Introduction
Materials and Methods
Cultivation environment and materials
Seedling growth and quality measurement
Chlorophyll fluorescence measurements
Statistical analysis
Results and Discussion
Cultivation environments under the different seedling production systems
Quality of seedlings grown in different seedling production systems before transplanting
Early growth after the transplanting of seedlings grown using different seedling production systems
Introduction
Plug seedling production aims to produce uniform, standardized, high-quality seedlings for planned production throughout the year (Lee et al. 2020). Seedling quality impacts not only the initial growth and yield outcomes after transplanting but also the quality of the fruit in later stages, highlighting the importance of high-quality seedlings. Superior seedlings also reduce the costs and time associated with various production factors, enhancing their significance even further (Kim and Park 2002). As of 2021, South Korea is home to approximately 1,431 nurseries covering a total area of about 482 hectares. Thus, the average area per nursery is approximately 3,369 m2 (KOSIS 2021a). The average sales revenue of nurseries selling vegetable seedlings is approximately 500 million won. Among these, the average sales revenue for tomato (Solanum lycopersicum L.) seedlings is around 1 million won, making it the second-highest income-generator among the 17 major vegetable crops (KOSIS 2021b). Tomato is also recognized as a high-value vegetable crop, and production and cultivation areas for the growing of tomatoes are steadily increasing worldwide (Kim et al. 2023).
Tomato seedling cultivation primarily occurs in specialized nurseries. While some large nurseries are adapting to changing environmental conditions by investing in their facilities, the majority of small and medium-sized nurseries are facing challenges as they attempt to produce high-quality seedlings due to an unstable external environment (An et al. 2021b). A plant factory with artificial lighting (PFAL) consists of well-insulated walls and utilizes artificial light sources to supplement natural lighting. Thus, PFALs are unaffected by external environmental conditions and are gaining attention as a novel alternative for seedling cultivation under suboptimal environments (Kozai and Niu 2020). The introduction of PFALs in major vegetable seedling production facilities in South Korea is currently under consideration (Lee et al. 2023; Moon et al. 2023a, 2023b).
Recently, studies of PFALs have attempted to identify the appropriate light intensity and temperature levels for cucumber seedling cultivation (Yang et al. 2023) and to analyze the effects of different light intensity levels on seedling quality, the evaporation rates of the main stem, and growth (Park et al. 2020). The effects of different CO2 concentrations and light intensity levels on growth outcomes have also been studied (Yun et al. 2023) in relation to cucumber and tomato seedlings. Additionally, researchers have assessed not only the quality of seedlings, including major varieties of vegetables and fruits, as well as grafted seedlings (Kwack and An 2021; Song et al. 2024), but have also assessed early growth following transplanting in order to evaluate the field applicability of PFAL seedlings (An et al. 2020). However, despite the satisfactory quality of PFAL seedlings and the stability of their early growth after transplanting, few studies have compared conventional plastic greenhouse-grown and PFAL-grown seedlings, causing farmers to be hesitant to adopt PFAL seedlings.
To ensure a stable supply of PFAL seedlings to farms, it is essential to verify any difference between conventional greenhouse and PFAL seedlings in terms of seedling quality and growth during the early stage after transplanting. Therefore, the purpose of this study is to assess the feasibility of using PFALs for seedling production by comparing the seedling quality and early-stage growth indicators of greenhouse seedlings grown utilizing triazole growth regulators with those of seedlings grown in a controlled PFAL environment.
Materials and Methods
Cultivation environment and materials
This study was conducted from April 11, 2023, to July 6, 2023, at the Hoban Agriculture Corporation located in Chuncheon, Gangwon Province (37°55’29” N, 127°47’04” E, elevation 85m). The study site features a PFAL, a seedling greenhouse, and the unheated linked vinyl greenhouse of Kangwon National University (L 28 × W 13 × side H 2.5 m; 37°52’18.6” N, 127°44’45.9” E, elevation: 123 m).
Solanum lycopersicum L. ‘TY Yeolgang’ (Hungnong Co., Ltd., Dangjin, Korea) seeds were sown in a 162-cell plug tray (L 540 × W 280 × H 45 mm, Bumnong Co., Ltd., Jeoneup, Korea). The tray was filled with a horticultural substrate material (Pindstrup Seedling Gold, Pindstrup, Denmark) with electrical conductivity (EC) of 0.47 dS·m-1 and a pH of 6.18. On April 11, 2023, following sowing, the trays were sufficiently watered using a top irrigation method and subsequently covered with vermiculite. The seeds were germinated in a dark germination chamber maintained at a temperature of 25–28°C and relative humidity exceeding 90% for 48 hours. On May 5, 2023, the seedlings were moved to either the PFAL or a nursery setting to compare their growth characteristics under these two environmental conditions. For seedlings that were cultivated in a nursery throughout the growing period, a triazole growth regulator (Diniconazole 5%, Dongbang Agro Co., Ltd., Seoul, Korea) was prepared at a concentration of 0.075 g·L-1 and applied as a foliar spray once two to three true leaves had expanded. For the seedlings cultivated in the PFAL, growth was regulated solely through environmental controls, without the application of the triazole growth regulator. In the PFAL, the temperature during the seedling period was set to 26/18°C (day/night), the photoperiod to 16/8h (day/night), the light intensity to 250 µmol·m-2·s-1, and the relative humidity to 60/70% (day/night) (Fig. 1). During the seedling period for both the PFAL and nursery seedlings, irrigation management was implemented using a nutrient solution (Technigro 13-2-13 Plus Fertilizer, Sun-Gro Horticulture, Bellevue, USA) adjusted to a pH of 5.5 and an EC of 1.4 dS·m-1, followed by subirrigation. On the 17th day after sowing, seedlings were transplanted into 50-cell plug trays (W 280 × L 540 × H 45 mm, Bumnong Co., Ltd., Korea) utilizing cylindrical biodegradable paper pots (Hydroponics, Ellegaard, Denmark).
Transplanting occurred on the 31st day after sowing. Seedlings were planted in two rows, each with five blocks containing one plant per treatment group (n = 10), alternating between left and right. This resulted in a total of 30 tomato plants being planted in rock wool substrate material (90 × 15 × 7.5 cm, SV75159, UR Media Co., Ltd., Seoul, Korea) within the linked Kangwon National University greenhouse. The environment of the cultivation greenhouse maintained an average daily temperature ranging from 20 to 30°C, average daily relative humidity exceeding 40%, a photoperiod of 14 hours, and a cumulative light amount of 300 to 1000 J·cm-2 from the transplanting date until the end of the growing period. The control group of seedlings, referred to as the “Greenhouse” (GH) seedlings, were grown in the nursery until transplanting, while the treatment group was divided into “Plant Factory to Greenhouse” (PG) seedlings, which were cultivated in a PFAL for 17 days before temporary planting in a nursery for an additional 14 days before transplanting, and “Plant Factory” (PF) seedlings, which were grown in a PFAL for 31 days before transplanting (Fig. 3). After transplanting, irrigation management was conducted using Dutch PBG tomato nutrient solution maintained at an EC of 2.5 dS·m-1 and a pH of 5.5 and applied at a rate of 120–150 mL·h-1. Irrigation was conducted an average of eight to ten times per day, beginning one to two hours after sunrise and continuing until three to four hours before sunset. The method of crop guidance was bottom-up, involving the training of a single stem and the removal of all lateral branches that emerged from the main stem. Cultivation was completed on the 56th day after transplanting.
Seedling growth and quality measurement
Before transplanting, a survey was conducted to compare seedling growth outcomes in the PFAL and the nursery. Survey measurements included the plant height, stem diameter, number of leaves, leaf area, chlorophyll content (SPAD), fresh weight, dry weight, compactness, the Dickson quality index (DQI), and the relative growth rate (RGR). A root zone analysis was also conducted. In total, ten samples per treatment were destructively analyzed for this survey. Leaf area was measured using a leaf area meter (LI-3100, LI-COR Inc., Lincoln, NE, USA). The root zone was analyzed for the surface area, average root diameter, number of root tips, and the root zone volume using the root zone analysis program WinRHIZO PRO 09 (Regent Instruments Inc., Quebec, QC, Canada). The chlorophyll content in each case was measured using a chlorophyll content meter (SPAD-502 Plus, Minolta Inc., Japan) on the third leaf from the growth point. Fresh weights were measured using an electronic scale (HS220S, Hansung Instruments Co., Ltd., Korea), and dry weights were measured after drying at 80°C for 72 hours in a convection oven (Sanyo Inc., Osaka, Japan). Compactness (Eq. 1), RGR (Eq. 2), and DQI (Eq. 3) were calculated using the equations of Jeong et al. (2020b), Yang et al. (2023), and Dickson et al. (1960), respectively:
H0 and H1: initial and final shoot fresh weight
t1 − t0: growing period (days)
After transplanting, non-destructive growth surveys were conducted every seven days, measuring the plant height, stem diameter, leaf length, leaf width, SPAD (leaf chlorophyll), and chlorophyll fluorescence and counting the number of nodes, flowering nodes, and leaves. The stem diameter was measured at the third node below the top of the stem, while nodes with fully open flowers were considered flowering nodes. The leaf length, leaf width, SPAD, and chlorophyll fluorescence were assessed at the third leaf below the top of the stem.
Chlorophyll fluorescence measurements
Using a portable chlorophyll fluorescence meter (FluorPen FP-110, Photon Systems Instrument, Czech Republic), a total of six measurements per treatment were taken from the middle three blocks, excluding the two end blocks. Before each measurement, dark adaptation was induced for 15 minutes using dark-adaptation leaf clips. The analysis was conducted using the FluorPen (Version 1.1.2.3, Photon Systems Instrument, Czech Republic) software. The resulting chlorophyll fluorescence parameters are summarized in Table 1 (Strasser et al. 2000).
Table 1.
Definitions and explanations of the chlorophyll fluorescence parameters obtained from the photosystem (PS) II chlorophyll fluorescence OJIP transition measurements used in this study
Statistical analysis
The experiment utilized ten plants for each treatment, with five plants repeated twice in a randomized block design. One-way ANOVAs were performed using SPSS version 26 (IBM, Armonk, NY). To assess the growth characteristics following transplantation, which were attributed to differences in seedling quality levels between the nursery- and PFAL-grown seedlings, we compared the control group (GH) means with those of the two treatment groups (PG and PF) to identify statistically significant differences (p < 0.05) using Duncan’s multiple range test. Furthermore, significant differences between the treatments were evaluated using two-way ANOVAs.
Results and Discussion
Cultivation environments under the different seedling production systems
The average daytime/nighttime temperatures in the PFAL (treatment PF) and the nursery greenhouse (treatment GH) were 25.9/18.3°C and 27.6/20.3°C, respectively, and the corresponding maximum/minimum daily average temperatures were 26.0/18.0°C and 32.2/19.6°C (Fig. 1A). The differences between the daytime and nighttime temperatures were generally greater in the greenhouse than in the PFAL. However, temperature fluctuations occurred more rapidly in the PFAL than in the greenhouse. This aligns with the findings of Song et al. (2024), which confirmed relatively high weekly temperatures and differences in day and night temperatures in plastic greenhouses.
The average daily relative humidity levels (Fig. 1B) were measured and found to be 66.6% in the PFAL and 58.9% in the greenhouse. The maximum/minimum relative humidity levels in the PFAL and greenhouse were 71.9/63.8% and 74.2/40.3%, respectively. Previous research has shown that the daily average variation in relative humidity is smaller in PFALs than in conventional nursery greenhouses (Hyeon et al. 2024). In the present study, the standard deviations of the daily average temperatures and relative humidity levels for each cultivation environment were 5.7°C and 16.5% in the greenhouse and 0.1°C and 1.9% in the PFAL. The PFAL exhibited only slight variations because it is insulated from external factors, whereas the large standard deviations in the nursery greenhouse environment indicate a greater influence of external conditions.
Quality of seedlings grown in different seedling production systems before transplanting
Plant height (Table 2) was lowest for GH seedlings, at 7.9 cm, while the PG and PF seedlings were 43% and 159% taller, respectively, than the GH seedlings. These results differ from those of a study conducted by Son (1995), which indicated that greater differences between day and night temperatures increase greenhouse-grown seedling growth, resulting in increased plant heights. However, research has indicated that the application of diniconazole during the seedling stage of peppers (Han et al. 2002) and tomatoes (Kim et al. 2016) suppresses growth parameters, including the plant height and leaf area. It is believed that this phenomenon stems from a growth inhibition effect caused by the diniconazole under greenhouse conditions. Stem diameters were thickest, at 4.0 mm, in the PF seedlings and thinnest, at 2.8 mm, in the GH seedlings. The number of leaves and leaf area tended to increase with the duration of seedling cultivation in the plant factory, aligning with the findings of Hyeon et al. (2024), which indicate that as seedling cultivation progresses in PFALs, both the number of leaves and the leaf area increase. SPAD values were highest, at 47.8, in the GH seedlings, with values of 39.4 and 41.8 measured in the PG and PF seedlings, respectively. This phenomenon is thought to arise from variations in the light quality, despite the fact that the total light exposure in the PFALs exceeded that in the greenhouses. Compactness is an indicator used to assess seedling quality, with higher values signifying superior quality (Seo et al. 2018; Jeong et al., 2020a; Yoon et al. 2021). Compactness was highest, at 39.8 mg·cm-1, in the PG seedlings, while it was lowest, at 28.5 mg·cm-1, in the GH seedlings. Thus, compactness is highest when seedlings were grown in the PFAL during their early growth stage and then transplanted to a nursery greenhouse environment, which does not align with the results of Hyeon et al. (2024) because in the stable environment of the PFAL (Fig. 1), while the leaf area increased, the variability of the greenhouse environment during the stage following temporary planting caused stress, which suppressed the growth height, resulting in improved seedling quality levels.
Table 2.
Plant height, stem diameter, no. of leaves, leaf chlorophyll (SPAD), leaf area, and compactness measurements for the tomato seedlings taken immediately before transplanting
| Treatmentz |
Plant height (cm) |
Stem diameter (mm) |
No. of leaves (ea) |
Leaf chlorophyll (SPAD) |
Leaf area (cm2) |
Compactness (mg·cm1) |
| GH | 7.9 ± 0.8 cy | 2.8 ± 0.4 c | 4.9 ± 0.3 b | 47.8 ± 3.2 a | 43.5 ± 10.0 c | 28.5 ± 6.6 b |
| PG | 11.3 ± 1.3 b | 4.0 ± 0.5 b | 5.1 ± 0.3 b | 39.4 ± 3.9 b | 107.0 ± 11.6 b | 38.9 ± 7.6 a |
| PF | 20.5 ± 1.6 a | 4.7 ± 0.2 a | 6.2 ± 0.4 a | 41.8 ± 3.6 b | 187.6 ± 17.0 a | 33.8 ± 3.8 ab |
| Significancex | *** | *** | *** | *** | *** | ** |
zTreatments include three growth environments: Greenhouse (GH), Plant Factory to Greenhouse (PG), and Plant Factory (PF).
Fig. 2 presents the DQIs and RGRs for each treatment. The DQI was developed as a seedling quality indicator and has been utilized in various applications recently (Mota et al. 2016). The PG seedlings exhibited the highest value, at 0.090, but there was no significant difference when compared to the PF seedlings. Conversely, the GH seedlings displayed the lowest DQI value among the treatments, at 0.016. In the RGRs (Fig. 2A), the PF seedlings exhibited the highest value, at 0.39 cm·cm-1·day-1, whereas the GH seedlings exhibited the lowest, at 0.012 cm·cm-1·day-1. Similarly, in work by Hyeon et al. (2024), the RGRs were highest when a PFAL was used. This suggests that the ultraviolet light from artificial light sources in PFALs is lower than that from natural light in seedling greenhouses, as experimental results have shown that overgrowth occurs in the event of insufficient light or ultraviolet rays (Ryu and Kim 2010). In contrast to a study assessing seedling RGRs and DQIs using definitional correlation (Seo et al. 2018), the present study found that the RGR in the PG caser was lower than that in the PF case, whereas the opposite was true for the DQI.
The results of a comparative investigation of the detailed growth characteristics of the roots (Table 3) indicated that the root surface area and average root length were greatest in the PG seedlings, measuring 21.7 cm2 and 1.6 mm, respectively. Regarding the number of root tips, PF exhibited the highest count, at 1445.3, but there was no statistically significant difference when compared to the PG seedlings. The total root length, including fine roots, was highest in the PG seedlings, at 226.4 cm.
Table 3.
Below-ground growth of tomato seedlings before transplanting
| Treatmentz |
Root surface (cm2) |
Average root diameter (mm) |
No. of root tips (ea) |
Root volume (cm3) | Root length (cm) | ||||
| < 0.5 mm | 0.5–1.0 mm | 1.0–1.5 mm | > 1.5 mm | Total | |||||
| GH | 8.6 ± 3.5 by | 0.4 ± 0.2 b | 687.7 ± 334.2 b | 0.6 ± 0.1 b | 99.1 ± 36.4 b | 19.9 ± 10.5 b | 3.8 ± 1.7 b | 7.4 ± 3.7 a | 130.3 ± 47.9 b |
| PG | 21.7 ± 3.3 a | 1.6 ± 0.4 a | 1255.0 ± 228.7 a | 0.9 ± 0.2 a | 166.6 ± 27.1 a | 43.5 ± 11.8 a | 7.2 ± 2.2 a | 9.0 ± 3.1 a | 226.4 ± 37.4 a |
| PF | 18.9 ± 4.4 a | 1.3 ± 0.4 a | 1445.3 ± 132.2 a | 0.9 ± 0.1 a | 169.8 ± 44.9 a | 35.3 ± 13.1 a | 7.3 ± 2.4 a | 8.3 ± 3.7 a | 220.7 ± 61.9 a |
| Significancex | *** | *** | *** | *** | ** | * | * | * | ** |
zTreatments include three growth environments: Greenhouse (GH), Plant Factory to Greenhouse (PG), and Plant Factory (PF).
The chlorophyll fluorescence index (Fig. 3) a non-invasive and reliable technique used to detect photosystem II damage in plants exposed to abiotic stresses. (Zhou et al. 2018). The Fv/Fm parameter shows the value of the maximum quantum yield and is frequently utilized in studies (An et al. 2021a; Shin et al. 2021; Lee and Oh 2023). When under stress, the Pi_Abs and ETo/RC parameters decrease, whereas ABS/RC, TRo/RC, and DIo/RC increase. If the value of Fv/Fm falls between 0.78 and 0.84, this indicates normal growth conditions (Kim et al. 2023). In the GH, PG, and PF seedlings, the values were 0.83, 0.84, and 0.81, respectively, indicating no growth abnormalities. The Pi_Abs parameter is more stress sensitive than other parameters, resulting in large differences in values across treatments (Lee et al. 2022). The PF seedlings exhibited the lowest Pi_Abs value, 2.95, among the treatments, whereas the PG seedlings demonstrated the highest, at 6.23. Both ABS/RC and DIo/RC exhibited only statistically insignificant differences among the treatments. Regarding the TRo/RC values, the PG seedlings exhibited the highest value at 1.52. Similarly, the ETo/RC values were highest in the PG treatment, whereas those of the PF and GH treatments were not statistically significant. Unlike the other treatments, the PF seedlings exhibited stress in all indicators except for Fv/Fm and DIo/RC. This phenomenon is attributed to the abrupt fluctuations between day and night temperatures within the PFAL (Fig. 1A), which are known to impact the stress levels of seedlings (Kwack and An 2021).
Early growth after the transplanting of seedlings grown using different seedling production systems
The growth parameters of the seedlings grown using the three seedling production methods are shown in Fig. 4. Immediately after transplanting, plant heights were greatest in the PF seedlings, at 28.5 cm, and shortest in the GH seedlings, at 16.1 cm. However, starting from the third week, the heights of the PG and PF seedlings were no longer statistically significant, and by the sixth week, there were no statistically significant differences among any of the treatment groups. In the first week, the lowest number of nodes was recorded in the GH seedlings, which had an average of 6.2 nodes. However, by the sixth week, the numbers of nodes for the GH, PG, and PF treatment group seedlings, 19.1, 18.9, and 19.4, respectively, were not significantly different. At the first appearance of flowering nodes, the average number was observed to be lowest, at 8.5 nodes, for GH seedlings and the highest, at 10.3 nodes, for the PG seedlings, and this difference became more pronounced as growth progressed. Interestingly, this trend contrasts with those of the plant height, stem diameter, and the number of nodes. Hyeon et al. (2024) stated in their study comparing PFAL and conventional methods for producing cucumber seedlings that growth was weakest in conventional greenhouse seedlings immediately after transplanting, but over time, growth in all treatment groups became comparable. This experiment found similar results; for plant heights and the numbers of nodes and leaves, the differences between PG and GH seedlings during week 1 were 9.5 cm (59%), 0.8 (13%), and 1 (14%), respectively, but the differences decreased by week 6 to 2.3 cm (2%), 0.2 (1%), and 0.1 (1%), respectively. This occurred because all treatments were exposed to the same greenhouse environment after transplanting and adapted to it, allowing the treatment differences to become smaller over time. In conclusion, this study determined that seedlings cultivated in a PFAL and then temporarily cultivated in a greenhouse before transplanting, without the use of diniconazole at any stage, exhibit excellent seedling quality, root establishment, and growth after planting, making them suitable for commercial seedling production.
