Introduction

Strawberry (Fragaria × ananassa Duch.) is one of the most delicious and widely consumed fruit crops. It is a rich source of vitamins and minerals and is known for its delicate flavor. It also contains significant levels of other bioactive compounds, including phenolics and flavonoids (Häkkinen and Törrönen 2000). Globally, the estimated market value of strawberries was approximately 11.5 trillion won. As of 2024, strawberries remain one of the top horticultural crops in Korea (Kim et al. 2023). In 2024, South Korea's strawberry production was approximately 190,820 metric tons (KOSTAT 2024).

While hydroponics aims to ensure stable production, pot hydroponics cultivation is also being explored as a method to reduce the disease susceptibility of strawberries. This system offers flexibility in design and can be constructed using various materials depending on cultivation requirements. Pot hydroponic systems have been shown to maximize plant density and productivity per unit area (Gomeza et al. 2012). This method creates an artificial rhizosphere, allowing roots to grow directly in a controlled environment. Pot hydroponic systems are diverse and can be adapted based on crop characteristics and cost considerations. Additionally, they help conserve water and minimize fertilizer use, contributing to sustainable agricultural practices (Treftz et al. 2015).

In hydroponic systems, substrates are crucial as they provide physical support, regulate moisture, and promote healthy growth. In addition, a proper substrate provides adequate support for root growth, facilitates oxygen supply, enhances nutrient uptake, and microbial activity (Beaudry 2000). Coir is considered one of the most promising substrates due to its adaptability and desirable physical properties. This substrate is effective in drainage management and substantial water-holding capacity, making it a suitable organic substrate for root growth (Lopez et al. 2004). Furthermore, artificial substrates are widely used due to their beneficial properties in promoting plant growth and improving horticultural crop production. Recently, biochar has been increasingly used in agriculture. It can easily interact with nutrients and microbes. Biochar improves water retention and aeration capacity, and it helps enhance the biological and physicochemical properties of the root zone (Jeffery et al. 2015).

Different strawberry varieties exhibit varying fruit quality, yield, and responses to environmental factors. In Korea, approximately 80% of strawberries belong to the Sulhyang variety. This variety exhibits vigorous vegetative growth, resulting in high yields, and is the first to ripen, providing fresh fruit during the winter months (Kim et al. 2023). Other commonly cultivated varieties include Kuemsil and Santa. The Kuemsil variety is economically viable for farmers due to its resistance to certain diseases and pests, making it highly demanded in both local and international markets (Jo et al. 2022). The Santha variety has a firm and dense texture, which enhances its resistance to damage during transport. It is known for its sweetness, firmness, and suitability for both fresh consumption and export (Jo et al. 2022). Moreover, the quality indices of strawberries are influenced by parameters such as size, firmness, soluble sugar content (SSC), and titratable acidity (TA). Changes in firmness are highly correlated with variations in SSC and other ripening indicators in strawberries. The fruit’s SSC tends to increase progressively during ripening. Modifications in firmness and pH significantly affect the quality and perceived sweetness of strawberries (Ornelas et al. 2013).

However, changes in growth and yield depend on the growing substrates, plant varieties, and their interactions. Despite these, few studies have investigated these factors in pot-based hydroponic systems. Therefore, the main objective of this study was to investigate the effects of different substrates on strawberry growth and fruit quality under pot cultivation. These findings enhance our understanding of the relationship between leaf and fruit quality, allowing for non-destructive, sustainable, and efficient assessment of strawberries, optimizing resource use and ensuring high-quality fruit.

Materials and Methods

Growing condition

This experiment was conducted in an 18 m × 8 m × 6 m (L × W × H) Venlo-type glass greenhouse at Kangwon National University in Chuncheon, Gangwon-do, from September 15, 2022, to March 31, 2023. Seedlings (leaf number with 3–5 each and plant height 25–20 cm) were sourced from a commercial farm in Pyeongchang, Gangwon-do, and transplanted into plastic pots (length 61 cm × width 20 cm × height 15 cm; De Sung P.P., Korea; Fig. 1A) on gutters. Each pot tray having four cells, each cell cultured two plants. Before transplanting, the cell was filled with growing substrates and buffered with University of Seoul (UOS) nutrient solution to EC 0.5 dS·m^-1. This nutrient solution, consisting of the macroelements N, P, K, Ca, Mg, and S of 7.0, 2.1, 4.0, 3.0, 1.5, and 1.5 me·L^-1 respectively, with the EC in the range of 0.8–1.5 dS·m^-1 and pH ranging of 5.5–6.0 was applied for the whole cultivation period through the drippers.

Temperature, relative humidity (RH), and integrated solar radiation (ISR) were measured using a temperature sensor and an RH sensor placed inside an instrument shelter within the greenhouse, and a light sensor installed on the roof, respectively. The values were recorded in real time by auto-control software (Ridder Synopta, Ridder, Harderwijk, The Netherlands). The range of temperature, relative humidity, and ISR was 11.2 to 21.4°C, 65.1 to 89.1% and 45.1 to 120.2 MJ·m^-2, respectively. Weekly ISR was lowest in December (45.1 MJ·m^-2), as were RH (65.1%) and temperature (11.2°C) (data not shown). Roof windows were opened over 22°C, and screens were closed at 600 and 800 W·m^-2. A heating system was operated when the greenhouse temperature dropped below 5–8°C at nighttime, and below 12°C during the daytime. General management was followed, and plants were pollinated by honey bees inside the greenhouse. Fruit thinning was conducted selectively, leaving six fruits per cluster.

Irrigation supply and management

One dripper was set between two plants for irrigation (four drippers per pot, Fig. 1). Irrigation was controlled automatically per pot by time and integrated solar radiation (ISR) in a Ridder HortiMaX system. The irrigation schedule was based on a timer from an hour after sunrise until 11 a.m., by ISR in the range of 100–180 J·cm^-2 for three hours of sunset. The irrigation amount per plant per day was 50–180 mL varying for the experimental periods.

Treatments

A complete randomized block design was implemented, and three replications per substrate were established. The experiment was conducted with two factors: cultivars and substrates. Three types of substrates, such as biochar (Fig. 1B), coir dust (Fig. 1C), and artificial substrate (Fig. 1D), and three cultivars, such as Sulhyang (Fig. 1E), Kuemsil (Fig. 1F), and Santa (Fig. 1G), were evaluated. The three types of substrates are denoted as T₁ (biochar, ari biochar Sang Lim Co. Ltd., Jeonbuk, Korea), T₂ (coir dust; coir dust 100%, Duck Yang Coco, Sri Lanka,), and T₃ (artificial substrate, a universal horticulture, Seoul Bio Co., Ltd., Eumseong-gun, Korea). The chemical properties of the substrates were measured and used as follows: the coir dust substrate had an EC of 0.5 dS·m^-1 and a pH of 5.5 (Shin and Son 2015); the biochar had an EC of 0.08 dS·m^-1 and a pH of 6.4; and the artificial substrate had an EC of 0.25 dS·m^-1 and a pH of 5.8 (Rajkovich et al. 2012).

Fig. 1.

A pot used for hydroponic strawberry cultivation system (A) with different substrates: biochar (B), coir dust (C), and artificial substrate (D), and with different cultivars: Sulhyang (E), Kuemsil (F), and Santa (G).

Plant growth measurement

Plant height was measured from the base of the plant to the highest point of its largest leaf. Petiole length, leaf length, and leaf width were determined using the third-newest leaf each month. Crown diameter was measured using a digital caliper (CD-20CPX; Mitutoyo Corp., Japan) at the widest part of the crown. Chlorophyll levels were measured monthly from the third leaf using a chlorophyll meter (SPAD-502, Konica Minolta, Japan). Leaf SSC and TA were measured using 0.5 g of fresh third-youngest leaf. In brief, the leaf was ground with sea sand, and 4.5 mL of distilled water was added. Then, 1.5 mL of the leaf juice was taken and centrifuged in a 1.5 mL centrifuge tube at 7000 rpm for twenty minutes. Next, 500 µL of the extract was used to measure leaf SSC (ATAGO PAL-1, Atago Co. Ltd., Japan). Subsequently, 4.5 mL of distilled water was added to the 500 µL extract for leaf TA measurement (PAL-BXIACID4, Atago Co. Ltd., Japan). The leaf EC and pH were measured from the same solution prepared for leaf SSC and TA. Then, 2 mL of the solution was mixed with 8 mL of distilled water for EC and pH measurements using a portable meter (HI9813-6, Hanna Instruments, Romania).

Fruit and yield measurement

Strawberry fruits (90 to 100% ripened) were harvested once or twice a week from November 2022 to March 2023. The number of fruits was counted manually and their weight was measured using an electronic balance (HS5200S, Hansang Instrument C. Ltd., Korea). Fruit firmness was measured using a cylindrical probe (3 mm) penetrated to the skin of the fruit to calculate the compression force in Newtons (N) with a fruit firmness tester (FR-5105, Lutron electronic enterprise Co. Ltd., Taiwan) from January to March 2023. To measure fruit SSC and acidity, the juice was extracted from the fruits; after that, SSC was measured using a pocket refractometer. To measure the acidity of the fruit, 1 ml of juice was first diluted with 49 ml of water (1:50).

Statistical analysis

Statistical analyses were performed using SPSS software version 26 (IBM Corp., USA). When the data was checked, the analysis was done using an F-test to evaluate the influence of two factors. Outcomes from an analysis of variance (ANOVA) and the mean differences were compared by a post-hoc test at a p level of < 0.05 according to Duncan’s multiple range test (DMRT). Principal component analysis (PCA) was conducted using the RStudio software (version 4.4.1; R Core Team 2024) using some R packages (factoextra and ggplot2). Data were analyzed using different samples for various parameters, including growth characteristics, leaf analysis, fruit number, and yield in 3 samples, fruit weight, and fruit quality in 10 samples for each substrate.

Results and Discussion

Growth parameter and leaf characters analysis

The effects of cultivars grown in substrates was significant, except for leaf length and petiole length (Table 1). However, substrates effect was only significant for crown diameter. The interaction between cultivars and substrates were significantly affected only plant height. The highest plant height (36.1 cm) was observed in T₃ substrate for the Sulhyang cultivar, whereas T₂ substrate was for Kuemsil (33.2 cm) and Santa (32.8 cm) cultivars. Similarly, the crown diameter in T₃substrate with Sulhyang showed the higher value than T₁ and T₂ substrates, and the T₂ substrate showed higher crown diameter for Kuemsil and Santa compared to T₁ and T₃ substrates. This may be due to a more favorable root environment and enhanced root expansion provided by these substrates, which supported the development of larger crown diameters in strawberry plants. Shin et al. (2023) reported that cultivating strawberries in different coir bags enhanced crown diameter, suggesting improved vegetative growth through efficient nutrient absorption, without negatively affecting other plant traits during winter cultivation. Regarding leaf width and petiole length, T₂ substrate with the Kuemsil cultivar showed the highest values, while the Sulhyang and Santa did not show any significant differences among the substrates. There were no significant differences in leaf length and SPAD values across the months. The observed differences in growth characteristics in this study may be due to variations in the substrates water-holding capacity and aeration among the cultivars tested. Similar to this study, Tehranifar et al. (2007) observed that the type of growing substrate, including combinations like peat and sand, sand alone, perlite, peat moss and perlite, peat moss alone, cocopeat and perlite, and cocopeat alone, had a significant impact on the vegetative growth of three strawberry cultivars such as Camarosa, Gaviota, and Selva.

The TA of the leaves was affected by cultivars, and the EC of the leaves was affected by substrates (Table 2). The interaction between cultivars and substrates did not shows significant variation. The T₃ substrate with Kuemsil cultivar was showed better leaf SSC than T₁, while there were no significant differences in Sulhyang and Santa cultivars. The higher SSC of the leaf helps the translocation of sugar from leaves to xylem cells through the plasmodesmata, and then its further transport to fruits. However, this transportation in hampered due to insufficient energy of phloem sap and lower value of TA (Remi et al. 2013). In Sulhyang cultivar, T₃ substrate showed the higher leaf TA than T₁, while T₁ and T₂ showed no significant differences. On the other hand, in Kuemsil cultivar, T₂ substrate showed higher leaf TA than T₁ and in Santa cultivar, there was no significance difference between T₂ and T₃. Furthermore, the substrate T₃ with Sulhyang and Santha cultivars was found to be the highest for leaf EC, while there were no significant differences in Kuemsil. The leaf EC helps to monitor the general health of the plant and optimal nutrient concentration (Langenfeld et al. 2022). Sulhyang showed the highest leaf pH 5.82 in T₂ substrate compared to T₁ and T₃. The substrate T₂ with Santa showed higher leaf pH than T₃, however, there were no significant differences in leaf pH among the substrates under the Kuemsil cultivar. These outcomes may be because the nature of substrates helps maintain optimal moisture levels in the root zone and provides a conducive environment for nutrient uptake.

Yield parameters

Fruit number, weight, and yield were influenced by different substrates and cultivars. The number of fruits in T₃ substrate (27.2) with Sulhyang was 20.9% higher than those under T₁ (22.5), while there was no significant difference between T₁ and T₂(Fig. 2A). Similarly, among the substrates, T₃ substrate with Sulhyang was found to be the higher weight (16.5g) of fruit (Fig. 2D) than T₁ and T₂, and the yield was found to be 59.0% and 57.3% higher in T₃ (514.9 g) substrate than those under T₁ (323.9 g) and T₂ (327.4 g), respectively (Fig. 2G).

T₁ and T₂ substrates with the Kuemsil cultivar for the number of fruits were found to be nearly 60.0% (24) higher than those under T₃ (15) (Fig. 2B). Among the three substrates, T₁ and T₂ with Kuemsil cultivar for weight of fruit were better than T₃ (10.7) (Fig. 2E). This could be attributed to the coir medium in the root zone maintaining sufficient moisture and nutrient transport, facilitating increased fruit production. Similar to this study, Tehranifar et al. (2007) reported that fruit weight was higher in coco medium than in other growing media.

Among the three substrates, T₁ and T₂ substrates with Kuemsil yielded nearly 50.0% (310.0 g) higher than those under T₃ (197.2 g) (Fig. 2H). This effect may be due to coir dust in the root zone maintaining adequate moisture, thus enhancing nutrient and ion absorption, ideal for fruit development. Similar to this sturdy. Jeon et al. (2006) showed that among growing media such as perlite, cocopeat and others, strawberries performed best on the cocopeat substrate. The substrate T₂ with Santa for the number of fruits was found to be nearly 25.0% (25.6) higher than those under T₁ (20.1) and T₃ (20.7) (Fig. 2C). The T₂ substrate with Santa for weight of fruit was better than T₁ and T₃(Fig. 2F). The T₂ substrate with Santa for yield was also found to be nearly 60.0% (438.3 g) higher than those under T₁ (278.1 g), and T₃ (278.1 g) respectively (Fig. 2I). Similar results were obtained by Rabbani et al. (2025), who reported that when Sulhyang was hydroponically grown for 5 months in various coir slabs, yields of 380 to 430 grams per plant were produced. Bartczak et al. (2010) reported that productive factors such as the quantity of fruit (number of fruits and weight) are directly correlated with crown diameter, which can predict plant yield potential.

Fig. 2.

Fruit number, weight of fruit and yield of strawberries for Sulhyang (A, D, G), Kuemsil (B, E, H) and Santa (C, F, I) cultivars, as affected by different growing substrates, were recorded from November 2022 to March 2023. The substrates T₁, T₂ and T₃ means by biochar, coir dust, and an artificial substrate, respectively. Means with different letters within the columns are significantly different according to Duncan’s multiple range test at p < 0.05 (n= 3).

Fruit quality parameters

Cultivars and substrates significantly affected the fruit soluble solid content (SSC) and firmness (Table 3). The interaction between cultivars and substrates also significantly influenced SSC and firmness. In the Sulhyang cultivar, the T₃ substrate showed the highest fruit SSC (12.8°Bx) and firmness (1.1 N/φ3 mm), while its TA was the lowest. Likewise, T₂ substrate with Kuemsil and Santa showed higher SSC and firmness than T₁ and T₃. However, the fruit TA for T₂ substrate with Kuemsil and Santa was significantly lower than T₁ and T₃. Additionally, the firmness of T₂ substrates in the Kuemsil cultivar was significantly 62.5% and 44.4% higher than T₁and T₃, respectively. Cao et al. (2015) worked on twenty-five strawberry cultivars and reported that total soluble solids content, which characterizes sweetness in fruits, is directly affected by environmental conditions and cultivation practices. Jafarnia et al. (2010) experimented on three strawberry cultivars (Kordestan, Fresnoand Selva) and three different growing media (100%: 0, 80%: 20%, and 60%: 40% v/v perlite and peat) and reported that SSC in strawberry fruits is influenced by cultivars and substrates. On the other hand, the lack of interaction for fruit acidity suggests that substrates did not cause substantial changes in TA across the cultivars. Small variations in TA are still observed among cultivars, which may reflect typical biological variability in agricultural experiments. This may have occurred because the type of substrates (e.g., artificial substrate and coir) affects the availability of nutrient solution, influencing how the plant synthesizes, stores SSC, and increases firmness (Jafarnia et al. 2010).

Principal component analysis

Principal Component Analysis (PCA) was conducted to assess the relationships between the biochemical characteristics of leaves and fruits, and their association with different varieties and growth substrates (Fig. 3). The first principal component (Dim1) explained 42.5% of the total variance, while the second component (Dim2) accounted for 28.3%. The graph indicates that the variables leaf EC, leaf TA, fruit SSC, and fruit firmness showed similar dynamics, with the highest values observed particularly for the ST₃, KT₂, and SaT₂ treatments. These aforementioned variables exhibited positive correlations, as indicated by the similar directions of their arrows. In contrast, leaf SSC and leaf pH showed a negative correlation based on the opposite directions of their arrows. Additionally, the KT₃ treatment was associated with higher leaf SSC values, while ST₁ and KT₁ treatments were located farther from the main cluster, indicating distinct characteristics. Long arrows contribute significantly to the variation in the data for all treatments, while short arrows contribute less significantly.

Fig. 3.

Principal component analysis (PCA) shows the relationships between treatments and different parameters of strawberries. The direction of the arrows indicates how each variable contributes to the principal components. The colored dots represent the values of the principal components for the different treatments. The lines starting from the center of the biplot show the positive or negative associations between different parameters and their degrees of correlation. There are six samples in each treatment group: ST₁ (Sulhyang and biochar), ST₂ (Sulhyang and coir dust), ST₃ (Sulhyang and artificial substrate), KT₁ (Kuemsil and biochar), KT₂ (Kuemsil and coir dust), KT₃ (Kuemsil and artificial substrate), SaT₁ (Santa and biochar), SaT₂ (Santa and coir dust), and SaT₃ (Santa and artificial substrate).

Conclusion

The findings of this study indicate that both the cultivars and substrates were correlated with the growth and productivity of strawberries cultivated in a greenhouse pot hydroponic. Cultivating Sulhyang strawberries in an artificial substrate and Kuemsil and Santa in coir dust significantly enhanced their growth, quality, and yield of fruit compared to other growing substrates. The analysis of leaf SSC, TA, EC, and pH showed significant variations. The results suggest that artificial substrates could be a valuable option for enhancing the yield and quality of strawberries of the Sulhyang cultivar. Similarly, using coir dust as a growing substrate for Kuemsil and Santa could be more advantageous than other substrates. Moreover, utilizing various growing substrates could enhance the agronomic and physiological performance of strawberries, leading to economic and environmental advantages. We suggest that further studies with different cultivars are necessary to draw general conclusions about the impact of growing substrates on the yield, quality indicators, and other characteristics of strawberry plants.