Introduction

Seedling quality is a critical factor in crop production and significantly influences plant performance, survival, stress resistance, growth, yield, and overall quality after transplantation (Johkan et al., 2010; Ritchie, 1984; Yan et al., 2019). High-quality transplants enable crops to better withstand environmental changes, improve survival rates, ensure uniform stand establishment, and enhance resource use efficiency, ultimately leading to higher and more profitable yields (Qin and Leskovar, 2020; Russo, 2006).

Seedling quality is influenced by various environmental factors during growth, with light, temperature, and soil moisture being primary determinants. In Korea, which is located in the mid-latitude climate zone of the Northern Hemisphere, distinct seasonal changes pose challenges for year-round vegetable seedling production. Climate change and unpredictable extreme weather events further complicate the production of high-quality seedlings (Leisner, 2020).

Conventional plug seedling systems, which are widely used in commercial nurseries in Korea, offer advantages in terms of space utilization and workability. However, these systems can lead to excessive elongation of seedlings under high temperature, low light, and high humidity conditions, potentially compromising seedling quality (Um et al., 2009). Moreover, removing seedlings from plug trays can cause mechanical damage to the root tips and hairs, disrupt the root/shoot balance, and potentially cause transplant shock (Qin and Leskovar, 2020).

Recently, there has been a growing interest in the production of seedlings using biodegradable paper pots (Jang et al., 2018, 2019, 2020; Knox and Chappell, 2014; Seo et al., 2017, 2018; Xu et al., 2020, 2021). These paper pots offer several advantages, including the prevention of root damage during transplantation, improved air circulation in the rhizosphere, and the ability to transplant younger seedlings (Jang et al., 2019; Seo et al., 2018).

However, the optimal environmental conditions required to produce seedlings in paper pots may differ from those required for conventional plug trays due to differences in the root environment. This study aimed to examine the growth responses of cucumber grafted seedlings grown in biodegradable paper pot trays compared to plug trays under varying conditions of light intensity, night temperature, and irrigation frequency.

Materials and Methods

1. Plant materials and biodegradable paper pots

Seeds of cucumber scion (Cucumis sativus cv. Joeunbaekdadaki, Farm Hannong Co., Ltd., South Korea) and pumpkin rootstock (Cucurbita maxima × C. moschata cv. Shintozwa, Nongwoo Bio Co., Ltd., South Korea) were sown in 128-cell plug trays (Beomnong Co., Ltd., South Korea) filled with commercial growing media (Heungnong Bio No. 1, Heungnong Seed company, South Korea). After two days of germination at 27°C in a germination room, seedlings were moved to a bench in a greenhouse (L 12 × W 12 m, side height 4.5 m, 144 m²).

Biodegradable paper pots (40 mm diameter, 40 mm height) were manufactured using a paper pot manufacturing device (Helper Robotec, South Korea) and Ellepot paper (Ellepot A/S, Denmark). The pots were filled with commercial growing media mixed with perlite (Pindstrup, Denmark) and placed in 40-cell dedicated trays (Beomnong Co., Ltd., South Korea). Control plants were grown in 40-cell plug trays (Beomnong Co., Ltd., South Korea) filled with commercial media (Heungnong Bio No. 1, Heungnong Seeds, South Korea). The growing media volumes were 68.8 ± 1.1 mL and 80.2 ± 1.5 mL for paper pots and plug trays, respectively.

After watering, the water potential and root-zone temperature of the media in paper pots and plug trays were measured over time by using a water potential meter (WP4, Decagon Devices, Inc., USA). The chemical properties of the media used for paper pots and plug trays were analyzed and are presented in Table 1. The EC value of the paper pot media was 0.90 dS·m^-1, approximately half that of the plug tray media. The paper pot media contained lower levels of nitrogen, available phosphate, potassium, and magnesium compared to the plug tray media. In contrast, the paper pot media exhibited higher organic matter and calcium contents than the plug tray media.

2. Grafting and healing

Seven days after sowing, cucumber scions were grafted onto pumpkin rootstocks using the one cotyledon splice grafting method (Lee and Oda, 2003). Grafted seedlings were then planted in either paper pot trays or plug trays. The grafted seedlings were healed and acclimatized for six days in a healing chamber set at 26°C and >90% relative humidity, with constant LED lighting (Red:Blue = 2:1, T&I Co., Ltd., South Korea) at approximately 20 μmol·m^-2·s^-1 photosynthetic photon flux (PPF).

3. Light intensity, night temperature, and irrigation frequency treatment

After healing and acclimatization, the grafted seedlings were placed in the greenhouse. Fig. 1 shows the process of producing grafted cucumber seedlings in a biodegradable paper pot tray. The grafted seedlings were subjected to eight treatments combining two levels each of light intensity, night temperature, and irrigation frequency for 14 days (Table 2). This experiment was conducted in both summer (June) and autumn (October) to account for seasonal variations in environmental conditions.

Light intensity was treated at two levels of shading or non-shading using a shading net with a shading level of 75%. Night temperature was treated at two levels of 15℃ treatment using a growth chamber (HB-301M, Hanbaek Science, South Korea) or ambient night temperature in the greenhouse from 8 pm to 6 am daily. Irrigation frequency was treated at two levels of once or twice-daily irrigations. In the twice-daily irrigation treatment, water was applied at 10 am and 2 pm, while the single irrigation treatment received water at 10 am. Every four days, seedlings were supplied with a nutrient solution having EC of 1.4 dS·m^-1 (‘Hanbang’ for seedling, N-P-K-Ca-Mg = 8.0-2.4-2.4-4.8-1.6 me·L^-1, Coseal Co., Ltd., South Korea). On these days, the nutrient solution application replaced the first scheduled irrigation.

Fig. 1.

The production of grafted cucumber seedlings in biodegradable paper pot trays.

4. Measurements

PPF, temperature, and relative humidity were continuously monitored using environmental sensors (LightScout Quantum Light sensor and external (soil)) temperature sensor, Spectrum Technologies, Inc., USA) and a data logger with built-in temperature and humidity sensor (WatchDog 1,000 Series Micro Stations, Spectrum Technologies, Inc., USA) from March to March of the following year, including the experiment period.

On day 14 of treatment, three plants per treatment were sampled to measure shoot length, number of leaves, leaf area (using a leaf area measuring instrument, LI-3100, Li-cor Inc., USA), SPAD value (using a chlorophyll measuring instrument, SPAD-502, Konica Minolta, Japan), root activity (Lee et al., 2017), and fresh mass of shoots and roots. Samples were then dried at 75°C in a dry oven (DS-80-3, Dasol Scientific, South Korea) for at least three days to determine dry mass. The Dickson quality index (DQI) was calculated using the following equations (Dickson et al., 1960).

After the 14-day treatment period, grafted cucumber seedlings were transplanted into 15 cm diameter pots filled with commercial media (Hungnong Bio No. 1, Heungnong Seeds, South Korea) and grown for an additional 14 days in a greenhouse. Measurements of growth parameters were taken, including leaf number, plant height, leaf area, fresh and dry mass, and female flower development at two weeks after transplanting.

5. Statistical analysis

Data were analyzed using SigmaPlot (v.14.0, Grafiti, Palo Alto, CA, USA) and SAS (v.9.4, SAS Institute, USA) statistical software. Analysis of variance (ANOVA) was performed to determine the effects of light intensity, night temperature, and irrigation frequency on plant growth parameters. Duncan’s multiple range test was used for mean separation at p ≤ 0.05.

Results and Discussion

1. Year-round greenhouse environmental conditions

The greenhouse environment exhibited significant seasonal variations in daily light integral (DLI), air temperature, and relative humidity, which are crucial factors affecting grafted cucumber seedling growth (Fig. 2). The average DLI throughout the year was 10.6 ± 7.2 mol·m^-2·day^-1, with notable seasonal fluctuations. The summer season from June to August showed the highest light levels with a peak of 18.4 mol·m^-2·day^-1. As the seasons transitioned, there was a noticeable decline in light intensity. The spring season from March to May experienced a 30% reduction in DLI compared to the summer peak, averaging 13.1 mol·m^-2·day^-1. The most dramatic decrease occurred during the autumn and winter season. From September through February, the DLI dropped to 5.5 mol·m^-2·day^-1, representing a substantial 70% decrease from the summer highs. This significant fluctuation in light levels throughout the year presents considerable challenges for maintaining consistent growing conditions for grafted cucumber seedlings across different seasons.

These values indicated that while the average annual DLI was sufficient for good seedling production, the autumn and winter levels fell below the optimal range of 10－12 mol·m^-2·day^-1 recommended for commercial vegetable seedling production (Poel and Runkle, 2017). Faust (2002) classified plant responses to DLI into five levels, noting that very low (≤5 mol·m^-2·day^-1) and low (5－10 mol·m^-2·day^-1) light conditions hinder normal growth and flowering. Under these conditions, flowering may be delayed, and plants may remain in the vegetative growth stage.

The greenhouse maintained an annual average temperature of 24.0 ± 3.9°C, which aligns well with the optimal range for cucumber growth of 22－28°C during the day and 15－18°C at night (RDA, 2019). Summer showed the highest temperatures, averaging 29.1°C with peaks reaching 39.3°C, occasionally exceeding ideal conditions. In contrast, autumn and winter brought cooler temperatures, averaging 22.5°C and 20.8°C, respectively. These cooler conditions, while potentially challenging, helped maintain seedling quality despite the concurrent low light levels.

Relative humidity levels in the greenhouse also varied by season, generally falling below the recommended range (70－80% during the day and 90% at night) for optimal cucumber growth (RDA, 2019). Summer and autumn maintained similar humidity levels of about 62%, while spring and winter saw significantly drier conditions, with humidity dropping to 37% and 32% respectively. These low humidity levels, particularly in spring and winter, could potentially hinder cucumber growth and development, underscoring the need for improved humidity management strategies during these seasons.

Fig. 2.

Daily light integral, maximum, mean, and minimum temperature, night temperature and relative humidity in a greenhouse.

2. Environmental conditions during treatment periods

The summer and autumn treatment periods exhibited markedly different environmental conditions, significantly impacting grafted cucumber seedling growth (Table 3). In summer, the average daily DLI was 21.3 ± 8.8 mol·m^-2·day^-1, with daily air temperature averaging 26.2 ± 1.3°C. The 75% shading treatment reduced DLI to 5.2 ± 2.3 mol·m^-2·day^-1. In contrast, autumn conditions presented a much lower average DLI of 5.3 ± 1.9 mol·m^-2·day^-1, with daily air temperature averaging 22.0 ± 2.1°C. The 75% shading in autumn further reduced DLI to 1.7 ± 0.9 mol·m^-2·day^-1.

These seasonal variations align with the year-round trends observed in the greenhouse, underscoring the challenges of maintaining optimal growing conditions across seasons. The substantial reduction in DLI due to shading treatments, particularly in autumn, highlights the need for careful light management to support optimal plant growth.

3. Water potential and root-zone temperature

The paper pot trays exhibited different water retention characteristics compared to plug trays (Fig. 3). While initial water potentials did not show a significant difference (–1.0 MPa for paper pots and –0.7 MPa for plug trays), paper pots showed a more rapid decrease in water potential over time. After 24 hours, paper pots reached –1.4 MPa compared to –0.9 MPa in plug trays. This suggests that paper pots may require more frequent irrigation to maintain optimal water availability for plant growth. This could be attributed to differences in the physical properties of the growing media or materials used in the trays, which can affect water retention and drainage (Argo, 1998).

The average root-zone temperatures were 28.9°C for the paper pot tray and 29.1°C for the plug tray, both approximately 1°C lower than the greenhouse temperature of 30.1°C. Root-zone temperatures in paper pot trays were slightly lower than in plug trays (maximum of 36.9°C and 37.4°C), potentially providing a marginal benefit in terms of mitigating thermal stress on root systems. The differences in root-zone temperatures indicate that the paper pot tray may offer a cooler environment compared to the plug tray and greenhouse. Cooler root-zone temperatures can benefit root health and overall plant growth, as excessive temperatures can lead to thermal stress and reduced performance. High root-zone temperatures (35－38°C) have been shown to inhibit cucumber growth, reduce leaf water content, and impair photosynthesis (Moon et al., 2007; Nada et al., 2003; Wang and Tachibana, 1996). Elevated root-zone temperatures are associated with increased levels of abscisic acid (ABA), which induces stomatal closure and reduces the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Nada et al., 2003).

These findings highlight the need for tailored irrigation and temperature management strategies based on the type of growing container used. Appropriate management is crucial for optimizing water availability and minimizing thermal stress, thereby enhancing cucumber seedling growth and overall plant health.

Fig. 3.

Media water potential (upper) and root-zone temperature (down) of paper pot trays or plug trays for a day after irrigation. Air temp. represents the air temperature inside the greenhouse.

4. Growth of grafted cucumber seedlings

Fig. 4 provides a comprehensive comparison of key growth parameters for grafted cucumber seedlings grown in paper pot trays and plug trays during both summer and autumn seasons. The parameters analyzed include total dry mass, leaf area index (LAI), and DQI.

Fig. 4.

The dry mass, leaf area index, and Dickson quality index of grafted cucumber seedlings grown in the paper pot trays or plug trays in summer or autumn.

The analysis revealed that paper pot trays consistently demonstrated lower average values for total dry mass and LAI compared to plug trays. This trend was observed across both summer and autumn seasons, suggesting that the growing media and tray type have a significant influence on these growth parameters.

Despite the differences in total dry mass and LAI, there were no significant differences observed in the DQI between the two tray types. This finding is particularly important as it indicates that while paper pots may result in lower overall biomass and leaf area, they do not negatively impact the overall quality of the seedlings as measured by the DQI.

One of the most significant findings of this comparison was the greater uniformity exhibited by seedlings grown in paper pot trays. These seedlings showed less variation among individuals across all measured parameters. This increased uniformity is a crucial factor in commercial seedling production, as it can lead to more consistent crop establishment and performance after transplanting.

A notable seasonal effect was observed in the growth parameters. In autumn, both the average values and individual deviations of the measured parameters decreased compared to summer for both tray types. This reduction can likely be attributed to the lower light levels and temperatures characteristic of the autumn season, as discussed earlier in the environmental conditions analysis.

Light intensity significantly affected shoot length, total and root dry mass, dry matter content, leaf mass ratio, root-to-shoot ratio, and DQI in both tray types (Table 4). Higher light intensity correlated with more compact and vigorous growth, aligning with previous research on the importance of light intensity during the seedling stage (Ji et al., 2020; Wang et al., 2021).

Paper pot seedlings showed heightened sensitivity to changes in light and temperature compared to plug tray seedlings. This was evident in the significant effects of both light intensity and night temperature on shoot length and leaf number in paper pot seedlings. Lower night temperatures (15°C) effectively suppressed shoot elongation, particularly under low-light conditions, indicating a potential strategy for controlling seedling morphology (Oh and Kim, 2010).

Grafted cucumber seedlings grown in paper pot trays consistently exhibited shorter shoot lengths and lower total dry mass compared to those in plug trays (Fig. 5). However, root dry mass was comparable to or greater in paper pot seedlings. This suggests that paper pots may promote a more balanced root-to-shoot ratio, potentially beneficial for transplant success and subsequent field performance.

Fig. 5.

The shoot length, total and root dry mass (A) of grafted cucumber seedlings grown in paper pot trays or plug trays under different light intensity, night temperature, and irrigation frequency and the ratio (B) of dry mass of 75% shading to non-shading. Different capital and lowercase letters indicate respectively a significant difference between treatments in paper pot trays (black diamond symbol) and plug trays (gray bar) according to Duncan’s multiple range test at p ≤ 0.05.

In summer, under non-shading conditions with a DLI of 21.3 mol·m^-2·s^-1, paper pot seedlings exhibited a shoot length of approximately 15 cm. However, when subjected to 75% shading (reducing DLI to 5.2 ± 2.3 mol·m^-2·s^-1), shoot length increased significantly by 54% to 23.1 cm. Interestingly, while night temperatures did not affect shoot length under non-shaded conditions, lowering the night temperature to 15°C under shaded conditions led to a reduction in shoot length by about 6 cm.

These findings suggest that lower night temperatures can effectively mitigate shoot elongation under low light conditions. This effect is likely due to reduced hormonal activity that typically promotes elongation (de Wit et al., 2016; Li et al., 2022). This insight presents a potential strategy for controlling shoot elongation in low-light environments without significantly impacting overall plant growth and metabolism.

The autumn experiments revealed a different pattern. Shoot length was influenced by a combination of factors including light intensity, night temperature, and irrigation frequency. Under non-shading conditions (DLI of 5.2 mol·m^-2·s^-1), paper pot seedlings reached about 14 cm in height. When subjected to 75% shading (reducing DLI to 1.9 mol·m^-2·s^-1), shoot length increased by 29% to 18.0 cm. The less pronounced difference in shoot length response to shading in autumn, compared to summer, can be attributed to the overall lower absolute light levels during this season.

These seasonal variations in shoot length response highlight the complex interplay between light intensity, temperature, and other environmental factors in determining seedling morphology. They also underscore the importance of season-specific management strategies in cucumber seedling production, particularly when using paper pot systems.

The dry mass of paper pot seedlings was reduced to 58% in summer and 65% in autumn under 75% shading compared to non-shading conditions. This reduction was less severe than in plug seedlings (reduced to 53%), suggesting that paper pot seedlings may be more adaptable to low-light conditions. While shoot length remained stable across seasons, autumn dry mass was only 50% of the summer levels, and root dry mass significantly reduced to 36% of the summer levels. These seasonal differences can be attributed to the reduced light intensity and temperatures in autumn.

Despite lower overall growth, paper pot seedlings demonstrated comparable or superior quality metrics in terms of dry matter ratio, leaf mass ratio, root-to-shoot ratio, and DQI (Fig. 6). Importantly, paper pot seedlings exhibited greater uniformity, with less variation among individual plants. This uniformity is a crucial factor in commercial seedling production, potentially leading to more consistent crop establishment and performance after transplanting.

Fig. 6.

The dry matter, leaf mass ratio, root to shoot ration and Dickson quality index of grafted cucumber seedlings grown in paper pot trays or plug trays under different light intensity, night temperature, and irrigation frequency. Different capital and lowercase letters indicate respectively a significant difference between treatments in paper pot trays (black diamond symbol) and plug trays (gray bar) according to Duncan’s multiple range test at p ≤ 0.05.

Fig. 7 showed the relationship between the mean DLI and dry mass of grafted cucumber seedlings grown in both paper pot and plug tray systems. As the DLI increased, there was a corresponding increase in the dry mass of the seedlings across both growing systems. While both systems showed increased dry mass with higher DLI, paper pot seedlings consistently exhibited lower dry mass compared to those grown in plug trays across the entire range of DLI values observed. This pattern suggests that despite the positive influence of increased light exposure, the paper pot system may impose some limitations on overall biomass accumulation. The observed differences could be attributed to factors including the characteristics of the growing media, root zone conditions, and variations in nutrient dynamics.

Fig. 7.

Relationship between mean daily light integral and dry of grafted cucumber seedlings grown in paper pot trays (black circle symbol) or plug trays (white circle symbol) under different light intensity, night temperature, and irrigation frequency.

Two weeks after transplanting, the number of unfolded leaves did not differ significantly between treatments (Fig. 8). However, dry mass and the number of female flowers were influenced by seedling growth conditions. The reduced number of female flowers under both low and high temperature conditions highlights the importance of maintaining optimal temperature ranges during the seedling stage for subsequent reproductive development.

Fig. 8.

The growth of grafted cucumber seedlings grown in paper pot trays or plug trays under different light intensity, night temperature, and irrigation frequency at two weeks after transplanting. Different capital and lowercase letters indicate respectively a significant difference between treatments in paper pot trays (black diamond symbol) and plug trays (gray bar) according to Duncan’s multiple range test at p ≤ 0.05.

Conclusion

This study demonstrates that biodegradable paper pots offer a viable alternative to conventional plug trays for grafted cucumber seedling production, with distinct advantages and considerations. The research also highlights the significant impact of seasonal variations in greenhouse environmental conditions on seedling growth. The year-round analysis of greenhouse conditions revealed substantial fluctuations in DLI, temperature, and humidity across seasons. While summer conditions generally provided sufficient light and warmth for optimal seedling growth, autumn and winter presented challenges with low DLI and humidity levels. These seasonal variations underscore the need for adaptive management strategies to maintain consistent seedling quality throughout the year.

Paper pot seedlings exhibited slower overall growth and lower total dry mass compared to those in plug trays, but consistently produced comparable or greater root dry mass and showed improved uniformity among plants. The heightened sensitivity of paper pot seedlings to environmental factors, particularly light intensity and temperature, necessitates more precise management of these variables during the seedling production period. The rapid decrease in water potential observed in paper pots suggests a need for more frequent irrigation compared to plug trays. However, the slightly lower root-zone temperatures in paper pots may provide a marginal benefit in terms of reducing thermal stress on root systems.

Despite lower overall growth, paper pot seedlings demonstrated comparable or superior quality metrics (dry matter ratio, leaf mass ratio, root-to-shoot ratio, and DQI) and greater uniformity, indicating potential advantages for transplant success and subsequent field performance. This underscores the importance of maintaining appropriate environmental conditions during the seedling stage. Future research should focus on optimizing environmental management strategies specific to paper pot systems, particularly in terms of irrigation frequency and light management. Additionally, long-term studies evaluating the post-transplant performance and yield of paper pot-grown seedlings in field conditions would provide valuable insights into the overall efficacy of this system for commercial cucumber production.

In conclusion, while the use of biodegradable paper pots for cucumber seedling production presents some challenges in terms of environmental management, it offers promising benefits in terms of seedling quality, uniformity, and potential ecological advantages. With further refinement of management practices and a comprehensive understanding of seasonal environmental impacts, paper pot systems could become a sustainable and effective alternative to conventional plug tray systems in commercial vegetable seedling production.