Introduction

The combined effects of population growth and fossil fuel consumption have accelerated energy demand and CO₂ emissions, resulting in global warming and widespread environmental, economic, and social consequences. In the agricultural sector, the transition to sustainable and energy-saving strategies has become a necessity (Chahidi et al. 2021). Greenhouses enable the cultivation of crops under controlled environmental conditions. They are essential for stable crop production regardless of external weather conditions. However, greenhouse operation requires considerable energy input, particularly for heating (Maraveas et al. 2023). Improving the thermal efficiency of greenhouse remains a major challenge for energy-saving greenhouse systems (Nasrollahi et al. 2021). To reduce energy consumption in a greenhouse, previous studies have adjusted the growth temperature or heating duration (An et al. 2013; Elings et al. 2005; Pollet et al. 2011).

Temperature is one of the most influential environmental factors affecting physiology and plant growth. Crassulacean acid metabolism (CAM) represents a photosynthetic adaptation that enhances water-use efficiency (WUE) by allowing atmospheric CO₂ uptake primarily at night when evapotranspiration is low (Borland et al. 2011;Osmond 1978). CAM plants exhibit significant flexibility in CO₂ uptake, which exists depending on temperature (Cushman and Bohnert 1999). For instance, Opuntia streptacantha plantlets showed a 44% increase in nocturnal acid accumulation after 15 days of cold exposure (Ojeda-Pérez et al. 2017). Moderately low temperatures can sometimes provide horticultural benefits. In ornamental CAM species such as Kalanchoë, moderately low temperatures have been reported to promote flower induction and improve floral quality, suggesting that controlled temperature reduction can be an effective strategy for both energy-saving and flowering regulation (Coelho et al. 2018). However, exposure to low temperatures can also impose physiological stress on plants by inhibiting photosynthetic activity (Fu et al. 2016;Liu et al. 2012). Therefore, it is essential to identify a temperature range that achieves energy savings without compromising photosynthetic performance or growth. Evaluating photosynthetic parameters under low temperature conditions allows for the determination of the lowest possible temperature that sustains active carbon assimilation while minimizing energy input.

Schlumbergera truncata (Cactaceae) is a commercially important CAM ornamental with numerous cultivars exhibiting diverse flower colors (Nobel and Hartsock 1990). In Korea, various cultivars of S. truncata have been developed and are widely cultivated for domestic and export markets. For flower induction of S. truncata, the optimal temperature range is known to be 20 to 25°C during the day and 15 to 20°C at night under 12-hour photoperiod (GARES 2020). However, it is also commonly believed that flower bud initiation can occur at temperatures as low as 10 to 15°C, regardless of photoperiod. To date, the physiological response of S. truncata 'Pink Dew' to such low temperatures remains unknown.

To reduce energy consumption in greenhouse cultivation, this study aimed to characterize the photosynthetic responses of reproductive growth stage S. truncata 'Pink Dew' grown under low-temperature conditions. Since S. truncata 'Pink Dew' shows distinct CO₂ uptake responses among phylloclade levels under optimal temperature (Jung et al. 2023), we examined the effects of low temperature on CO₂ exchange across the phylloclade level.

Materials and Methods

Plant Material

Six-month-old cuttings of Schlumbergera truncata 'Pink Dew' were acquired from the Gyeonggi-do Agricultural Research and Extension Services in July 2022. In August 2022, the cuttings were transplanted into pots with a diameter of 8 cm. The pots were filled with a soil mixture consisting of perlite and peatmoss at a 1:1 volume ratio. Two seedlings were allocated to each pot. The plants were irrigated with tap water in the Information and Communication Technologies Greenhouse at Seoul Women's University in Seoul, Korea (latitude 37°N, longitude 127°E). Thermal curtains were utilized in the winter from 17:00 to 09:00 to preserve the nighttime temperature.

Experimental Environment Conditions

Temperature and relative humidity were monitored using sensors (SH-VT250, Soha Tech, Seoul, Korea) installed in the greenhouse. Light intensity was recorded with a photosynthetically active radiation (PAR) sensor (SQ-214, Apogee Instruments Inc., Logan, UT, USA). The average day and night temperature inside the greenhouse was maintained at 15/11°C in January and 21/12°C in March 2025. The average relative humidity was 56 and 58% in January and March, respectively. Daily light integral (DLI) is a measure of the total photosynthetic photon flux density (PPFD) delivered over the course of 1 day (Faust and Logan 2018). DLI was 2 and 7 mol·m^-2·day^-1 in January and March, respectively.

Gas Exchange Measurements

Gas exchange measurements were conducted in January 2025 with early flowering period (36-month-old plants) and in March 2025 with late flowering period (38-month-old plants). For each measurement, photosynthetic parameters were recorded from two phylloclade levels: the top and the second phylloclades. The top phylloclade refers to the one located immediately beneath the apex, and the second phylloclade represents the segment directly below the top phylloclade. All measured top phylloclades had visible flower buds or were in bloom at the time of measurement. Measurements were conducted on light green phylloclades that did not exhibit any visible symptoms of chilling injury.

Net photosynthetic rate, transpiration rate, and stomatal conductance were recorded using a portable infrared gas analyzer (LI-6400XT, LI-COR Inc., Lincoln, NE, USA). The measurements were taken over a 24-hour period. Water-use efficiency (WUE) was calculated as follows:

water-use efficiency (WUE) = net CO₂ uptake ÷ transpiration rate

The measurements were conducted with a 2 × 3 cm clear chamber (6400-08, LI-COR Inc., NE, USA), maintaining a flow rate of 500 μmol·s^-1 and a CO₂ concentration of 500 μmol·mol^-1. The temperature and light intensity were not artificially regulated, reflecting the ambient conditions of the greenhouse (Fig. 1).

Chlorophyll Fluorescence Measurements

The chlorophyll fluorescence parameter, the maximum photochemical efficiency of PSII (F_v/F_m), was measured with a portable pulse amplitude modulation fluorometer (JUNIOR PAM, Heinz Walz GmbH, Effeltrich, Germany). Before conducting the measurements, the phylloclades were subjected to a 15-minute dark adaptation period. Measurements were taken from both the top phylloclades and the second phylloclades. F_v/F_m values were recorded between 11:00 and 14:00 with five replicates.

Plant Growth and Biomass Parameters

The number of branches, the number of phylloclades, shoot fresh weight, root fresh weight, shoot dry weight, root dry weight, the length and width of the top phylloclade, the length and width of the second phylloclade, and the number of flower buds were measured on March 2025. The roots of all plants were blotted dry and the fresh weight of each plant was recorded. The number of phylloclades and flower buds of ≥ 5 mm was counted. The dry mass of these parts was determined after the fresh plants were dried in an oven at 70°C for three days. All measurements were conducted on five randomly selected replicates.

Results

During the early-flowering period, the top phylloclades of Schlumbergera truncata 'Pink Dew' showed CO₂ uptake rates ranging from 0.11 to −0.62 μmol·CO₂·m^-2·s^-1, whereas values in the second phylloclades varied between 0.17 and −0.24 μmol·CO₂·m^-2·s^-1 (Fig. 2A). In the late-flowering period, the range of CO₂ uptake expanded, with the top phylloclades recording values between 0.32 and −0.35 μmol·CO₂·m^-2·s^-1 and the second phylloclades showing higher rates overall (0.43 to −0.20 μmol·CO₂·m^-2·s^-1; Fig. 2B). These results indicate enhanced CO₂ assimilation in the second phylloclades, particularly during the late-flowering stage.

Stomatal conductance during the early-flowering period fluctuated between 0.01 and −0.02 mol·H₂O·m^-2·s^-1 in the top phylloclades and between 0.007 and −0.005 mol·H₂O·m^-2·s^-1 in the second phylloclades (Fig. 2C). A narrower range was observed in the late-flowering period, with values of 0.01–0.002 mol·H₂O·m^-2·s^-1 in the top phylloclades and 0.008–0.003 mol·H₂O·m^-2·s^-1 in the second phylloclades (Fig. 2D). Overall, stomatal conductance remained relatively stable regardless of phylloclade position or flowering stage.

Water use efficiency (WUE) exhibited broad variability during the early-flowering period, ranging from 14.68 to −31.52 μmol·CO₂·mmol^-1·H₂O^-1 in the top phylloclades and from 10.44 to −21.31 μmol·CO₂·mmol^-1·H₂O^-1 in the second phylloclades (Fig. 2E). In contrast, WUE values during the late-flowering period were less variable, with ranges of 12.49 to −9.95 μmol·CO₂·mmol^-1·H₂O^-1 in the top phylloclades and 9.90 to −7.22 μmol·CO₂·mmol^-1·H₂O^-1 in the second phylloclades (Fig. 2F). No consistent differences in WUE were detected between phylloclade positions or flowering periods.

In the early-flowering period, total net CO₂ exchange was negative in both phylloclade positions. The top phylloclades recorded −1.53 μmol·CO₂·m^-2·s^-1 during the day and −1.73 μmol·CO₂·m^-2·s^-1 at night, while the second phylloclades showed −0.60 and −1.72 μmol·CO₂·m^-2·s^-1, respectively (Fig. 3). By the late-flowering period, however, net CO₂ uptake was evident at both positions. Daytime and nighttime values for the top phylloclades were −0.46 and 1.21 μmol·CO₂·m^-2·s^-1, respectively, whereas the second phylloclades exhibited positive uptake during both periods (0.99 and 0.55 μmol·CO₂·m^-2·s^-1).

Consistent with these patterns, daily total net CO₂ uptake was negative during the early-flowering period, amounting to −3.20 mmol·CO₂·m^-2·day^-1 in the top phylloclades and −2.32 mmol·CO₂·m^-2·day^-1 in the second phylloclades (Fig. 4). In contrast, daily carbon balance shifted to positive values in the late-flowering period, reaching 0.75 mmol·CO₂·m^-2·day^-1 in the top phylloclades and 1.54 mmol·CO₂·m^-2·day^-1 in the second phylloclades. Thus, during late flowering, total CO₂ assimilation exceeded respiratory carbon losses at both phylloclade positions.

The maximum quantum yield of PSII (F_v/F_m) increased from the early- to late-flowering period (Fig. 5). Early-flowering plants exhibited F_v/F_m values of 0.51 in the top phylloclades and 0.44 in the second phylloclades. These values rose during the late-flowering period to 0.57 and 0.55, respectively.

Representative images of S. truncata plants are presented in Fig. 6. Phylloclades displayed a range of colors from light green to reddish green, while roots showed no visible signs of injury (Fig. 6A, B, C). The darkest green coloration was typically observed in phylloclades closest to the substrate, and occasional brown lesions were noted on some segments (Fig. 6D).

Morphological traits of S. truncata 'Pink Dew' are summarized in Table 1. On average, plants produced 37.6 ± 4.31 branches and 71.0 ± 6.18 phylloclades (n = 5). Shoot fresh and dry weights were 50.7 ± 5.35 g and 4.2 ± 0.46 g, respectively, while root fresh and dry weights reached 8.9 ± 0.19 g and 0.8 ± 0.08 g. The top phylloclades measured 54.8 ± 6.30 mm in length and 39.5 ± 1.00 mm in width, whereas the second phylloclades were slightly smaller (50.1 ± 2.29 mm and 34.7 ± 2.94 mm). All plants used for gas exchange analysis bore flowers or floral buds, with an average of 3.0 ± 0.63 buds per plant.

Discussion

During the experiment, the greenhouse was maintained at average day/night temperatures of 15/11°C during the early-flowering period in January and 21/12°C during the late-flowering period in March (Fig. 1). These temperature regimes were below the reported optimal range for floral induction in Schlumbergera truncata, which is 20–25°C during the day and 15–20°C at night, with 15°C representing the minimum temperature for normal growth (GARES 2020). Exposure to low temperatures is known to disrupt essential photosynthetic processes, including stomatal regulation, carbon reduction, and carbohydrate allocation (Allen and Ort 2001;McConnell and Sheehan 1978). In CAM plants, chilling conditions can impair carbon fixation, thereby reducing overall photosynthetic capacity (Daems et al. 2022).

Evidence from other CAM species supports this response. In the CAM orchid Phalaenopsis 'Edessa', stomatal conductance and CO₂ uptake declined to nearly zero after three weeks of exposure to 10°C (Daems et al. 2022). Similarly, in the present study, S. truncata 'Pink Dew' exhibited negative total net CO₂ uptake during the early-flowering period, reflecting suppressed carbon assimilation under low-temperature conditions. As daytime temperatures increased during the late-flowering period, total net CO₂ uptake shifted to positive values, indicating a recovery of carbon gain (Figs. 3 and 4).

The maximum quantum yield of PSII (F_v/F_m) is widely used as a diagnostic indicator of photosynthetic performance under temperature stress (Hussain et al. 2023;Mehmood et al. 2021). This parameter reflects the efficiency of PSII photochemistry, with reductions typically indicating photoinhibition or damage to the photosynthetic apparatus (Guidi et al. 2019). Under optimal, non-stress conditions, F_v/F_m values generally range from 0.75 to 0.81 across C₃, C₄, and CAM species (Björkman and Demmig 1987). Pronounced declines in F_v/F_m have been reported in various plant species subjected to chilling stress. In Phalaenopsis 'Edessa', prolonged exposure to 10°C caused F_v/F_m to decrease below 0.50 after 8 days and to less than 0.25 after 22 days (Daems et al. 2022). Comparable trends have been observed in C₃ plants; for example, Arabidopsis thaliana (WT) exhibited a gradual decline in F_v/F_m during exposure to 4°C, reaching values below 0.25 after 60 h (Takeuchi et al. 2025). In the same study, Cucumis sativus cv. High Green 21 showed an even more rapid response, with F_v/F_m dropping to approximately 0.20 after only 24 h of chilling.

In contrast, S. truncata 'Pink Dew' maintained relatively higher F_v/F_m values despite prolonged exposure to low temperatures. Plants had been subjected to temperatures below 10°C for more than two months prior to measurement, beginning on 1 November 2024. During the early-flowering period, F_v/F_m values ranged from 0.51 in the top phylloclades to 0.44 in the second phylloclades (Fig. 5). These values increased during the late-flowering period to 0.57 and 0.55, respectively. The comparatively moderate decline in F_v/F_m suggests either a reduced sensitivity of PSII to chilling stress or a degree of acclimation and recovery during prolonged low-temperature exposure. Further investigation is required to clarify the underlying mechanisms.

Throughout the experimental period, phylloclades of S. truncata 'Pink Dew' exhibited a range of colorations, including light green, yellowish green, and reddish green (Fig. 6). In addition, brown lesions were observed on the surface of some phylloclades. Similar visual symptoms have been reported in the cladodes of Nopalea cochenillifera cv. Maya exposed to low temperatures, where yellowing, orange discoloration, and surface lesions were associated with chilling injury (Kondo et al. 2023). These observations suggest that the discoloration and lesions observed in S. truncata 'Pink Dew' may also represent visible manifestations of cold stress.

Temperature conditions during the experiment remained below the reported optimal range for S. truncata, with average day/night temperatures of 15/11°C in January and 21/12°C in March. Flower bud induction in S. truncata typically occurs under short-day photoperiods of less than 12 h; however, temperatures below 15°C can inhibit bud formation even under inductive photoperiods. Conversely, floral initiation can occur at temperatures between 10 and 15°C regardless of day length (GARES 2020). The observed number of flower buds per plant (Table 1) indicates that flowering in this study was likely influenced not only by short-day conditions but also by the prevailing low-temperature environment.

Overall, relatively few studies have examined the temperature-dependent limitations of photosynthetic processes in CAM plants across a broad thermal range. Investigating CAM photosynthesis under low-temperature conditions is therefore valuable, as it enhances our understanding of the temperature sensitivity of CAM carbon fixation and identifies potential constraints on photosynthetic performance under suboptimal environmental conditions.