Introduction

Senecio haworthii, also known as Caputia tomentosa, is a succulent plant belonging to the Asteraceae family. S. haworthii exhibits high drought tolerance (WOS, 2023) and possesses a remarkable white hairy covering over the entire plant, presumed to serve as a mechanism for reflecting intense direct sunlight in arid and high-temperature environments, resulting in its striking and fascinating appearance, which makes it highly valuable for horticultural interest. Plants within the Senecio genus are known to contain pyrrolizidine alkaloids, which exhibit toxic properties to animals and humans, categorizing them as poisonous plants (Cheeke, 1988; McLean, 1970). Various cases of Senecio poisoning have been reported for about a hundred years (Basson, 1987; Gava and Barros, 1997; Giles, 1983; Selzer and Parker, 1951; Stillman et al., 1977; Van Schalkwyk et al., 2021; Willmot and Robertson, 1920). On the other hand, from a pharmacological perspective, Senecio species have shown effects in anti- microbial and anti-bacterial (Pérez et al., 1999), anti-fungal (Loizzo et al., 2004), anti-inflammatory (Pérez González et al., 2013; Zahoor et al., 2023), anti-tumor (Dou et al., 2017), anti-diabetic (Ayoola et al., 2019; Mahmoud et al., 2011), and anti-oxidant (Faraone et al., 2018; Lounis et al., 2018), suggesting their potential for future medicinal purposes. Therefore, under appropriate management, Senecio species can be considered suitable for use as ornamental succulents or medicinal crops. Despite these advantages, little is known about the optimal growth conditions for Senecio species, necessitating research to identify the most suitable environment for cultivation.

Growth temperature is one of the critical factors influencing plant growth and quality (Morison and Lawlor, 1999; Oh et al., 2022). Each plant species has specific temperature preferences and study on the day/night temperature range is challenging due to its complex nature, requiring various experimental approaches. Succulent plants have developed unique physiological abilities to adapt to extreme habitats and dry environments (Nam et al., 2016). Succulent plants have a unique carbon dioxide assimilation pathway called Crassulacean acid metabolism (CAM) (Grace, 2019; Ting, 1989; Winter and Holtum, 2014), and they are known to be particularly sensitive to night temperature increases during cultivation (Sutton, 1975; Yamori et al., 2014). Particularly, high night temperatures have been reported to significantly inhibit CO₂ nocturnal uptake and accumulation of titratable acidity in succulent plants (Neales, 1973). Therefore, determining the suitable growth temperatures for each succulent species is necessary. Previous studies have investigated the effect of temperature on flowering in S. bellum (Heinze, 1971), and the impact of temperatures on the growth and anthocyanin content in Echeveria species (Cabahug et al., 2019). However, studies on the optimal day/night temperature setting for cultivating Senecio species are virtually nonexistent, necessitating research to address this knowledge gap.

For ornamental plants, leaf color is an important factor that significantly influences their ornamental value in addition to plant growth. Leaf color serves as an element enabling consumers to intuitively evaluate the external quality of plants, it is an essential consideration (Lee and Nam, 2023b). CIELAB is a color space created by the International Commission on Illumination (CIE) in 1976 (Lee et al., 2022c). Developed to overcome the limitations of Hunter Lab, which was introduced in 1958, CIELAB is characterized by the inclusion of asterisks in its notation, represented as L^*, a^*, and b^* as opposed to L, a, and b in Hunter Lab (Lee and Nam, 2022a). CIELAB has been widely applied in various horticultural studies, such as external quality evaluation of fruits and vegetables (Cho et al., 2021; Kim et al., 2022; Lee et al., 2022a; 2022e), ornamental flower crops (Jang et al., 2023; Park et al., 2023; Shin et al., 2022), and succulent plants (Lee, 2023; Lee and Nam, 2022b; Nam et al., 2022), and serves as an excellent parameter for describing the qualitative quality of plants. Therefore, CIELAB is expected to be useful for evaluating leaf color in succulent plants.

Accordingly, we selected S. haworthii, one of the most widely distributed Senecio species in the South Korean ornamental succulents market, as our experimental plant, and investigated and analyzed the effects of day/night temperatures on the growth and leaf color of S. haworthii.

Materials and Methods

Plant Material

Senecio haworthii cultivated within the experimental greenhouse of the Department of Environmental Horticulture at Sahmyook University, were used for this study. The plant sizes, measured as shoot height and shoot width, were about 5 and 3 cm, respectively.

Day/Night Temperature Treatments

This study was conducted for 10 weeks, inside growth chambers (KGC-175VH, KOENCON, South Korea) located in the experimental greenhouse of the Department of Environmental Horticulture at Sahmyook University in South Korea. Four different temperature treatments were designed, with day/night temperatures set at 20/15, 24/19, 28/23, and 32/27°C, respectively. The photoperiod was set at 14/10 hours (day/night). We utilized light-emitting diodes (LEDs) with a ratio of 4:2:8:2 in red:blue:white:far-red as the internal light sources for the growth chambers. The light intensity inside the growth chamber was adjusted to a photosynthetic photon flux density (PPFD) of 500 µmol m^-2 s^-1 using a portable spectroradiometer (SpectraPen mini, Photon Systems Instruments, Czech Republic). The plants were planted in a succulent media, consisting of decomposed granite, river sand, and fertilized horticultural substrate (Hanareumsangto, Shinsung Mineral, South Korea) mixed in a ratio of 6:3:1 (v/v/v). Rectangular pots measuring 48.5 × 33 × 8 cm (length × width × height) were used. The plants were watered once a week with 1.5 L of purified water per pot.

Parameters and Measurement Methods

The parameters assessed in this study included survival rate, shoot height, shoot width, stem diameter, root length, number of leaves, leaf length, leaf width, shoot and root fresh weight, shoot and root dry weight, shoot and root moisture content, CIELAB values (L^*, a^*, and b^*), and CIE76 color difference (ΔE^*_ab). Shoot height was measured as the distance from the soil surface to the highest point of the plant. Shoot width was measured as the widest part of the plant when observed from above. Root length was measured as the length of the longest root among the plant’s roots. Fresh weight was measured after the soil attached to the plant was washed off and then naturally air-dried in a sealed space for 24 hours. Dry weight was measured after heat drying the plants using a heat drying oven (HK-DO135F, HANKUK S&I, South Korea) at 85°C for 24 hours. Moisture content was calculated based on the comparison between fresh weight and dry weight, and it was determined using the following equation (Eq. 1).

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

For CIELAB measurements, we referred to the leaf color measurement method described by Lee et al. (2022b) and obtained CIELAB L^*, a^*, and b^* values with a spectrophotometer (CM-2600d, Konica Minolta, Japan) set to CIELAB D65/10° with the specular component included (SCI). To convert the CIELAB L^*, a^*, and b^* values to visually assessable colors, we used the Converting Colors tool operated by Zettl (2023) for color conversion. To compare the effect of temperature levels on the leaf color of S. haworthii in the day/night temperature treatments, we calculated color differences (ΔE^*_ab) using CIE76 color difference formula, setting each day/night temperature level as a reference. The formula (Eq. 2) used for ΔE^*_ab in this study was as follows (CIE, 2004).

(In this study, ΔE^*_ab ≤ 1.5 is regarded as ‘no color difference’ or ‘subtle color difference’, 1.6-3.0 as ‘very small color difference’, 3.1-6.0 as ‘small color difference’, 6.1-9.0 as ‘color difference’, 9.1-12.0 as ‘big color difference’, and ≥ 12.1 as ‘very big color difference’ or ‘completely different color’)

Data 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 five plants assigned to each treatment in three replications.

Results and Discussion

Plant Sizes Analysis

Senecio haworthii, subjected to different day/night temperatures, exhibited diverse growth responses (Fig. 1 and Table 1). The survival rate of S. haworthii was 100% within the day/night temperature range of 20/15-28/23°C; however, at 32/27°C, the survival rate declined to 86.6%. The results of the plant sizes analysis revealed that shoot height and shoot width showed no significant differences within the day/night temperature range of 20/15-28/23°C, based on Duncan’s multiple range test. Consequently, it is estimated that the shoot sizes of S. haworthii plants are minimally affected unless exposed to high temperatures, such as 32/27°C. The stem diameter was found to be the thickest at 0.56 cm under 20/15°C conditions. Meanwhile, the root length reached its maximum at 9.29 cm under 28/23°C conditions, while under the relatively higher temperature of 32/27°C, it sharply decreased to 6.35 cm. The orchid cultivar Phalaenopsis cv. KS Little Gem, known for its Crassulacean acid metabolism (CAM) plant, was reported to exhibit the lowest root length at the highest temperature treatment of 30°C within the temperature range of 20-30°C (Vo et al., 2021). The specific root length, a parameter used for fine roots, was reported to exhibit a negative response as the growth temperature increased (Ostonen et al., 2007). Consistent with this previous study report, it was observed that at excessively high temperatures beyond the plant’s optimal growth range, the root length could sharply decrease. The number of leaves reached its maximum at 26.2 under 20/15°C conditions, while conversely, at 32/27°C treatment, it exhibited the lowest number of leaves at 5.7, similar to other parameters indicating plant size. The Peperomia caperata, within the temperature range of 15-21°C, an increase in temperature was reported to lead to an increase in leaf length, leaf width, and leaf number (Brøndum and Friis, 1990). For succulent plants, it has been reported that with an increase in night-time temperature, the stomatal conductance pattern is altered, resulting in reduced photosynthetic efficiency (Neales, 1973). Succulent species with thicker leaf tissues, like S. haworthii, are considered to experience decreased leaf function under excessively high-temperature conditions compared to their optimal growth temperature (Medina et al., 2021; Schoffl et al., 1998; Schroda et al., 2015), emphasizing the need for caution when setting prolonged exposure to high-temperature conditions. The leaf length showed comparable significant levels within the temperature range of 20/15-28/23°C, consistent with the results of shoot height and shoot width. However, leaf width, unlike the leaf length, appeared broader at 0.77 and 0.75 cm under 20/15-24/19°C treatments, respectively. A previous study on Chrysanthemum cv. Orange Egg subjected to shade with temperature treatments showed a similar result to this study, as leaf length increased with a decrease in relative growth temperature (Park and Kim, 2021). According to Vo et al. (2021), the P. cv. KS Little Gem exhibited a decrease in both leaf length and width as the growth temperature increased within the temperature range of 20-30°C, which aligns with the findings of this study.

Fig. 1.

Shape of Senecio haworthii plants as affected by day/night temperatures for 10 weeks.

In conclusion, when considering the overall analysis of plant sizes, a growth temperature of 20/15°C was deemed most favorable for significant enlargement of the shoot part of the plants. However, for root length, the results were contrary to those of the plant shoot sizes, as a growth temperature of 28/23°C was considered most advantageous for significant root elongation.

Plant Biomass Analysis

In the results of biomass analysis, the shoot fresh weight was heaviest in the 20/15°C treatment, measuring 16.15 g (Fig. 2). Conversely, in the 32/27°C treatment, it was considerably lower at 0.97 g, representing a reduction of approximately 94.0% compared to the 20/15°C treatment. On the other hand, the root fresh weight did not show significant differences between the 20/15-28/23°C treatments, but it was the lowest in the 32/27°C treatment, measuring 0.63 g. This finding aligns with a previous study on the P. cv. KS Little Gem, where the lowest root biomass was observed in the treatment with the highest growth temperature of 30°C (Vo et al., 2021), which is consistent with the results of our study. Similarly to the shoot fresh weight, the shoot dry weight was highest in the 20/15°C treatment at 1.12 g. Conversely, in the 32/27°C treatment, it was the lowest at 0.31 g, indicating a significant reduction of approximately 72.4% compared to the 20/15°C treatment. This considerable decrease in shoot biomass allocation under high-temperature conditions indicates likely heat damage to the shoot part, suggesting that the growth conditions exceed the optimal growth temperature range. The root dry weight was relatively high in the 20/15 and 24/19°C treatments, measuring 0.35 and 0.34 g, respectively. Despite the relatively lesser decrease in root biomass compared to shoot biomass, the negative effects on root development under high-temperature conditions appeared to be similar. Both shoot and root moisture content did not show significant differences within the temperature range of 20/15-28/23°C. However, in the 32/27°C treatment, both shoot and root moisture contents were the lowest, measuring 68.1% and 54.9%, respectively. In a previous study by Way and Oren (2010), trees grown in high-temperature environments were reported to allocate less biomass to the roots.

Fig. 2.

Plant biomass and moisture content of S. haworthii as affected by day/night temperatures for 10 weeks: A) shoot fresh weight; B) shoot dry weight; C) shoot moisture content; D) root fresh weight; E) root dry weight; and F) root moisture content. Vertical bars indicate standard error and asterisks (*, **, and ***) indicate significance at p < 0.05, 0.01, or 0.001, respectively. Different lowercase letters indicate significant differences at p < 0.05 based on Duncan's multiple range test.

According to the results of the biomass analysis, S. haworthii exhibited higher biomass in both shoot and root under the 20/15°C treatment. Therefore, it appears that S. haworthii performs more carbon dioxide assimilation under lower temperature conditions. Based on the results of this study, we suggest cultivating S. haworthii at a relatively low temperature, approximately around 20/15°C, to encourage the growth of sturdy plants. However, it is important to note that further research may be necessary to explore the potential for even better growth at lower temperatures. Additionally, it is imperative to take into account that uniform irrigation conditions were consistently applied throughout this study, regardless of growth temperatures.

Leaf Color Analysis

In the leaf color analysis using CIELAB values, the lightness parameter of CIELAB L^* was observed to increase slightly with the rise in S. haworthii’s growth temperature, measuring 78.01 and 77.29 for the 28/23 and 32/27°C treatments, respectively (Table 2). Previous studies have reported an inverse relationship between L^* and plant growth parameters (Lee and Nam, 2023a; Lee et al., 2022d), and our study also showed similar results. The green-red opponent color parameter of CIELAB a^* was highest at -0.61 under the 32/27°C treatment, suggesting that under high-temperature conditions, the leaf color tends to shift towards the red. On the other hand, the blue-yellow opponent color parameter of CIELAB b^* was highest at 7.80 in the 20/15°C treatment, indicating a subtle yellowing tendency of the leaves under lower temperature conditions. Furthermore, the CIE76 color difference (ΔE^*_ab) analysis revealed various leaf color differences in S. haworthii according to growth temperature. Specifically, the ΔE^*_ab values between the 20/15 and 24/19°C treatments were ΔE^*_ab = 1.76, and between the 28/23 and 32/27°C treatments, it was ΔE^*_ab = 1.43, indicating ‘very small color difference’ and ‘subtle color difference’, respectively, within each set of treatments. However, the ΔE^*_ab values between the 20/15 and 24/19°C treatments and the 28/23 and 32/27°C treatments were in the range of ΔE^*_ab = 4.47-4.95, showing ‘small color difference’ Therefore, even with significant differences in day/night temperature levels, noticeable differences in leaf color were not observed.

Table 2.

Leaf color reading values of CIELAB, converted color, and CIE76 (ΔE^*_ab) values of S. haworthii as affected by day/night temperatures for 10 weeks

Temperatures (day/night)	CIELAB values (leaf color)			Converted colorz (color chip)	CIE76 (ΔE*ab) by day/night temperatures
	L*	a*	b*		20/15°C	24/19°C	28/23°C	32/27°C
20/15°C	73.48 by	-2.37 c	7.80 a		Reference	1.76	4.85	4.55
24/19°C	73.06 b	-2.06 bc	6.11 b		1.76	Reference	4.95	4.47
28/23°C	78.01 a	-1.85 b	6.14 b		4.85	4.95	Reference	1.43
32/27°C	77.29 a	-0.61 a	6.03 b		4.55	4.47	1.43	Reference
Significancex	*	***	**

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

^yMeans separations within columns were performed using Duncan’s multiple range test, with a significance level set at p < 0.05.

^x*, **, and ***: significant at p < 0.05, 0.01, or 0.001, respectively.

Based on the comprehensive evaluation of the leaf color analysis results, S. haworthii exhibited an increase in L^* and a^* at higher day/night temperature ranges, while b^* showed a decreasing trend. Additionally, leaf color differences were evaluated as ‘small color difference’ when comparing the 28/23 and 32/27°C treatments to the 20/15 and 24/19°C treatments based on ΔE^*_ab values.

Conclusion

In this study, we investigated the optimal day/night temperature range for enhancing the growth and ornamental quality of Senecio haworthii. Four day/night temperature treatments were designed as follows: 20/15, 24/19, 28/23, and 32/27°C, respectively. Based on the plant sizes and biomass analysis, it is determined that the best growth performance can be achieved under the relatively lower temperature treatment of 20/15°C. Conversely, the 32/27°C treatment resulted in the lowest growth rates, likely attributed to decreased leaf function caused by prolonged exposure to high-temperature conditions. Additionally, leaf color analysis using the CIELAB color space revealed that CIELAB L^*, representing lightness, and CIELAB a^*, indicating green-red opponent colors, proportionally increased with the rise in day/night temperature levels. Previous studies have reported an inverse relationship between L^* and plant growth parameters. Consistent with this finding, the treatment at 32/27°C, which exhibited the lowest growth, showed higher L^* values, aligning with the tendencies observed in previous research. On the other hand, the CIELAB b^* value, representing blue-yellow opponent colors, was highest under the 20/15°C treatment, suggesting leaf yellowing tendencies in relatively lower temperature. Based on the color difference analysis using CIE76 color difference (ΔE^*_ab), temperatures of 20/15 and 24/19°C were used as reference points, under these conditions, the temperatures of 28/23 and 32/27°C were evaluated as having ‘small color differences’. Therefore, it was concluded that even with significant differences in day/night temperatures, there were no noticeable differences in leaf color. Taking all these findings into consideration, it can be concluded that cultivating S. haworthii in relatively lower temperature conditions around 20/15°C would be advantageous for enhancing both plants growth and ornamental quality compared to higher-temperature conditions like 32/27°C.