% growth rate respectively. The above results showed that when the lighting was insufficient, indoor planting obviously enhanced the CO 2 concentration, and therefore polluted indoor air to a certain extent, which indicated that the lighting control was particularly important. Table 3 compared indoor CO 2 concentration and the corresponding reduction rate for each 24 h period among the five plants in the poor lighting environment. To lower the influence from the initial CO 2 concentration, the data were also analyzed in the second and third 24 h. The highest, lowest and average values of CO 2 concentration were from 818 ppm to 1672 ppm, from 428 ppm to 1178 ppm, and from 647 ppm to 1434 ppm, and the corresponding reduction rates were from −160.8% to −26.6%, from −102.8% to 8.3%, and from −132.4% to 4.2% in the second and third 24 h, compared to the CO 2 concentration in the sealed chamber without plants. By comparing the five plants, the most unsuitable plants to be grown indoors were Neottopteris nidus and Schlumbergera truncate in the poor lighting environment, and their reduction rates of average CO 2 concentration were from −83.9% to −132.4% and from −39.3% to −63.2%, respectively.   When the light was insufficient, the plants increased the CO2 concentration in the sealed chamber. The LED light was adopted to enhance the photosynthesis of plants.

Effect of Plants on Indoor CO 2 Concentration in the All-Day Lighting Environment
When the light was insufficient, the plants increased the CO 2 concentration in the sealed chamber. The LED light was adopted to enhance the photosynthesis of plants. Figure 5 shows the variation of CO 2 concentration over time in sealed chambers with plants during 72 h in the all-day lighting environment. It can be seen that the CO 2 concentration was stable at about 700 ppm in the sealed chamber without plants, while the CO 2 concentration decreased rapidly and gradually stabilized at 100 ppm to 200 ppm after 24 h in the sealed chamber with plants, and the balance between photosynthesis and respiration made the CO 2 concentration relatively stable. Figure 5 shows the variation of CO2 concentration over time in sealed chambers with plants during 72 h in the all-day lighting environment. It can be seen that the CO2 concentration was stable at about 700 ppm in the sealed chamber without plants, while the CO2 concentration decreased rapidly and gradually stabilized at 100 ppm to 200 ppm after 24 h in the sealed chamber with plants, and the balance between photosynthesis and respiration made the CO2 concentration relatively stable. From the CO2 concentration fluctuation curve, the CO2 concentration showed a small increase under natural light. This phenomenon was mainly related to the fact that the plant stomata were closed to photosynthesis when the natural light intensity was strong. The nighttime supplemental light used 100 xl LED full-spectrum light, so the CO2 concentration showed a decreasing trend when the nighttime supplemental light was applied. From the CO 2 concentration fluctuation curve, the CO 2 concentration showed a small increase under natural light. This phenomenon was mainly related to the fact that the plant stomata were closed to photosynthesis when the natural light intensity was strong. The nighttime supplemental light used 100 Lx LED full-spectrum light, so the CO 2 concentration showed a decreasing trend when the nighttime supplemental light was applied.
All five plants performed better during nighttime supplemental light. Scindapsus aureus, Chlorophytum comosum and Neottopteris nidus could reduce the CO 2 concentration by from 82.7% to 89.9%, while Bambusa multiplex and Schlumbergera truncate could reduce the CO 2 concentration by 67.4−76.8%. Since photosynthesis usually varied according to the balance of light quantity and CO 2 concentration in plant cells, more-intense light induced higher photosynthetic rates and lowered CO 2 concentration in cells. If the CO 2 concentration was limited, the stomatal conductance and photosynthesis rate would also be limited [1]. Table 4 compared indoor CO 2 concentration and the corresponding reduction rate for each 24 h period among the five plants in the all-day lighting environment. To lower the influence from the initial CO 2 concentration, the data were also analyzed in the second and third 24 h. Compared to the chamber without plants, the highest, lowest and average values of CO 2 concentration were from 90 ppm to 286 ppm, from 47 ppm to 169 ppm, and from 66 ppm to 206 ppm, and the corresponding reduction rates were from 60.4% to 84.6%, from 74.4% to 92.9%, and from 70.3% to 90.5% in the second and third 24 h, compared to the CO 2 concentration in the sealed chamber without plants. The above data showed that the plants can significantly reduce the CO 2 concentration in the sealed chamber in the all-day lighting environment.