Introduction

Urbanization and industrialization have led to an increasing number of people residing indoors. As a consequence, there is a rise in energy consumption and greenhouse gas emissions in cities, resulting in air pollution. Consequently, the demand for indoor plants and environmentally friendly elements like green buildings and green walls (or bio walls) is steadily rising (Hovhannisyan and Khachatryan, 2017; Irga et al., 2018; Kwon et al., 2021). Previous studies have indicated that indoor plants are beneficial for human physical and mental health (Han, 2019; Hung and Chang, 2021) and positively impact life satisfaction (Kim et al., 2019). Moreover, they have been shown to improve work productivity and job satisfaction among indoor workers and enhance emotional stability by reducing stress in patients (Dijkstra et al., 2008; Dravigne et al., 2008). Green walls adorned with indoor plants have been found to suppress human sympathetic nerve activity, promoting mental stability (Youn et al., 2022).

Peperomia is one of the genera within the Piperaceae family, known for its rich species diversity (Frenzke et al., 2016). It comprises approximately 1600 species, mostly found in tropical regions (Samain et al., 2009; Smith et al., 2008). Peperomia, a type of foliage and succulent plant, exhibits various unique colors and forms across different species and cultivars, making it a popular indoor ornamental plant. Moreover, some Peperomia species are known for their excellent shade tolerance and easy cultivation. Additionally, certain Peperomia species have been reported to be used as traditional remedies for ailments such as cancer, inflammation, and infections (Al-Madhagi et al., 2018). Previous studies have shown that key compounds synthesized by Peperomia exhibit potential effects as anthelmintic, anti-fungal, anti-bacterial, and anti-proliferative (Gutierrez et al., 2016; Ware et al., 2022; Wei et al., 2011). Recently, various phytochemicals extracted from Peperomia have been reported to have positive effects on glaucoma and cataract treatments, indicating their potential value as medicinal plants (Ho et al., 2022).

Light-emitting diodes (LEDs) are widely used as artificial light sources for indoor plant cultivation due to their ease of spectral distribution control, easy combinations, lower heat emission, and longer lifespan compared to fluorescent lamps (Morrow, 2008). LED usage has been increasing not only in plant factories and plant production facilities but also in general households, where LED lighting has become relatively prevalent compared to fluorescent lamps (Matsushima et al., 2011). LED lights with various color temperatures, ranging from 2700 to 6500 K, are available in the market (Jang et al., 2023). However, there are relatively few studies that have applied white LEDs with different color temperatures for indoor plant cultivation.

Chlorophyll fluorescence analysis is a non-destructive testing that allows the investigation of plant photosynthetic responses. Various photosynthetic parameters (F_v/F_m, Φ_Do, ABS/RC, DI_o/RC, PI_ABS, and etc.) enable easy evaluation of plant photosynthetic performance, and its usefulness has been demonstrated in various studies (Lee et al., 2021; 2022d; Park et al., 2023; Vosnjak et al., 2021). While the conventional methods of assessing plant growth parameters were based on measurements of plant sizes or biomass, recent studies have increasingly also utilized leaf color reading values (such as CIELAB, Hunter Lab, and RGB) and leaf pigments analyses (Cabahug et al., 2017; 2019; Nam et al., 2016; 2022) as well as chlorophyll fluorescence analysis (Lee and Lee, 2023; Oh et al., 2022; Yang et al., 2022) for assessing plants vitality.

Therefore, in this study, we selected different Peperomia species and cultivars, P. obtusifolia, P. caperata cv. Eden Rosso, and P. caperata cv. Napoli Nights, as experimental plants, investigated the effects of different white LED light sources with different color temperatures of 3000, 4100, and 6500 K, respectively, on the growth, visual qualities, and photosynthetic parameters of Peperomia.

Materials and Methods

Plant Materials

To investigate the impact of the different color temperatures of white LEDs on the growth and photosynthetic parameters of Peperomia plants, species and cultivars, namely P. obtusifolia, P. caperata cv. Eden Rosso, and P. caperata cv. Napoli Nights were selected as experimental plants (Fig. 1). The plants were potted in commercial round plastic pots measuring 11 × 10.5 cm; diameter × height, filled with fertilized horticultural substrate (Hanareumsangto, Shinsung Mineral, South Korea).

Fig. 1.

Peperomia species and cultivars used in this study: A) P. obtusifolia; B) P. caperata cv. Eden Rosso; and C) P. caperata cv. Napoli Nights.

Environmental Setup

The study was conducted for 5 weeks in the plant physiology laboratory located in the annex of the experimental greenhouse of the Department of Environmental Horticulture at Sahmyook University in South Korea. The artificial light sources used in the study were commercial white T5 LEDs (Zhong Shan Jinsung Electronic, China), which are the most widely distributed in South Korea, with different color temperatures of 3000 (warm white LED; peaks at 455 and 600 nm), 4100 (natural white LED; peaks at 455 and 590 nm), and 6500 K (cool white LED; peaks at 450 and 545 nm), respectively. We verified whether the color temperature of the LEDs provided by the manufacturer matched the actual color temperature using a spectroradiometer (SpectraPen mini, Photon Systems Instruments, Czech Republic). The LEDs used in this study were 1.2 m long with 20 W power consumption, AC 220 V rated voltage, and 60 Hz frequency. The photoperiod was set at 14 hours of day and 10 hours of night for all treatments. The photosynthetic photon flux density (PPFD) (in this study, 350－800 nm) was adjusted to 100 µmol m^-2 s^-1 between the plants and the LEDs using a portable spectroradiometer (SpectraPen mini, Photon Systems Instruments, Czech Republic), as shown in Fig. 2 and Table 1. Shade films were installed to prevent light interference between different LED light treatments. Throughout the experimental period, a temperature of 20 ± 1°C was maintained using a temperature control system, and the relative humidity was kept at 63.7 ± 15.2%. The plants were watered once a week with 500 mL of purified water per pot.

Fig. 2.

Spectral intensity of white light-emitting diodes (LEDs) used in this study: A) warm white, 3000 K white LED; B) natural white, 4100 K white LED; and C) cool white, 6500 K white LED.

Parameters Analysis

To analyze of growth and photosynthetic parameters of Peperomia species an cultivars, we investigated various parameters, including shoot height, shoot width, root length, ground cover, leaf length, leaf width, SPAD units (chlorophyll content), CIELAB values (L*, a*, and b*) of leaf color, fresh weight, dry weight, moisture content, and various photosynthetic parameters (F_v/F_m, Φ_Do, ABS/RC, DI_o/RC, and PI_ABS). For shoot height, measurements were taken from the highest point of the plant above the potting media surface. Shoot width was measured as the widest part of the plant when viewed from above. Root length was measured based on the longest root of the plants. Ground cover was calculated as the square of shoot width. SPAD units were measured using a portable chlorophyll meter (SPAD-502Plus, Konica Minolta, Japan). The CIELAB values were measured by referring to the leaf color measurement method proposed by Lee et al. (2022b). We used a spectrophotometer (CM-2600d, Konica Minolta, Japan) set to CIELAB D65/10°, and obtained CIELAB L*, a*, and b* values, including the specular component (SCI). For both SPAD units and CIELAB values, random selections of leaf samples were made from areas where leaf veins did not pass through the leaf center. In addition, the leaf color was converted into a transformed color using the Converting Colors developed by Zettl (2023), which converts the average CIELAB L*, a*, and b* values into converted colors. For fresh weight measurement, plants were washed and naturally dried in a sealed space for 24 hours before being weighed. Dry weight was measured after subjecting the samples to hot-air drying at 85°C for 24 hours using a hot-air dryer (HK-DO135F, HANKUK S&I, South Korea). Furthermore, the moisture content was calculated by comparing the fresh weight and dry weight using the following equation (Eq. 1).

(x is moisture content, A is fresh weight, and B is dry weight)

For the analysis of chlorophyll fluorescence responses influenced by the different color temperatures of white LEDs, chlorophyll fluorescence analysis was performed using a portable chlorophyll fluorometer (FluorPen FP 110/D, Photon Systems Instruments, Czech Republic) on the leaf’s central part, where the leaf veins were absent, using random selection method. Prior to measuring chlorophyll fluorescence indices, the plants were dark-adapted for approximately 15 minutes using dark-adaptation leaf clips according to the manufacturer’s guidelines (PSI, 2023). To assess the quantum yield, plant stress indicators, and performance index of the plants, five photosynthetic parameters were selected and applied, including F_v/F_m (maximum quantum yield of photosystem II) (Eq. 2), Φ_Do (probability of absorbed photons being dissipated) (Eq. 3), ABS/RC (absorption flux per reaction center) (Eq. 4), DI_o/RC (dissipated energy flux per reaction center) (Eq. 5), and PI_ABS (absorption basis performance index) (Eq. 6). These photosynthetic parameters were calculated using the following formulas (PSI, 2023; Stirbet and Govindjee, 2011).

Statistical Analysis

The results were analyzed using SAS 9.4 (SAS Institute, USA) with analysis of variance (ANOVA). Duncan’s multiple range test at the p < 0.05 significance level was performed for mean comparisons. The study was in a completely randomized design, with three plants assigned to each treatment in three replications.

Results and Discussion

Changes in Plant Growth Parameters

Different color temperatures of white LEDs exposed to Peperomia species and cultivars resulted in various changes in plant sizes (Table 2). Regarding shoot height, P. obtusifolia and P. caperata cv. Napoli Nights (hereafter ‘Napoli Nights’) exhibited the highest values at 25.06 and 11.29 cm, respectively, under the 4100 K (red/blue ratio = 1.77) white LED treatment. However, under the 3000 (red/blue ratio = 3.28) and 6500 K (red/blue ratio = 0.83) white LED treatments, which had large differences in the distribution ratio of red and blue wavelengths, a tendency of reduced shoot height was observed. On the other hand, for P. caperata cv. Eden Rosso (hereafter ‘Eden Rosso’), the shoot height reached the highest value at 9.20 cm under the 6500 K white LED treatment, which had a relatively high ratio of blue wavelengths. Shoot width was the widest for P. obtusifolia at 22.65 cm under the 6500 K white LED treatment, indicating a higher influence of blue wavelengths. However, the other two Peperomia cultivars showed no significant differences among the treatments. As for root length, ‘Eden Rosso’ and ‘Napoli Nights’ exhibited the longest values at 9.54 and 10.78 cm, respectively, under the 6500 K white LED treatment, suggesting a significant influence of blue wavelengths on root development. A previous study by Lee and Nam (2023c) demonstrated that, for Pachyphytum, the use of 6500 K white LED was relatively suitable for promoting root development, survival rate, and rooting success rate, similar to some succulent species like Peperomia and Pachyphytum. In contrast, for rosemary (Rosmarinus officinalis), it was reported that blue light led to relatively faster root formatting compared to other light qualities (Gil et al., 2021), and for Populus sieboldii × P. grandidentata, the blue LED treatment induced deep-rooted growth (phreatophyte) (Kobayashi et al., 2022). Conversely, king protea (Protea cynaroides) demonstrated that red light promoted root development (Wu and Lin, 2012), and grapevines (Vitis vinifera) showed increased rooting percentage and root numbers under red LED light conditions (Poudel et al., 2008), indicating that the influence of red and blue light on root growth varies among different plant species. The result for ground cover revealed that P. obtusifolia exhibited the widest area at 521.6 cm² under the 6500 K white LED treatment, suggesting that increasing the ratio of blue wavelengths within white LED is relatively advantageous for significant ground cover expansion. As for leaf length, ‘Eden Rosso’ and ‘Napoli Nights’ indicated the longest values at 4.72 and 4.52 cm, respectively, under the 3000 K white LED treatment, while leaf width was widest for P. obtusifolia at 7.04 cm under the 4100 K white LED treatment. Meanwhile, ‘Eden Rosso’ showed wider leaf lengths at 2.53 and 2.42 cm under the 3000 and 6500 K white LED treatments, respectively. Chlorophyll content (SPAD units) is an indicator directly affecting plant growth and development (Bui et al., 2022) and is generally positively correlated with plant growth parameters. The chlorophyll content results indicated that both P. obtusifolia and ‘Napoli Nights’ exhibited the highest values under the 4100 and 6500 K white LED light treatments, implying that an increase in the ratio of red wavelengths might decrease the chlorophyll content per unit area. However, ‘Eden Rosso’ showed no significant differences among treatments. A previous study reported that purple basil (Ocimum basilicum cv. Red Rubin) and red kale (Brassica oleracea cv. Scarlet) showed an increase in chlorophyll concentration with higher blue wavelength ratios (Dou et al., 2020).

White LEDs with different color temperatures had diverse effects on the fresh weight and dry weight of Peperomia species and cultivars (Table 3). For the fresh weight, P. obtusifolia showed the highest value at 185.13 g under the 4100 K white LED light treatment. Similarly, ‘Napoli Nights’ exhibited higher fresh weights at 68.26 and 69.46 g under the 3000 and 4100 K white LED treatments, respectively. Therefore, it is presumed that there is a positive relationship between the proportion of red wavelengths and the moisture content. This observation was consistent with a previous study on Sedum album cv. Athoum, exhibited the heaviest fresh weight under the 3000 and 4100 K white LED treatments (Lee and Nam, 2022a). A previous study on the ratio of red and blue LED light for Lactuca sativa cv. Sunmang and L. sativa cv. Grand Rapid TBR also reported that an increase in the proportion of red wavelengths led to an increase in both fresh weight and dry weight (Son and Oh, 2013), aligning with the findings of this study. Additionally, Centella asiatica cv. Urban indicated a similar trend of decreasing fresh weight with a decrease in the proportion of red wavelengths (Song et al., 2022). On the other hand, ‘Eden Rosso’ showed relatively higher preferences for the high distribution of blue wavelengths, as it exhibited higher fresh weights of 50.05 and 58.45 g under the 4100 and 6500 K white LED light treatments, respectively, compared to ‘Napoli Nights’. In terms of dry weight, P. obtusifolia demonstrated the highest value at 9.54 g under the 4100 K white LED treatment, consistent with the results of fresh weight. A previous study on Orostachys japonica reported similar results, with high dry weight observed under the 4100 K white LED treatment (Lee et al., 2022c). ‘Eden Rosso’ exhibited the highest dry weight at 2.45 g under the 6500 K white LED treatment, showing consistency with the results of fresh weight. On the other hand, ‘Napoli Nights’ showed the heaviest dry weight at 2.80 g under the 3000 K white LED light treatment, consistent with the results of fresh weight, suggesting a higher influence of red wavelengths within white LED light. Regarding moisture content, P. obtusifolia exhibited higher values of 94.8 and 94.7% under the 3000 and 4100 K white LED treatments, respectively. In contrast, ‘Eden Rosso’ and ‘Napoli Nights’ both showed the highest moisture content under the 4100 K white LED treatment. A previous study, Sempervivum cv. Black Top was reported to increase moisture content alongside vigorous growth (Lee and Nam, 2023b), and it is postulated that some Peperomia species also tend to have increased moisture content concurrently with higher growth rates.

After a comprehensive analysis of plant growth parameters for the different Peperomia species and cultivars, it is recommended to cultivate P. obtusifolia under 4100 and 6500 K white LEDs, ‘Eden Rosso’ under the 6500 K white LED treatment, and ‘Napoli Nights’ under the 3000 K white LED light to significantly enhance plant sizes and biomass.

Changes in Visual Qualities

Different color temperatures of white LEDs had diverse effects on the leaf color quality of Peperomia species and cultivars (Table 4). CIELAB, introduced by the International Commission on Illumination (CIE) based in France, made its debut in 1976 (Lee et al., 2022c), is characterized by the representation of L*, a*, and b* parameters, including an asterisk, unlike the previously used Hunter Lab (Lee and Nam, 2022a). CIELAB is currently employed for quality assessments in horticultural industries, such as floral crops (Jang et al., 2023; Shin et al., 2022), ornamental plants (Lee, 2023; Lee and Nam, 2022b; 2023a), and edible plant products (Cao et al., 2021; Kim et al., 2022; Lee et al., 2022a; 2022e). The CIELAB L*, representing color lightness, showed the highest values of 32.44 and 43.72 for P. obtusifolia and ‘Napoli Nights,’ respectively, under the 6500 K white LED light treatment. The CIELAB a* indicates redness with positive values and greenness with negative values. For P. obtusifolia, the highest a* value of -6.88 was observed under the 4100 K white LED treatment. For ‘Eden Rosso,’ the values were higher under the 3000 and 6500 K white LED treatments, at -2.67 and -2.79, respectively, but a* was lowest at -3.32 under the 4100 K white LED treatment, contrasting with the results for P. obtusifolia. The CIELAB b* represents yellowness with positive values and blueness with negative values. P. obtusifolia showed higher b* values of 11.77 and 11.22 under the 3000 and 6500 K white LED treatments, respectively. For ‘Eden Rosso,’ higher values were observed under the 3000 and 4100 K white LED treatments, at 5.69 and 5.80, respectively. On the other hand, ‘Napoli Nights’ indicated the highest b* value of 5.50 under the 4100 K white LED treatment, exhibiting distinct trends. Ultimately, plant size parameters and biomass showed an inverse relationship with b*, which is consistent with a previous study reporting a negative correlation between b* and plant growth parameters in plants (Lee et al., 2022c).

Table 4.

Leaf color reading values of CIELAB (L^*, a^*, and b^*) and converted color of Peperomia species and cultivars grown under white LEDs with different color temperatures (3000, 4100, and 6500 K) for five weeks

Plants	Color temperatures (K)	CIELAB values (leaf color part)			Converted colorz (color chip)
		L*	a*	b*
P. obtusifolia	3000	30.70 by	-7.47 b	11.77 a
	4100	31.58 ab	-6.88 a	9.09 b
	6500	32.44 a	-7.59 b	11.22 a
P. ‘Eden Rosso’	3000	29.75 a	-2.67 a	5.69 a
	4100	29.90 a	-3.32 b	5.80 a
	6500	30.17 a	-2.79 a	3.94 b
P. ‘Napoli Nights’	3000	39.12 b	-2.65 a	5.21 b
	4100	40.78 ab	-2.69 a	5.50 a
	6500	43.72 a	-2.68 a	5.36 ab
Significancex	Plants (A)	***	***	***
	Color temperatures (B)	*	**	**
	(A) × (B)	NS	**	***

^zColors converted using CIELAB L^*, a^*, and b^* values.

^yStatistical significance within individual species or cultivars were assessed for each treatment using Duncan’s multiple range test at a significance level of p < 0.05; same lowercase letters indicate no significant difference.

^xStatistical significance using factorial analysis; NS, *, **, and ***: non-significant or significant at p < 0.05, 0.01, or 0.001, respectively.

In the analysis of leaf color quality using CIELAB values, both P. obtusifolia and ‘Napoli Nights’ displayed an inverse relationship between L* and plant growth parameters, such as plant sizes and biomass, consistent with previous studies findings. This indicates that the lightness of leaves may increase relatively when plant growth is suppressed, and conversely, the lightness of leaves decreases when the plant is vigorous and well-developed. The results for b* also similar to this observation.

Changes in Photosynthetic Parameters

Different color temperatures of white LEDs had varied effects on the photosynthetic parameters of Peperomia species and cultivars (Table 5). The parameter F_v/F_m represents the maximum quantum yield of photosystem II (PSII) photochemistry and is used to assess the photosynthetic physiological state of plants (Sharma et al., 2015). For unstressed higher plants, this parameter is known to range from 0.78 to 0.84 (Bjorkman and Demmig, 1987; Butler and Kitajima, 1975; Genty et al., 1989; Govindjee, 1995; 2004; Paillotin, 1976; Yoo et al., 2012), and in this study, it ranged from 0.77 to 0.81. Specifically, for P. obtusifolia and ‘Eden Rosso,’ their values fell within the reported normal range, whereas ‘Napoli Nights’ exhibited a mean value of 0.77 under the 4100 K white LED treatment, although there was no statistically significant difference. The parameter Φ_Do, representing the probability of absorbed photons dissipated, was significantly higher for P. obtusifolia and ‘Eden Rosso’ under the 6500 and 3000 K white LED treatments, respectively, with values of 0.20 and 0.21. Specifically, ‘Eden Rosso,’ which exhibited lower plant sizes and biomass under the 3000 K white LED treatment, appeared to be negatively affected by the higher proportion of red wavelengths within the white LED light, similar to other plants of the same category. This suggests that the increased distribution of red wavelengths within white LED light has a detrimental impact on the growth and photosynthetic physiology of ‘Eden Rosso’ and similar species and cultivars. The parameter ABS/RC, representing the absorption flux per reaction center, was highest for P. obtusifolia under the 6500 K white LED treatment (1.20), while ‘Eden Rosso’ showed the highest value (1.49) under the 3000 K white LED treatment, indicating contrasting results. Similarly, the parameter DI_o/RC, representing the amount of energy dissipated per reaction center, also displayed the highest value for P. obtusifolia under the 6500 K white LED treatment (0.24), while ‘Eden Rosso’ showed the highest value (0.34) under the 3000 K white LED treatment, indicating different preferred color temperatures of white LEDs for each plant. The performance index PI_ABS, expressed on an absorption basis (Srivastava et al., 1999), was higher for P. obtusifolia under the 4100 and 6500 K white LED light treatments, with values of 11.37 and 10.95, respectively. For ‘Eden Rosso,’ the highest value (10.71) was observed under the 6500 K white LED treatment, aligning with the trends observed for plant sizes and biomass. Particularly, the higher preference for the 6500 K white LED treatment, with a higher proportion of blue wavelengths, was consistent with previous research showing a significant increase in photosynthetic efficiency per unit leaf area in rose (Rosa × hybrida) under similar conditions (Terfa et al., 2013).

In conclusion, based on the changes in growth, visual qualities, and photosynthetic parameters, it is recommended to cultivate P. obtusifolia under 4100－6500 K white LEDs, while ‘Eden Rosso’ and ‘Napoli Nights’ should be grown under the 6500 and 4100 K white LEDs, respectively.

Conclusion

The genus Peperomia, belonging to the family Piperaceae, comprises around 1600 species, primarily distributed in tropical regions. Peperomia is commonly cultivated as an indoor or succulent plant and is widely distributed in the ornamental plant market due to its shade tolerance, making it suitable for indoor cultivation. In this study, we applied three different color temperatures of white light-emitting diodes (LEDs) and analyzed their effects on the growth and photosynthetic parameters of Peperomia species and cultivars. Specifically, we used white LEDs with different color temperatures of 3000, 4100, and 6500 K, respectively, and evaluated species and cultivars namely: P. obtusifolia, P. caperata cv. Eden Rosso (‘Eden Rosso’), and P. caperata cv. Napoli Nights (‘Napoli Nights’). The results indicated that plant sizes were most favorable for P. obtusifolia and ‘Napoli Nights’ under 4100 and 6500 K white LEDs, while ‘Eden Rosso’ exhibited the best growth under the 6500 K white LED. Biomass production was highest for P. obtusifolia under the 4100 K white LED. Additionally, ‘Eden Rosso’ and ‘Napoli Nights’ showed the best biomass production under the 6500 and 3000 K white LEDs, respectively. Regarding leaf color, L* and b* exhibited trends opposite to those of plant biomass, consistent with a previous study showing that, under growth-inhibiting conditions, leaf lightness (L*) increases, and leaf color becomes more yellow. Chlorophyll fluorescence results indicated that the maximum quantum yield of PSII (F_v/F_m) ranged from 0.77 to 0.81 across all treatments, generally acceptable values. Stress indices, including Φ_Do, ABS/RC, and DI_o/RC, were higher for P. obtusifolia under the 6500 K white LED, while ‘Eden Rosso’ exhibited higher values under the 3000 K white LED, contrasting with the growth results. The performance index PI_ABS was higher for P. obtusifolia under the 4100 and 6500 K white LEDs, while ‘Eden Rosso’ showed the highest value under the 6500 K white LED, indicating that preferred color temperatures can vary even within the same species. In conclusion, we recommend cultivating P. obtusifolia under 4100－6500 K white LEDs, while ‘Eden Rosso’ and ‘Napoli Nights’ are better suited for cultivation under 6500 and 4100 K white LEDs, respectively. These findings offer valuable insights into the specific color temperature preferences of different Peperomia species, which can aid in their successful indoor cultivation.