Suitability of Foliage Plants for Indoor Decoration Based on CO2 Emission and Absorption Rate and Stomata Density

Use of foliage plants for indoor decoration is pursuing tradition in several cultures. With contemporary living patterns, frequent replacement of indoor plants has become impractical. Therefore, this experiment was conducted to identify indoor plants with low carbon dioxide (CO2) emission or CO2 absorption ability in nights to be kept continuously indoors. Five common indoor plants (Cryptanthus sp., Dieffenbachia seguine, Dracaena sanderiana, Sansevieria trifasciata and Zamioculcas zamiifolia) were placed separately in 1000 L airtight chambers for 12 h in the dark. The CO2 level in each chamber was measured before and after the experiment and the difference was calculated. The stomatal count of both adaxial and abaxial surfaces was taken in each plant type to determine the relationship between CO2 emission/absorption efficiency and stomatal density of tested ornamental species. From the test plant species, D. seguine, D. sanderiana and Z. zamiifolia, showed positive CO2 equilibrium in the chambers and the CO2 increments were 0.16 ppm cm-2, 0.39 ppm cm-2 and 0.18 ppm cm-2 of leaf area, respectively. Both Cryptanthus sp. and S. trifasciata showed negative CO2 equilibrium at around -0.20 ppm cm-2 of leaf area. Sansevieria trifasciata and D. sanderiana possessed stomata in both adaxial and abaxial surfaces, while stomatal number in adaxial surface of other three test plant species was negligible. The average number of stomata Cryptanthus sp. was 5.56x104 cm−2, D. seguine 5.03x104 cm−2, D. sanderiana 9.05x104 cm−2, S. trifasciata 5.25x104 cm−2 and Z. zamiifolia 3.51x104 cm−2. Stomata in Cryptanthus sp. and S. trifasciata close during day time and open at night. Present study concludes that potted Cryptanthus sp. and S. trifasciata (plants with CAM photosynthesis pathway) used for indoor decoration absorb CO2 during the night, and hence, are safe to keep indoors during day and night.


Introduction
Use of foliage plants in indoor decorations has shown a positive trend in the recent past. Beautification of living environment, and the natural effect of plants contributing positively to mental health, physical health and safe indoor environment (Lohr, 2007) are their other beneficial effects. Further, plants can substantially improve indoor environmental quality by reducing the major types of urban air pollutants (Burchett et al., 2011). The volatile organic compounds (VOC) and CO2 are the two major classes of air pollutants in indoors and plants can significantly reduce these Sri Lanka Journal of Food and Agriculture (SLJFA) two pollutants (Soreanu et al., 2013). Generally, the indoor CO2 levels are about 10 times higher than that of the outdoor levels. Living under high CO2 concentration could lead to several health problems such as sick building syndrome (Milton et al., 2000). The city dwellers spend more than 80% of their time inside the buildings, and face higher risk due to indoor air pollution (Torpy et al., 2014). Growing plants indoors is one of the potential remedies in converting concrete buildings into living-friendly environments.
There is a wide array of plants used in indoor decorations, however, past records on effects of those plants on human health are not commonly available. Burchett et al. (2011) reported that several test plants actively removed VOC from air while the CO2 reduction was accomplished by green part of the plants at adequate light levels. Photosynthesis was actively performed by Juniperus conferta (a hanging indoor plant) in spring and summer in Japan absorbing large amounts of CO2 than in winter (Fujii et al., 2005). These results revealed that, though plants can fix CO2, their contribution to indoor CO2 reduction has to be studied thoroughly as CO2 emission and absorption depend on several plant-based factors.
Aerobic respiration continues in plant cells, using O2 and releasing CO2. In contrast, photosynthesis uses CO2 and produces O2, but the process occurs only in light and stops in dark. This is a prominent mechanism in plants with C3 or C4 photosynthesis pathways. All these plants release CO2 during nights due to dark respiration (Bader and Abdel-Basset, 2002) (Burchett et al., 2011, Holtum et al., 2007. Production of plants for indoor decoration (indoorscape) represents more than 50% of the floriculture industry in Sri Lanka. Knowledge on beneficial indoor plants will thus be important to these producers to determine their production targets. This study was conducted to identify CO2 emitting and CO2 absorbing plants to facilitate the decision making process of producing such plants in large scale and promoting them to be continuously kept indoors.
They were individually planted in 30 cm diameter plastic pots containing a homogenized potting mixture and few pots were without plants were used as the control. The plants were allowed to grow under a uniform condition at 2000 -2200 lux light in a shade house for 3 weeks. Visually uniform plants from each species were used as replicates.
Plastic barrels of 1000 L covered with transparent polythene from the top were modified as air tight chambers to study CO2 emission or absorption by the test plants during night. Selected plants were carefully placed in the chambers. Soon after placing the pots, the open end of the chambers was covered with a 200 μm polythene film fixing it tightly using several layers of sticky tapes. The chambers with plants were kept for 12 h in the dark under a black cloth cover to cease photosynthesis. After 12 h (overnight), a CO2 detector (MS-CO2 Model 1204003) was inserted to chambers without allowing any gas exchange and CO2 concentration, temperature and relative humidity of each chamber were recorded. Pots with only the growth medium (control) were used to study the CO2 emissions or absorption by soils and to calculate emission or absorption by the five tested plants. A set of empty barrels also served as controls to provide baseline data on CO2 concentration. The total photosynthetic area of each plant was measured using grid method to calculate the net CO2 emission or absorption of plant per unit photosynthetic area.
The experiment was carried out in a Complete Randomized Design with four replicates. The experiment was continued for 3 weeks and data were collected 3 days per week. At the end of the experiment, the stomata number of each species was counted from a nail polish imprint of adaxial and abaxial surfaces (Jacobsen et al., 2012). The same method was used to prepare slides to study the behaviour of stomata in day and night. Stomata density, length and width were measured at 100 x magnification.

Results and Discussion
The characteristics of indoor plants used in the experiment are presented in Table 1. All plants used in the experiment were shade tolerant and could acclimatize at 2000-2200 lux shade level at the floriculture shade house at HORDI.
Dieffenbachia seguine, D. sanderiana, S. trifasciata and Z. zamiifolia showed new leaf initiation during the acclimatization period, but leaf initiation or flowering was not observed in Crypanthus spp. The CO2 absorption and emission varied with the plant species used. The control treatment (pots with only the medium) also has emitted CO2 while the CO2 concentration in the empty growth chamber has not changed.    Both Crypanthus sp. and S. trifasciata has tightly closed stomata during the day time ( Figure 2) and abaxial surface of Crypanthus sp. was thoroughly covered with powder-like layer, which masks the exposure of stomata to the outer environment. Crypanthus sp. Did not possess stomata in the adaxial surface, but the cumulative stomatal density was higher than that of S. trifasciata. All the experimental chambers showed higher RH values than the open environment indicating that the pots with or without plants released water to the air. This could be the evaporation + transpiration from the medium and plant. The results proved that all five plant species used in the study are well adapted to indoor condition and can be easily be grown indoors. Although the main objective of this experiment was to study the air quality due to emission or absorption of CO2 by plants kept indoors, the results showed that not only the plants, even the growing media (soil) also greatly influence on the indoor air quality. In the present study, the control pots (without plants) emitted considerable amount of CO2 during the night and the difference of CO2 concentrations between empty growth chamber and chamber with pot was 564 ppm. This is probably due to the soil respiration of dwelling organisms, including microorganisms, in the media. A similar observation was made by Burchett et al. (2011) who emphasized that organisms in potting media could significantly contribute to indoor air quality. Bond-Lambarty and Thomson (2010) showed that the increasing temperatures would result in an increase in net release of CO2 from soil by triggering microbes to speed their consumption of plant debris and other organic matter. This shows that not only the plants, but also the potting medium should be considered in assessing the indoor air pollution. Therefore, the net CO2 emission or absorption by tested plants in the experiment was calculated by deducting CO2 emission by pot (growth medium) in each case.
The findings of this study clearly demonstrated the significant variations of emission or absorption of CO2 among the selected indoor plants. It was clear that Crypanthus sp. and S. trifasciata absorb CO2 in the dark. Both these species show absolute CAM photosynthesis, which absorbs CO2 during the night, store in the vacuole as malic acid, and then utilize it in the Calvin cycle during the day time (Yamori et al., 2014). As observed in the microscopic studies (Figure 2), the stomata in both of these species were closed during the day time and thus, allowing all gas exchanges to take place during the night when stomata are open. Therefore, it is clear that Crypanthus sp. and S. trifasciata do not contribute to enhance indoor CO2 levels during day time. The CAM plants open their stomata during the cool nights and close them during the hot, dry-day time as an adaptation to live in dry harsh environments. Closing stomata during the day minimizes the loss of water but, because H2O and CO2 share the same diffusion pathway, CO2 must then be taken up by the open stomata at night. When the stomata are closed, CO2 generated from the metabolic activities does not escape from the leaf. Thus, stomata closure not only helps A B

C D E
conserving water, but also assists in the building up of internal concentration of CO2, to be utilized in photosynthesis and emitting O2 when stomata open at night (Kluge and Ting, 1978).
Previous studies have also shown that S. trifasciata is one of the best indoor plants due to its highest CO2 utilization ability at night (Wolverton et al., 1989). However, the present study revealed that CO2 absorption ability per unit leaf area of S. trifasciata is not significantly different to that of Crypanthus sp. (Table 2). Furthermore, a strong relationship between the presence of stomata in leaf surfaces and CO2 emission could not be identified in those two species in this study. Presence of stomata in adaxial and abaxial surfaces may be a species-specific character, which is related to the morphology and physiology of plant (Bader and Abdel-Basset, 2002). Though indoor CAM plants absorb CO2 at night, the microcosm is not adequate to compensate emissions of the dwellers through breathing. Therefore, the targeted net CO2 reduction cannot be achieved by having 1-2 plants. Use of these plants in large scale, i.e. as live walls, vertical gardens, etc.), could be effective in achieving such beneficial effects to some extent (Soreanu et al., 2013).
The present study also provided clear evidence on the CO2 emitting plants at night. Although the emission of these plants does not significantly influence on the indoor air quality, retaining a number of such plants (with absolute C3 photosynthetic pathway) at night may contribute to increase in the indoor CO2 concentration. During the day time these plants emit O2 and absorb CO2 and the dark respiration process taking place in the absence of light, which is the most responsible event for CO2 generation at night (Bader and Abdel-Basset, 2002). It is evident that CO2 absorption or emission has a direct relationship with the light level of the surrounding environment. In Valladares and Niinemets (2008) have shown that shadeloving plants photosynthesize at low light levels and contribute to air purification indoors. Therefore, further research is needed to identify low CO2 emitting as well as shade-loving plants for healthy indoor decoration. Among the used C3 plants in the present study, D. sanderiana showed a higher CO2 emission at night, followed by Z. zamiifolia and D. seguine (Figure 1) suggesting that D. seguine is better for indoor decoration than the other two species. However, the photosynthesis capacity of these plants should be measured under different light levels prior to drawing any conclusions.
Though there are evidence that Z. zamiifolia (Holtum et al., 2007) and D. sanderiana (Burchett et al., 2011) act as facultative CAM plants under certain environmental conditions, the present study showed that both these species emitted a significant amount of CO2 as normal C3 plants (Table 2). High temperature and drought condition could make Z. zamiifolia to be a facultative CAM plant (Holtum et al., 2007). Hence, water management could be an essential practice to induce CO2 emission of Z. zamiifolia and D. sanderiana. Further studies are required on these aspects that would help using of these plants as beneficial plants indoors.
The results of this experiment have shown that the indoor plants have not only affected the air quality, but also the indoor RH levels. Pots with or without plants, which were in chambers, have increased RH by about 20-25% (Table 2). Increasing RH indoors is the main issues in bio-purification using plants. Prolong high humidity level would lead to mould development and wall deterioration indoors (Torpy et al., 2014). Hence, use of shade-loving xerophytes (CAM plants) would have high scope in indoor decoration than C3 and C4 plants.

Conclusion
The present study concludes that D. seguine, Z. zamiifolia and D. sanderiana plants neither close stomata at day time nor absorb CO2 at night under normal environmental condition. Potted Cryptanthus sp. and S. trifasciata (plants with CAM photosynthesis pathway) keep stomata closed during day time but absorb CO2 at night, hence can effectively be used in indoor air purification at night. However, the soil-based growth medium releases CO2 at night. Dracaena sanderiana possess higher number of stomata in leaves and shows higher emission of CO2 at night. However, the relationship between stomata number and CO2 will need to be studied further. Different indoor plants and their growth medium influence the indoor air quality with respect to CO2 concentration.