As one of the essential components in ornamental crops, foliage plants are grown for their attractive leaves with various shapes, colors, styles, and textures, which are typically used for outdoor landscapes or interior scaping (Henny and Chen 2003), add aesthetic beauty to a living space (Kochlar 2009), provide functional features such as a partitioning and screen device (Randani et al. 2017), and has been found to improve task performance and act as a mood booster (Shibata and Suzuki 2002). Thus, it is not a surprise that foliage plants have continued to boost their market share in the floriculture industry and have continued to surge in market demands for the last decade (USDA 2019).
Aside from homes and offices, a large market of ornamental foliage plant species is being purchased to be cultivated and grown indoors under continuous lighting within commercial spaces, buildings, and other public infrastructures that operate day and night, such as malls, hospitals, and airports. These ornamental plants have often been placed or landscaped around attractions or relaxing places for guests arriving at these facilities as green walls, center islands, and plant boxes. Nowadays, smart green buildings and homes are continuously developing and improving technologies in assisting people and businesses to provide favorable conditions to facilitate the growth and development of indoor plants, thus it is important to optimize and determine plant species’ responses in these given situations (Chen et al. 2017;Shagol et al. 2018).
One of the most challenging aspects for indoor management for these foliage plants is the light conditions. Light is the most critical factor influencing foliage plant performances under interior conditions (Conover and Poole 2011), more so for extended light periods under light-emitting diodes (LED), as it affects both photosynthetic rate and assimilate accumulation. Several research studies conducted on indoor light intensities of foliage plants have ranged from < 20 - 300 μmol m-2 s-1 (Park et al. 2010a;Park et al. 2010b;Shagol et al. 2018) which could be classified into different categories as low, medium, and high (Pennisi 2017); however foliage plants generally favor low to medium light intensities.
Depending on plant tolerance, plant responses to continuous lighting may cause varying effects such as foliar chlorosis, limited or reduced plant growth and productivity, sudden onset of senescence, and early flowering (Sysoeva et al. 2010;Van Gestel et al. 2005). On the other hand, other studies have shown that continuous light exposure instigated a higher and faster developmental rate in spring wheat (Zhukov and Romanovskaja 1980), chickpeas (Sethi et al. 1981), and Arabidopsis (Massa et al. 2007). However, reviews of Sysoeva et al. (2010) suggest that light intensity is another modifying factor that affects continuous lighting regimes which must also be taken into consideration.
Mechanisms fundamental to the tolerance of interior light intensities vary among different types of foliage plants (Chen et al. 2004). Low light intensities were generally reported to increase leaf drop and reduce leaf quality (Conover and Poole 1997;Sawwan and Ghunem 1999), however, other studies suggest otherwise and have increased fresh and dry weights (Shen and Seeley, 1983) and higher chlorophyll content (Kubatsch et al. 2007;Lance and Guy 1992). Thus, proper trials must be done on foliage types and species in order to determine their response and appropriate light intensities that would allow them to thrive in indoor conditions.
An important tool that is widely used and powerful in both basic and applied research among plant physiological studies is the use of chlorophyll fluorescence analysis (Kupper et al. 2019). This analysis is based on the principle that light energy absorbed by chlorophyll molecules may be used for photosynthesis (photochemistry), dissipated as heat (non-photochemical quenching), or re-emitted as light (chlorophyll fluorescence), and by determining the yield of the latter data, information about the efficiency of the other metabolic processes can be attained (Maxwell and Johnson 2000). This data is done to determine the effects of physiological stresses in various economic plants such as Aloe vera (Hazrati et al. 2016), roses (Bayat et al. 2020), lotus (Bordenave 2019), fern (Oliwa and Skoczowski 2019), etc.
Although light quality and duration have been often studied among ornamental crops, continuous lighting and light intensity levels have had scant attention despite the increasing exposure of foliage plants to such indoor abiotic conditions. Hence, this study deemed to determine the impact on plants’ performance, chlorophyll fluorescence, and growth under continuous lighting and different light intensity levels in various foliage plants.
Materials and Methods
Six ornamental foliage plants, namely Hoya carnosa f. variegata, Epipremnum aureum f. variegata, Rhapis excelsa, Hedera helix, Chamaedorea elegans, and Spathiphyllum wallisii (Fig. 1) which were taken from a plant nursery in Gyeonggi-do, South Korea. Experimental units were then taken to the Plant Physiology Laboratory, Department of Environmental Horticulture, Sahmyook University.
Before the conduct of the experiment, plants were repotted into premixed horticultural soil (Shinsung Mineral, South Korea) to homogenize planting media. These were then placed in the greenhouse for 3 weeks to allow the establishment of plants into the new media.
Experimental design and treatment
The study was conducted in Completely Randomized Design (CRD) with three light intensity levels of 60, 120, and 180 μmol m-2 s-1 under continuous lighting conditions using white LED bars (CPH40W, Pulse Tech, South Korea) with 6500 K color temperature which was installed in a growth facility. These treatments were replicated thrice with three plants per replication.
Potted foliage plants were placed in different growing facilities with their respective light intensity treatments. Conditions were set at 20/18°C (± 1°C) D/N with a relative humidity of 75% (± 10%) for 4 weeks from March to April 2021.
Plant growth parameters
Growth and quality evaluation was done by collecting plant height, width, fresh and dry weight. For determining the dry weight of samples, leaves and roots were oven-dried at 85°C for 24-hours wherein the weight became constant. To determine ornamental quality, leaf color was assessed using the CIELAB color value using a spectrophotometer (CM-2600d, Konica Minolta, Japan) which utilizes L*, a*, and b* color space values indicating the lightness, hue, and saturation, respectively. In addition, CIELAB values were processed using the CIELAB– RHS converter (CIELAB-RHS Colour Converter, Oregon State University, USA) in order to determine the Royal Horticulture Society (RHS) color value and group.
SPAD chlorophyll content measurement
Chlorophyll content was determined using a portable SPAD chlorophyll meter (SPAD-502, Konica Minolta, Japan). Calibration was done by carefully clamping the instrument onto the central part of the leaf to obtain chlorophyll readings. In each plant, three leaves were tagged as representative and served as replications. SPAD readings were taken in a two-week interval on the same leaves.
Chlorophyll fluorescence measurement
Using a portable fluorometer (FluorPen FP 110/D, Photon Systems Instruments, Czech Republic), fluorescence transient data were taken from fully developed leaves which were selected and tagged for measurement. The leaves were dark-adapted for 30 min before starting the measurements as a standard protocol indicated by the manufacturer. Measurements were done three times for each replication on the adaxial leaf surface at 13:00 o’clock in the afternoon at a 2-week interval.
Parameters of the OJIP fluorescence, indicating transient and specific energy fluxes values, were calculated as shown in Table 1 with their respective formulas and further descriptions. These measurements were adapted from the studies of Stirbet and Govindjee (2011) and Kasemsap (2014).
Data from plant growth parameters, chlorophyll content, and chlorophyll fluorescence measurement were organized and subjected to standard analysis of variance (ANOVA) using SPSS Statistics 22.0 (IBM, USA). To compare mean differences, Duncan’s multiple range test was applied with a 5% level of significance. Likewise, the comparison of photon yield and specific energy fluxes were presented in radar charts using Microsoft Excel (Microsoft, USA), while OJIP fluorescence transient peaks were derived from the FlourPen software (Photon Systems Instruments, Czech Republic).
Results and Discussion
Plant growth and CIELAB values
Different levels of light intensity under continuous lighting conditions significantly affected the respective growth parameters of various foliage plants (Table 2).
Hoya carnosa f. variegata had significantly the tallest (18.10 cm) and bigger plants (27.24 cm) when subjected to 180 μmol m-2 s-1 and were also found to have the highest values in terms of their dry weight (shoot and root) and shoot fresh weight. These were then followed by plants grown with 120 and 60 μmol m-2 s-1.
Growth parameters collected from Epipremnum aureum f. variegata were significantly affected by lighting conditions. Plants grown under 180 μmol m-2 s-1 had the tallest plants (27.54 cm), fresh weight of shoots (63.34 g) and roots (27.89 g), and dry weight of shoots (7.11 g) and roots (3.38 g).
The highest values of growth parameters were observed from Hedera helix from those of the 180 μmol m-2 s-1 group. Treatments at higher light intensities (180 and 120 μmol m-2 s-1) had significantly the tallest plants. The highest shoot fresh and dry weight were taken from those treated at 180 μmol m-2 s-1 which did not significantly differ from those at 120 μmol m-2 s-1, followed by those of 60 μmol m-2 s-1. Significant differences were found in the root dry weight with the highest value (1.00 g) taken at 180 μmol m-2 s-1 which significantly differed from those of 120 (0.81 g) and 60 (0.71 g) μmol m-2 s-1. Likewise, similar trends were observed from those of Spathiphyllum wallisii which statistically had the tallest (40.16 cm) and bigger plants (28.63 cm), and the highest value for shoot fresh weight (43.34 g), and shoot (7.43 g) and root (3.16 g) dry weight.
Rhapis excelsa foliage plants had significantly the tallest plants treated under 120 μmol m-2 s-1 with 38.66 cm, followed by those grown under 180 and 60 μmol m-2 s-1 with 37.86 cm and 36.56 cm, respectively. These were similar to those of their fresh and dry weights, however, mean differences were found to be insignificant. Likewise, Chamaedorea elegans also had favorable growth under 120 μmol m-2 s-1 light intensity which had the tallest plants (28.16 cm), and highest shoot (14.06 g) and root (4.55 g) fresh weight. These were followed by plants grown under 60 and 180 μmol m-2 s-1, which did not significantly differ from each other.
Although foliage plants are generally tolerant to shaded conditions, this ability of plants can be traced in their efficiency to utilize light energy during photosynthesis and the rate of respiration under different low light intensities (Corre 1983). Under continuous lighting conditions, the use of a 24-h photoperiod with relatively low photosynthetic photon flux (PPF) has been reported to have benefited tolerant crops (Sysoeva et al. 2010). The increase of plant growth including fresh and dry weight favoring higher light intensities is indicative of the treatment’s importance to the physiological processes involved in its development (Chen et al. 2004). Taiz and Zeiger (1991) explain that at higher lux/PPF values, the movement of the chloroplast is parallel to the indecent light increasing plant height. In addition, it has been well established that appropriate high light exposures would enhance photosynthetic activity which would, in turn, have higher growth rates (Adams and Early 2004). Similar studies have been observed in other potted ornamental plants such as in succulents (Cabahug et al. 2017;Nam et al. 2016) and H. helix (Park et al. 2010a). However, in more shade-tolerant plants, wherein their light saturation point is lower, even at low light levels would prompt an increase in height and leaf expansion as means to adapt to low light conditions (Steinger et al. 2003;Zhang et al. 2003).
Based on the results of the CIELAB values, significant color differences were observed from E. aureum f. variegata and R. excelsa. Results showed that brighter, lighter hue, and more saturated in color at higher light intensities. Among foliage plants, only those of E. aureum f. variegata and R. excelsa have been significantly affected by different lighting intensities in their CIELAB values as shown in Table 3. E. aureum f. variegata plants which were treated under 180 μmol m-2 s-1 significantly had the highest a* and b* values with -8.14 and 16.48, respectively. In their L* values, those grown at the highest light intensities had significantly differed from other light levels with 36.80. However, based on the RHS color group and value, they are still categorized as green. R. excelsa plants, on the other hand, had significantly changed in RHS color group and value which is indicative of its high CIELAB values at 180 μmol m-2 s-1. Lower light intensities were tagged as yellow-green while at the highest level was categorized as green.
Changes in colors for ornamental foliage plants have been of great importance due to their aesthetic value. Studies by Kim et al. (2012) suggested that some foliage plants under low light conditions may have less or no leaf color changes at all.
Chlorophyll content (SPAD units) was significantly affected by the different light intensities under continuous lighting conditions as shown in Fig. 2.
Results showed that most of the foliage plants, H. carnosa f. variegata, H. helix, C. elegans, and S. wallisii had significantly higher chlorophyll content when subjected to 180 μmol m-2 s-1, which was then followed by those treated at 60 and 120 μmol m-2 s-1. On the other hand, E. aureum f. variegata had the highest chlorophyll content from those of 60 μmol m-2 s-1 (58.21), followed by those of 120 and 180 μmol m-2 s-1 with 52.76 and 49.45 which did not statistically differ from each other. R. excelsa plants grown under 120 μmol m-2 s-1 significantly had the highest chlorophyll content of 61.40, followed by those subjected at 60 and 180 μmol m-2 s-1 with 57.16 and 54.70, respectively.
One of the most essential factors wherein light condition affects plants is the leaf pigment. Color-leaf plants are found to be sensitive to light and mostly produce anthocyanins in response to the light being absorbed especially as a response to environmental changes (Chen et al. 2008;Michal 2009). According to Zhao et al. (2012), low-light environments are likely to interfere with normal photosynthetic processes by affecting the synthesis and ratio of various plant pigments which include chlorophyll. Research studies dealing with low light intensity largely reported that as light intensity decreases chlorophyll contents significantly increases such as that of Physocarpus species (Zhang et al. 2016), Brassica campestris (Zhu et al. 2017), Sage (Rezai et al. 2018), and other foliage plants such as Dieffenbachia, Anthurium, and Ficus (Chen et al. 2005), which was attributed to the correlated increase of specific leaf area and enlarging leaf area.
However, this has not been the case for plants under continuous lighting conditions. Langton et al. (2003) suggested that, in some plant species, prolonged photoperiod also results in increases in leaf area and chlorophyll content per unit of leaf area which would mirror the increase in biomass production. This trend in continuous light conditions has been observed in the production of mini-cucumbers (Lanoue et al. 2021), Platycodon grandiflorum (Xiao et al. 2007), among others. Notably, reviews of Sysoeva et al. (2010) reported that leaf pigment content as affected by photoperiod has been found to be varied depending on species.
Chlorophyll fluorescence analysis
According to the reviews of Stirbet and Govindjee (2011), OJIP chlorophyll transient has been able to determine and monitor the effects of environmental stressors on the photosynthetic apparatus and their respective functions. Each step of the OJIP curve corresponds to specific activities involved in the photosystems. Starting from the initial fluorescence (Fo) or the level O, the induction curve appears toward levels J and I emitted after 2 and 60 ms, which is then followed by Fm or level P (Strasser and Strasser 1995). The reduction of the acceptors of PS II is indicated by the O-J section, reduction of plastoquinone (PQ) pool by the J-I section, and reduction of acceptors of PS I by the I-P section (Ripoll et al. 2016).
Results on the effect of different light intensities under continuous lighting conditions are shown in Fig. 3. The relative fluorescence intensity in the leaves of plants treated under the lowest light intensity (60 μmol m-2 s-1) was higher than that of 120 and 180 μmol m-2 s-1, respectively, from all foliage species at all points in the OJIP curve. Moreover, evident differences from light intensity levels are observed starting from the J, I, and P peaks. The increased values of the fluorescence value at these points, especially at points J and I, indicate a reduced photochemical activity of the PSII (Zhang et al. 2016). For R. excelsa (Fig. 3c), C. elegans (Fig. 3e), and S. wallisii (Fig. 3f), the large decrease in the fluorescence value of 120 and 180 μmol m-2 s-1 resulting in a flatter OJIP curve. This was, likewise, observed among H. helix at 180 μmol m-2 s-1 (Fig. 3b).
Differences in the radial plot analysis were observed from OJIP parameters and energy fluxes were mostly observed from those of Fm/Fo, Fv/Fo, ABS/RC, and DIO/RC as affected by light intensity levels under continuous lighting conditions (Fig. 4). The Fm/Fo and Fv/Fo were higher at lower light intensities. These parameters are indicative of chlorophyll fluorescence quenching activities which are often associated with stress-induced changes in photosynthetic activities (Cen et al. 2017). Dan et al. (2000) suggested that Fv/Fo is a good indicator of the number and the size of photosynthetic reaction centers. Although there was a decline of these parameters in higher light intensities, these may have been attributed to the continuous light exposure. Although continuous light has been applauded for short-term exposure, the long-term impacts on photosynthesis have been reported to be contradictory citing a reduction in photosynthetic activity resulting in an inefficient accumulation of starch, imbalance of source-sink ratio, etc. (Sysoeva et al. 2010). Similar results were found in continuous light treated mung bean in which decline of the Fv/Fo values was attributed to the shift in the rate of electron transport from PSII to the principal electron acceptors with the decrease in the number and size of the RC (Kumar et al. 2020).
Specific energy fluxes (i.e., ABS, TR, and ET) are expressions of fully active PS II reaction center (RC). Among specific energy flux parameters, the absorption flux per RC (ABS/RC) and dissipated energy flux per RC (DIo/RC) had higher values in 180 μmol m-2 s-1 compared to those grown under lower light conditions (60 and 120 μmol m-2 s-1). ABS/RC is said to be the measure of the average absorption per active RC and of the average amount of absorbing chlorophylls per RC, i.e., of the apparent antenna size (Strasser et al. 2004). On the other hand, DIo/RC denotes the proportion of the total dissipation of untrapped excitation energy from all active RCs, wherein the dissipation transpires as heat, fluorescence, and energy transfer to other systems (Grieco et al. 2015).
Opposing views in some studies dealing with the values of ABS/RC and DIo/RC. Studies by Zhang et al. (2016) resulted in an increase in its value as an adaptive response to low light conditions which have been attributed to the enhanced activity of antenna pigments per reaction center. Results of the studies of Kumar et al. (2020) indicated that under continuous light conditions there would be a decrease value indicating an increased number of active centers. Likewise, similar explanations are both given for the values of DIo/RC. Given that the foliage plants used in this study are considered to be low-light tolerant, the increase of ABS/RC and DIo/RC is attributed to the exposure to excess irradiance causing the inactivity of reaction centers.
In summary, foliage plants showed a stronger response to continuous lighting conditions at higher light intensities. Plant parameters were found to have favorable growth and mass yield in higher light intensities. However, differences in light intensities under continuous light conditions did not affect the hue, saturation, and lightness of colors of the majority of foliage plants. In evaluating the OJIP curve, suggest strong stress responses were evident in low light conditions. However, in the analysis of fluorescence parameters, continuous lighting conditions with high light intensities may provide stress to low-light tolerant plants. A future prospective study may be deemed necessary for investigating other ornamental plants to determine responses in such conditions.