As people today spend more time indoors, typically 80–90% of their time, the design of their indoor environment directly affects their health (Deng and Deng, 2018). For this reason, the use of ornamental indoor foliage plants to improve indoor spaces has increased, leading to steady growth in the market size in South Korea (Park et al., 2010). Previous studies have reported that placing ornamental foliage plants indoors not only reduces psychological stress and improves workers’ health (Fjeld et al., 1998; Toyoda et al., 2020), but also promotes the physical and mental health of patients undergoing rehabilitation (Raanaas et al., 2010). Moreover, indoor foliage plants have been shown to be effective in reducing carbon dioxide (CO₂) and fine dust (PM_2.5) in the air, which helps improve the indoor environment (Yang et al., 2022). Among these plants, studies on the genus Philodendron, which is widely used as an indoor plant, have focused on indoor vertical gardens (Gautam et al., 2021; Phonpho and Saetiew, 2017), particulate matter reduction (Jeong et al., 2020), and hydroponic nutrient solutions (Dhanraj, 2020).

The genus Philodendron has over 700 species distributed throughout the Americas. It is the second most species-rich genus in the family Araceae, after the genus Anthurium (Croat, 1997; Gauthier et al., 2008). Most species in the genus Philodendron are characterized by their growth as epiphytes or climbers (Mayo et al., 1997). Of these, Philodendron hederaceum var. oxycardium is available on the South Korean market as an ornamental foliage plant. This species is relatively unknown academically, except for a study on the effects of shading levels and fertilizer sources on its growth and quality (Conover and Poole, 1974), and reports on various diseases affecting it (Knauss et al., 1970, 1972; Wang et al., 2016). There are currently no physiological studies with this species. It is, therefore, necessary to determine an efficacious cutting propagation method and to study approaches to improving the rooting and survival rates of stem cuttings, as well as their growth using exogenous auxin, in order to promote the distribution of P. hederaceum var. oxycardium.

Auxin, a phytohormone commonly referred to as a rooting promoter, plays multiple roles in regulating plant growth and development (Leyser, 2018). This includes localized responses such as endocytosis, cell polarity, cell cycle control, cell elongation, and differential growth, as well as macroscopic phenomena like embryogenesis, tissue patterning, and the de novo formation of organs (Sauer et al., 2013). Since the 1930s, the most common methods used to apply auxin in vegetative propagation within the horticultural industry have been the basal quick-dip, powder application, and dilute soak methods (Blythe et al., 2007). Of these, the quick-dip method is an easy way to apply auxin that is highly effective and is still widely used by farmers today. It is recognized as a useful method for stable mass propagation using cuttings with traits identical to the mother plant.

A previous study reported that in a cutting propagating experiment with Pseudolysimachion ovatum, indole-3-butyric acid (IBA) was more effective than 1-naphthaleneacetic acid (NAA), with the best effect on cutting propagation and growth at a concentration of 500–1000 mg·L⁻¹ (Kwon and Cho, 2023). Hwang et al. (2021) found that cuttings of the strawberry cultivar "Maehyang" had the highest number of leaves at an IBA concentration of 100 mg·L⁻¹, and the highest number of primary roots at an indole acetic acid (IAA) concentration of 100–150 mg·L⁻¹. Meanwhile, rooted grape cuttings were reported to have the highest root fresh and dry weights at an IBA concentration of 4000 mg·L⁻¹ (Galavi et al., 2013). As such, the concentration of auxin required for cuttings varies depending on the species, and different experimental approaches are needed to determine the appropriate auxin concentration for each species.

In this study, P. hederaceum var. oxycardium, commonly used as an ornamental indoor foliage plant in the horticultural industry, was selected as the experimental subject. The rooting ability of stem cuttings and the potential for enhancing growth were assessed based on the type and concentration of exogenous auxin.

The visual evaluation image of Philodendron hederaceum var. oxycardium stem cuttings affected by different auxin types and concentrations in this study are shown in Fig. 1. The survival rate of P. hederaceum var. oxycardium affected by different auxin types and concentrations was 100% for the IAA₁₆₀ (160 mg·L⁻¹ IAA), IBA₅₀₀ (500 mg· L⁻¹ IBA), and IBA₁₀₀₀ (1000 mg·L⁻¹ IBA) treatments, while it was the lowest at 91.4% for the control and NAA₁₀₀₀ (1000 mg·L⁻¹ NAA) treatments (Fig. 2). Meanwhile, the rooting rates were 97.1% for the IAA₁₆₀ and IAA₃₂₀ (320 mg·L⁻¹ IAA), and NAA₅₀₀ (500 mg·L⁻¹ NAA) treatments, but 91.4% for the control and NAA₁₀₀₀ treatments, indicating a tendency for lower rooting rates. However, there was no significant difference among the treatments.

Oh and Lee (2022) reported that the rooting rate of Sedum takesimense cuttings treated with auxin was 95% or higher at an IBA concentration of 250–500 mg·L⁻¹ and an NAA concentration of 500 mg·L⁻¹. Meanwhile, Kwon and Cho (2023) observed that Pseudolysimachion ovatum stem cuttings exhibited a survival rate of 90% or higher, regardless of the auxin concentration. It was reported that Ginkgo biloba cuttings had the highest survival rate at a concentration of 10.0 μM IBA (Pandey et al., 2011). In another study, Viburnum lantanoides exhibited a rooting rate of 51.1% in the control group, but a rooting rate of 70.0–73.3% at a concentration of 1000–8000 ppm IBA. In contrast, Eubotrys racemosa showed a higher rooting rate in the control group compared to the above IBA concentration range (Lubell and Gardner, 2017). As such, it was determined that the effects of auxin on survival and rooting rates vary depending on the plant species. Additionally, since the duration of dipping may also have an effect (Bak et al., 2023), different experimental approaches are needed.

An examination of the parameters related to plant size showed that the shoot length and width of rooted cuttings of P. hederaceum var. oxycardium under IAA₃₂₀ treatment were the highest at 20.98 and 18.91 cm, respectively (Table 2), whereas the shoot length and width in the control were significantly lower at 11.32 and 10.77 cm, respectively. Cabahug et al. (2016) reported that Echeveria subsessilis, a type of ornamental succulent, had the greatest shoot length and width at a concentration of 500 ppm IBA, but Echeveria runyonii had the greatest shoot length and width at a concentration of 0–100 ppm IBA. On the other hand, the results of this study showed that the stem diameter of P. hederaceum var. oxycardium was the thickest at 0.40 cm under the NAA₁₀₀₀ treatment, and the root length was the longest at 19.16 cm under the IBA₂₅₀ (250 mg·L⁻¹ IBA) treatment. Both stem diameter and root length were the lowest in the control at 0.24 and 15.32 cm, respectively. Consequently, all the auxin-treated groups were found to be effective in increasing the size of P. hederaceum var. oxycardium propagated by stem cuttings compared to the control, regardless of the auxin type or concentration. Leaf length was observed to be 9.34 and 9.37 cm under the IAA₈₀ (80 mg·L⁻¹ IAA) and IAA₁₆₀ treatments, respectively, and leaf width the widest at 5.68 cm under the IAA₁₆₀ treatment. Among the chlorophyll fluorescence parameters, F_v/F_m, which indicates the maximum quantum yield of photosystem II, was highest at 0.823 under the IAA₃₂₀ treatment. In contrast, the control had the lowest F_v/F_m value of all treatment groups at 0.796. However, it fell within the range of 0.780–0.840, indicating that the cuttings were not under stress (Björkman and Demmig, 1987; Park et al., 2023; Shin et al., 2023, 2024; Yoo et al., 2012).

An analysis of fresh weight and relative moisture content showed that the IAA₃₂₀ treatment had the highest shoot fresh and dry weights at 9.14 and 0.99 g, respectively, while the IBA₂₅₀ treatment had the highest root fresh and dry weights at 1.54 and 0.33 g, respectively (Table 3). In the control, the fresh and dry weights of the shoots were found to be significantly lower than those of the other treatments (4.56 and 0.63 g, respectively). These findings are consistent with observations regarding plant shoot size and maximum quantum yield of the shoots. Previous studies on cutting propagation of ornamental floricultural crops have suggested that excellent root growth, including root number, length, and both fresh and dry weights, can increase initial survival and promote shoot growth (Kim and Kim, 2015; Lee et al., 2011; Oh and Lee, 2022). However, despite the excellent root growth observed in P. hederaceum var. oxycardium cuttings, shoot growth exhibited relatively different trends. Meanwhile, the relative moisture content of the shoots and roots of P. hederaceum var. oxycardium under the NAA₂₅₀ (250 mg·L⁻¹ NAA) treatment was the highest at 90.2 and 85.6%, respectively. Conversely, the control had the lowest average shoot and root moisture content of 86.4 and 75.1%, respectively. These trends may result from poorer initial rooting conditions, leading to lower moisture absorption rates and efficiency.

A previous study found a positive correlation between the chlorophyll content and SPAD units of Betula, Triticum, and Solanum tuberosum (Uddling et al., 2007). In this study, the relative chlorophyll content, expressed in SPAD units, reached its highest value of 35.56 under the NAA₁₀₀₀ treatment. This suggests that the high concentration of NAA absorbed from the base increases the density of leaf chlorophyll within the same unit area (Table 4). Czerpak et al. (2002) reported that auxins such as phenylacetic acid (PAA) or NAA generally decrease the chlorophyll and carotenoid content of leaves. Other studies have reported that wheat coleoptiles treated with IAA alone exhibited a relatively lower chlorophyll content compared to the control (Volfová et al., 1978), and NAA treatment resulted in a reduction of chlorophyll content in the Momordica charantia cultivar "Makiling" (Tolentino and Cadiz, 2005). However, in this study, as mentioned above, chlorophyll content was increased by treatment with relatively high concentrations of NAA, which is contrary to the findings of previous studies. This suggests that the effect of auxin on chlorophyll content may vary depending on the species. Moreover, this study showed inconsistent results between F_v/F_m, which indicates maximum quantum yield, and chlorophyll content. Therefore, based on the results of this study, it appears that despite elevated chlorophyll levels, the maximum quantum yield may not necessarily increase in proportion, and the variation of F_v/F_m is more closely associated with the shoot growth level of P. hederaceum var. oxycardium stem cuttings.

The external qualities of plants, including leaf and flower colors, can serve as indicators for intuitive evaluation (Lee and Nam, 2023). They can also be affected by changes in pigment content, which can be caused by various hormones (Xia et al., 2021). Karatas et al. (2010) reported that the application of auxin resulted in a reduction in the concentration of chlorophyll and carotenoids in Tropaeolum majus. Li et al. (2017) reported that exogenous auxin was also involved in the inhibition of pigment, total phenolic content, and flavonoid accumulation in tomato fruits. Parameters, including CIELAB, Hunter Lab, and RGB, are employed to assess the external quality of plants. Among these, CIELAB has been useful in evaluating leaf color (Park et al., 2023; Shin et al., 2023, 2024), flower color (Jang et al., 2023; Zhang et al., 2020), and root color (Kim et al., 2024a, 2024b). In this study, a comparison of the leaf color measurements between the auxin-treated groups and the control, using CIELAB color space coordinates, showed a tendency for the lightness and yellowness of the leaves to decrease, regardless of auxin type and concentration. In the control, the L^* coordinate, which indicates lightness in the color space, and the b^* coordinate, which indicates yellow (+) and blue (−), exhibited the highest values at 45.30 and 25.50, respectively. In contrast, in the NAA₁₀₀₀ treatment, those values were significantly the lowest mean values, at 38.29 and 14.85, respectively. Meanwhile, the a^* coordinate representing red (+) and green (−) was lower in the control, IAA₈₀, IAA₁₆₀, and IBA₅₀₀ treatments at statistically the same level of significance, indicating that it was relatively closer to green, whereas in the NAA₁₀₀₀ treatment, it reached its highest at −8.54. This was likely due to variations in intracellular pigment content caused by exogenous auxin (Sunderland, 1966).

When taken comprehensively, the findings of this study suggest that the application of auxin, regardless of its type or concentration, has a positive effect on the growth promotion of P. hederaceum var. oxycardium propagated using stem cuttings compared to the control. Overall, immersion treatment at an IAA concentration of 320 mg·L⁻¹ was most effective for increasing shoot size and enhancing shoot fresh and dry weights. For leaf size, an IAA concentration of 160 mg·L⁻¹ was optimal. Additionally, IBA at 250 mg·L⁻¹ was most effective for increasing root length as well as fresh and dry weights. According to the F_v/F_m results, the plants showed no signs of stress, regardless of the type or concentration of auxin, including in the control group. Leaf color quality results showed that, on average, the auxin-treated groups outperformed the control, regardless of auxin type or concentration. The highest quality was observed at an NAA concentration of 1000 mg·L⁻¹.

This study aimed to determine the effects of different types and concentrations of auxin applied to the base of Philodendron hederaceum var. oxycardium stem cuttings on their growth and rooting ability. Three types of auxins were applied: indole acetic acid (IAA), 3-indolebutyric acid (IBA), and 1-naphthylacetic acid (NAA). IAA was used at concentrations of 80, 160, and 320 mg·L⁻¹, while both IBA and NAA were used at concentrations of 250, 500, and 1000 mg·L⁻¹. The results showed that the mean survival rate was 97.4% in the auxin treatment groups, compared to 91.4% in the control. Similarly, the mean rooting rate was 94.5% in the auxin treatment groups, compared to 91.4% in the control. Meanwhile, shoot height, width, and both fresh and dry weights were found to be the highest at an IAA concentration of 320 mg·L⁻¹, and mean leaf length and width were the highest at an IAA concentration of 160 mg·L⁻¹. In contrast, the highest root length as well as fresh and dry weights were observed at an IBA concentration of 250 mg·L⁻¹. Based on the F_v/F_m results, the plants showed no signs of stress, regardless of the type and concentration of auxin, not even in the control group. In the CIELAB color space analysis, the control exhibited the highest L^* (lightness) and b^* (yellowness) values, indicating a lower ornamental value. In conclusion, the application of auxins was found to be effective for the propagation of P. hederaceum var. oxycardium by stem cuttings, regardless of the type and concentration of auxins. An IAA concentration of 320 mg·L⁻¹ was most effective for promoting shoot growth, while an IBA concentration of 250 mg·L⁻¹ was most effective for promoting root growth.