Introduction
Hemerocallis thunbergii Baker (HTB) is a perennial herbaceous plant that takes 1.5–2.5 years to mature, depending on available resources (Chung et al., 2007). Its flowers bloom pale yellow and have a scent (Hwang and Kim, 2012). Recently, as interest in native plants has risen, studies have been conducted to increase the usability of HTB, a native plant in Korea, including on productivity improvement (Nam et al., 2009). In addition, for the purpose of generating income, farmers cultivate and harvest HTB whose tuberous root, flowers, and young leaves can be used as food/medicine (Korea Forestry Promotion Institute, 2014).
As native plants have been adapted to the Korean climate over a long period, they have excellent environmental adaptability and high resistance to pests and diseases in terms of management (Ryu, 2003); ornamental and floral materials for landscape planting are now required to be developed using native plants, replacing foreign-introduced species (Lee et al., 2020). As ornamental plants are selected and cultivated for a variety of purposes beyond simple ornamental use, including decontamination, environmental improvement, food and medicine, and extraction of functional substances, a need for mass production technology to meet the increasing demand is anticipated (Park et al., 2021). In several developed countries, seedling raising in multi-celled plastic plug-trays has already developed as an industry, and the growth rate of seedlings is affected by the container cell size and volume when planting plants in a typical container (Singh et al., 2005).
For plug seedlings growing under limited environmental conditions, the setting of the nursery period and the growth environment are important (Jeong et al., 2016), so studies targeting various plant species for efficient production in plant factories and greenhouses with limited temperature, humidity, and light quantity are in progress. Lee et al. (2021) evaluated the effect of cell diameter and height of plug trays on the growth and morphology of ginseng seedlings. They reported that the growth of the underground and above-ground parts (shoots and roots) was most efficient at a cell diameter of 30 mm, and the further growth rate not significantly different between the diameter and the maximum diameter. In addition, an analysis of the growth change of spinach using various plug cell volumes found that the larger the volume of plug cells in the pre-transplant stage, the greater the growth of the plant (Di Matteo et al., 2015). Other studies mainly targeted agricultural crops for the purpose of increasing mass productivity (Kim et al., 2001; Singh et al., 2005; Lee and Lee, 2019). Furthermore, for flowering herbaceous plants, raising seedlings for the production of standardized seedlings using plug trays has been attempted. In a study analyzing the effect of plug-tray cell size and media on gerbera seedling growth, Cho et al. (2003) reported that the larger the plug-tray cell size, the better the seedling growth. As for Impatiens walleriana, an herbaceous flower crop, the larger the cell volume of plug trays, the greater the dry weight and main root length and thickness at the sale stage (Di Benedetto and Klasman, 2004). Veronica pyrethrina Nakai (VPN), a native plant species in the Korean peninsula, showed no difference in the growth of shoots while raising seedlings in plug trays for 6 weeks; but sufficient root pruning for moving was achieved in the smallest plug cell of 10 mL, deriving an effective cell volume with a large number of roots despite the small cell size (Kwon et al., 2021). On the other hand, as Veronica pusanensis Y.N. Lee, which corresponds to the same genus as VPN, did not show a significant difference in the growth of seedlings according to plug cell size, it was found to be economically superior to produce its seedlings in a 200-cell plug tray with the smallest cell size of the experimental groups (Oh et al., 2022). As such, many studies have aimed to develop seedling technology using plug trays. However, as even plants belonging to the same genus may see different effects on their growth depending on medium, irrigation, and environmental conditions (Jang et al., 2014; Jeong et al., 2020; Ma et al., 2020), it is important to determine the appropriate plug-tray cell size and nursery period for each plant species for efficient production under various conditions.
Compared to direct sowing, plug seedlings not only require less seedling area and seeds, but also enable consistent systemization of raising seedlings, and facilitate planting (Yeoung et al., 2004). However, for seedling containers with a large cell size, sowing time and cost are increased, a longer period for root pruning for transplantation is required, and more labor is involved in the process (Kim et al., 2019). In addition, since the fewer cells and larger cell size in plug trays reduce planting density and productivity per unit area, it is important to select an appropriate number and size of cells (Jang et al., 2014). Therefore, targeting Hemerocallis thunbergii Baker (HTB), which has been spotlighted not only as an ornamental plant, but also as a landscaping and medicinal plant (Ahn et al., 2003; Lee et al., 2012), this study aimed to determine the appropriate size and number of plug-tray cells and the optimal nursery period by examining the growth characteristics of its seedlings according to the plug tray cell size, and the number of days after sowing.
Research Methods
Materials
An experiment was conducted using 32-cell (143 mL/cell, 4 × 8 cells), 50-cell (70 mL/cell, 5 × 10 cells), 105-cell (18 mL/cell, 7 × 15 cells), and 200-cell (13 mL/cell, 10 × 20 cells) plug trays (54 cm × 28 cm) in a greenhouse of Konkuk University Glocal Campus in Chungju-si, Chungcheongbuk-do, and one HTB seed was sown for each cell. Experimental groups were arbitrarily arranged in 3 repetitions per treatment on the bed in the glass greenhouse, and 10 specimens per tray were selected and measured. For soil materials, a light soil medium for seedlings (Baroker, Seoulbio Co., Ltd., Eumseong, Korea) was used, which accelerates the early growth rate, shortens the nursery period, and has a high porosity that is good for rooting. The prepared experimental groups were subjected to overhead irrigation three times a week to prevent the soil from drying out.
Methods
By referring to existing studies to determine the seedling growth characteristics of HTB, an herbaceous plant, according to the cell size of plug trays (Kim et al., 2001; Yeoung et al., 2004; Nam et al., 2022), the experimental period was set up to September starting in July, one month after sowing, a sufficient period of time. As for the growth of seedlings, leaf number (LN), leaf width (LW), and leaf length (LL), and root length (RL) were examined at intervals of one month after sowing. On average, the environmental conditions in the greenhouse were temperature of 29.7°C, 56.5% humidity, and illumination of 783.2 lux. The LN was counted visually, and the LL and LW were measured using a 30 cm stainless steel ruler (SB, Korea), and the RL using a digital tape measure (SD-TM40, Sincon, China). In addition, using a microelectronic balance (SF-400C, Electronic Compact Scale, China), the fresh weight of shoots and roots was measured, and then the dry weight was measured after drying in a drying oven (C-DF, Changshin Sci Co, Korea) at 70°C for 72 hours. The plant water content was calculated from the measured fresh and dry weights (fresh weight - dry weight).
Statistical analysis
The data collected in this study were analyzed using SPSS Statistics ver. 27 (SPSS Inc., Chicago, USA). To verify the significance of the means of each treatment group depending on the cell size of plug trays and the number of days after sowing, one-way ANOVA was performed at the level of p ≤ .05, and Duncan’s multiple-range test was conducted to check the differences between the means. Graphs were presented using Sigmaplot ver.12.3 (Systat, San Jose CA, USA).
Results and Discussion
Changes in the growth of HTB depending on the cell size of plug trays
LN, LW, LL, and RL
The LN, LW, and RL of HTB showed the best growth in the 143 mL cells at 90 days after sowing. The LN was highest in the 143 mL cells, followed by the 18, 13, and 70 mL cells. For the 143 and 13 mL cells, the growth decreased in terms of the LN as days went by, while for the 70 and 18 mL cells, it was the highest at 60 days after sowing, and then tended to decrease. The LW and LL tended to increase in proportion to the cell size, but the RL, a measure of the growth of roots, continued to increase as time passed regardless of the cell size. This result is similar to the findings reported by Kim et al. (2001) that in plug trays with the same cell size, the longer the nursery period, the greater the overall growth. Plugs keep more root hairs, that quickly absorb water and nutrients, and this active root system allows more uniform and faster plant growth after transplanting (Durner et al., 2002). In particular, the RL did not show a significant difference between the 70 and 18 mL cells, while it showed a significant difference and changed in the 18 mL cell 60 days after sowing. The LL was longest for the 70 mL cell, followed, in order, by the 18, 143 and 13 mL cells (35.4, 32.4, 31.8, and 27.5 g, respectively). For the 70, 18, and 143 mL cells, but not the 13 mL cell, the LLs were similar, showing no significant difference between the means, but they tended to be the longest at 60 days after sowing and decrease thereafter. Lee et al. (2019) found in a study to investigate the nursery period of Rehmannia glutinosa plug seedlings, a perennial herb, that the LW increased up to 50 days after sowing, but decreased at 60 days. This seems to result from leaf aging, considering that the leaf color fades over time after sowing. In addition, in a study that investigated plug seedling raising methods of Angelica dahurica, an herbaceous plant, it was reported that the growth of 63-day old seedlings was the best, but the dry weight-height ratio (compactness) of 70-day old seedlings including plant height decreased (Nam et al., 2022). On the other hand, compared to other treatment groups, the LN for 13 mL/cell, the smallest soil volume per cell, decreased steadily over time, while the LL, LW, and RL continued to increase. In particular, the growth progressed and the leaves fell, so the LN of all groups tended to decrease 60 days after sowing (Fig. 1). Considering the finding that the more leaves, the better the above-ground part growth and rooting after planting plug seedlings (Yoo and Roh, 2009), LN is considered to be an important measure of the growth in raising plug seedlings using plug trays. The LN is the most distinct measure of growth that can be visually observed in the above-ground part. Compared with the early growth phase, the LN only increased (18.9%) for 18 mL/cell (105-cell trays) during the experiment period, and the LW, LL, and RL increased by 29.3%, 53.3%, and 31.2%, respectively; the growth rate was the highest of all treatment groups.
Fresh and dry weights
The fresh weight of HTB shoots was 0.8 g for the 143 mL cell, which was the heaviest of the experimental groups, followed in order by the 13, 18, and 70 mL cells (0.6, 0.5, and 0.4 g, respectively); however, there was no significant difference. The dry weight of HTB shoots was the heaviest at 0.2 g in the 143 mL cell, followed in order by the 70, 18, and 13 mL cells, in proportion to the cell size of plug trays. The fresh weight of HTB roots was the heaviest at 1.7 g for the 143 mL cell, followed in order by the 70, 18, and 13 mL cells (1.5, 1.1, and 0.7 g, respectively). The dry weight of HTB roots was the heaviest at 0.4 g in the 143 mL cell, followed in order by the 70, 18, and 13 mL cells, similar results to the shoots (Fig. 2). The dry weight of shoots, and the fresh and dry weights of roots, excluding the fresh weight of shoots, were found to be lighter as the cell size decreased, showing a significant difference according to the cell size (Fig. 3). It seems that the restriction imposed on the root system, when plants are grown in smaller plug cell trays, decreased the dry weight accumulation (Di Benedetto and Klasman, 2004). In particular, the root development of plug seedlings is closely related to plug-tray cell size; as seedling quality is affected by substrate volume in the rhizosphere, the length and number of roots are also greatly affected by the cell size (Lee and Lee, 2019). That is, plant growth in the rhizosphere in raising plug seedlings varies depending on the cell size of plug trays, and the difference in the early growth affects the growth even after planting (Lee et al., 2020), suggesting that plant growth in the rhizosphere in plug trays before planting is very important.
Plant water content (PWC)
The PWC of HTB shoots and roots depending on plug-tray cell size was as follows (Fig. 4). The PWC of both shoots and roots were highest in the 143 mL cell, but the PWC of shoots and roots had different trends. For HTB shoots, the PWC was found to be lower in the order of the 143, 13, 18, and 70 mL cells at 90 days after sowing, but there was no significant difference depending on plug-tray cell size. For HTB roots, it was found to be higher as the cell size increased in the order of 143, 70, 18, and 13 mL/cell (1.3, 1.2, 0.9, and 0.6 g, respectively), showing a significant difference depending on plug-tray cell size (p ≤ .05). That is, compared to the PWC of HTB shoots, the PWC of HTB roots showed distinct differences in proportion to plug-tray cell size. As cell diameter decreased, the restriction of tap root growth became evident, and the lateral roots might have had to compete for space in cell. The length and diameter of roots, as well as the fresh and dry weights, improved as the cell diameter increased (Lee et al., 2021). Therefore, due to the limited space for bed soil in the cell that characterizes plug trays, even in terms of PWC, it appears that the cell size of plug trays exerted a greater effect on the growth of the roots than that of the shoots, showing a significant difference based on the cell size.
Changes in HTB seedling growth according to the number of days after sowing
By analyzing the growth of HTB according to the number of days after sowing, significant differences were found in LN, LW, LL, and fresh weight, excluding RL (Table 1). In addition, the RL and fresh weight of roots, which are growth items of roots, showed the best growth at 90 days after sowing at 12.97 cm and 1.26 g, respectively, but the LN, LW, LL, and fresh weight of shoots showed the best growth at 60 days after sowing during the experiment. That is, based on 60 days after sowing, the growth of shoots was reduced, and that of roots also began to slow down in terms of the fresh weight and RL.
For excellent quality, the appropriate pot size and growing period are required for each plant. When the size of pots is smaller, the growth of roots is inhibited, which affects plant growth; it is thought that under such conditions the growth state is relatively deteriorated as the growing period is longer. As for some herbaceous native plants including Hemerocallis fulva, the quality of 60-day old seedlings was found to be higher than that of 90-day old seedlings after planting (Jeong et al., 2016). This suggests that they can be transplanted and cultivated after 60 days of raising seedlings, considering the growth of shoots and roots when sowing HTB in plug trays.
Conclusion
This study was conducted to determine the growth characteristics of HTB seedlings according to the number of days after sowing and cell size of plug trays. The LW and RL of HTB showed a growth state in proportion to the cell size, and the LN was highest in the 143 mL cell at 90 days after sowing, followed by the 18, 13, and 70 mL cells. The LL was the longest for the 70 mL cell, followed by the 18, 143, and 13 mL cells, but there was no significant difference except in the 13 mL cell. Based on an analysis of the fresh and dry weights and PWC of shoots and roots, it was found that compared to those of shoots, the larger the cell size, the better the growth and PWC of roots, showing a significant difference according to the cell size of plug trays. On the other hand, as for the change in the growth of HTB according to the number of days after sowing, the growth of shoots was most vigorous at the 60th day after sowing, while the RL and fresh weight of roots, which can be considered indicators of the growth of roots, increased steadily during the experiment, but slowed down after 60 days of sowing.
Compared with 143 mL/cell (32-cell plug tray) with the largest volume per cell among the experimental groups, the LN only increased for 18 mL/cell during the experiment, and showed no significant difference from that for 143 mL/cell, and the LL was rather longer. In addition, compared to the early growth, the growth rate in terms of LW, LL and RL was found to be the highest of the experimental groups. Therefore, considering the actual production period and economic efficiency of the seedlings, it is considered that a nursery period of 60 days or more in plug trays with 18 mL/cell (105-cell plug trays) is appropriate for HTB. In the future, it seems that over the long term, field application studies are needed on suitable management methods for each species for the production of standard seedlings, as well as post-transplant rooting.
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