Introduction

Watermelon (Citrullus lanatus) produces 119 million tons worldwide, accounting for about 7% of fruit vegetable production, and is the largest industrial crop among the top 5 fruits consumed (Guo et al., 2019). Watermelon cultivated in most soils in a crouching posture (horizontal crop) can easily have a severe replantation problem and is prone to gummy stem blight, anthracnose, fruit and vine rot, fusarium wilt, and viruses when continuously cropped in the soil (Kwon et al., 2005; Park et al., 1996). To solve this problem, grafts are utilized that are resistant to soil-borne diseases and have been primarily applied to 91.2% of watermelon cultivation since 1920 (Jang et al., 2019; Lee, 1989; Lee, 1994; Lee et al., 2010). However, the occurrence of replanting problems is recently increasing even with grafted seedlings (Davis et al., 2008; Ko, 1999). Therefore, countermeasure is required.

Hydroponic cultivation is an effective way to improve the soil crop problem in watermelons and actively control the root-zone environment. Hydroponic cultivation has the advantage of not experiencing replanting problems because it does not use soil. Furthermore, the temperature, pH, EC (Electrical Conductivity), and composition of nutrients can be adjusted, and the yield and quality of fruits can be improved (Dorai et al., 2001; Lim et al., 2021). However, research on watermelon-related hydroponics is insufficient. Even though studies on soil and quality comparison (Park et al., 1998b), substrate type (Park et al., 1999), and nutrient solution development (Kim and Kim, 1999) have been conducted, research is still extremely rare. A previous study from 25 years ago reported that the preferred size of a watermelon is changing from a large watermelon (10 kg) to small and medium-sized watermelon (> 6 kg), or a small watermelon (< 2 kg) due to the increase in single-person households and nuclear families (Huh et al., 2020). To address these changes in consumption trends, hydroponic cultivation techniques targeting small and medium-sized watermelons are required.

Substrate culture (3,906 ha) occupies 91% of Korea’s hydroponics area (4,271 ha) in contrast to the water culture (364 ha) (KOSIS, 2021), which is primarily applied to leafy vegetables during a single cultural season. In the case of fruit vegetables, the range of EC and pH change is conspicuously large due to the rapid absorption of potassium post the fruiting stage, and most of them are cultivated with substrate culture for stable substrate management (Lee et al., 2021). Substrate culture is divided into coir, perlite, rock wool, peat moss, and charcoal based on the type of growing medium. Coir (1,845 ha), an eco-friendly organic medium made by processing coconut husk, is mostly utilized at 47% (KOSIS, 2021). Research on substrate culture for watermelon hydroponics is also limited. It is, therefore, necessary to research the substrate culture of hydroponics which reflects on the recent new consumption trend of watermelon.

This study aimed to establish a hydroponic method based on the mixing ratio of artificial substrates that affect the root zone environment, as part of the growth environment management technology for year-round production of high-quality small and medium-sized watermelons in greenhouse.

Materials and Methods

1. Plant materials and growth conditions

This study was conducted in a greenhouse at the Watermelon Research Institute in 2021. The study was divided into the semi-forcing culture in mid-March, harvested in mid-June, retarding culture in early July, and harvested in mid-September. Arch-shaped vertical hydroponics devices were installed in four lines (height 2 m, width 0.4 m) in a single-standing glasshouse (594 m²) with a width of 9 m and a length of 26 m (Fig. 1).

Fig. 1

Vertical hydroponic device (patent registration number: 10-2359307) suitable only for fruits such as watermelon cultivated by attracting stems horizontally. (A) arch-shaped structure, (B) stem vertical attraction to the net, (C) fruit support structure, (D) schematic diagram of a device, and (E) watermelon cultivation using vertical hydroponic systems (Jeon et al., 2022).

The two cultivars utilized for the study: the ‘Royal-Black’ (Jinandosinongbu Co., Ltd., Jinan, Korea), a small and medium-sized watermelon, and the ‘Dalkomi-Mini’ (Partner seed Co., Ltd., Gimje, Korea), a small watermelon. Regardless of the cultivar and cultivation period, 768 seedlings with three to four normal leaves were transplanted at 25 cm intervals. Two of the second vine stems were then vertically induced and cultivated. Fertilization was performed after 30 days of transplanting by introducing bees for 10 days, when three female flowers bloomed. Small watermelons were cultivated to yield two fruits per plant, while medium-sized watermelons were cultivated to yield one fruit per plant. A greenhouses environment control system (MAGMAPLUS-1000, Green Control System Co., Ltd., Damyang, Korea) was utilized to control the greenhouses environment and record the cultivation environment data. A previously developed nutrient solution for watermelon (Kim and Kim, 1999) was utilized in hydroponic cultivation. The solution strength was set to ‘1.0-1.5-2.0-2.5 dS·m^-1’ according to the growth stage of ‘vegetative period-flowering-fruit enlargement-enhancing sweetness in fruit’. The nutrient solution supply was performed using volumetric moisture content control based on the substrate moisture content sensors for 2 h after sunrise to 2 h before sunset, via a nutrient solution supply system (NMC-PRO PERTIKIT3G, Netafim, Negev, Israel). A substrate moisture content sensor, a frequency domain reflectometry sensor (NetaSence, Netafim, Negev, Israel; minimum value 7%, maximum value 45%), was inserted horizontally in the middle of the medium, and the nutrient solution was supplied through feedback control through monitoring at 30-min intervals. The substrate moisture content was set to ‘21－24%－24－15%－24－29%－29－15%’ according to the growth stage of ‘vegetative period-flowering-fruit enlargement-enhancing sweetness in fruits’ in all the substrates.

2. Mixing ratio of substrates

Coir (Dutch Plantin B.V., Boekel, Netherlands) was mixed with perlite (Parat, Kyungdong-One Co., Ltd., Seoul, Korea) to improve the low cation exchange capacity of the perlite substrates and increase irrigation efficiency. Perlie and coir were mixed in the ratio of perlite: coir = 10:0 (Perlite, single substrates), 8:2, 6:4, 4:6, 2:8, 0:10 (Coir, single substrates) and treated six of hydroponics on watermelon was implemented (Fig. 2). The water content and temperature of the substrate were collected for three treatments: 10:0, 4:6, and 0:10, based on previous studies indicating a gradual increase in water content and temperature with higher mixing ratios of perlite and coir. The water content of the perlite substrates was adjusted as per the irrigation starting point according to the growth stage, and the irrigation amount of all the substrates was the same. The amount of irrigation per plant during the whole growing period was 244 L (3.0 L·day^-1) for small and medium-sized watermelon (Royal-Black) during semi-forcing culture, 233 L (2.9 L·day^-1) for small-sized watermelon (Dalkomi-Mini), and 276 L (4.0 L·day^-1) for small and medium-sized watermelon during retarding culture.

Fig. 2

Various types of substrates such as perlite (A), coir (B), and substrates mixed with perlite and coir (C). Substrate mixing ratio (perlite:coir) is 10:0 (A), 0:10 (B), 8:2, 6:4, 4:6, 2:8 (C).

3. Plant growth, fruit, and yield characteristics

The growth characteristics of watermelon were investigated at weekly intervals (seven times in total) from two to eight weeks after planting. Plant height, number of nodes, number of leaves, leaf length, leaf width, petiole length, node length, and stem diameter were investigated for a total of eight items for the second vine stem. The leaf-related characteristics, such as the leaf length, leaf width, and petiole length were investigated while the leaf was attached to the 3^rd female flower of the watermelon, and the node length and stem diameter of the node immediately below the 3^rd female flower were investigated. Fruit and yield characteristics of watermelon were examined once at approximately 80 days after transplanting (approximately 50 days after fertilization) in semi-forcing culture and approximately 70 days after transplanting (approximately 40 days after fertilization) in retarding culture. A total of 10 fruit-related parameters were investigated: fruit weight, fruit circumference length, fruit length, fruit width, pericarp thickness, sugar content, hardness, fruiting rate, commodity fruit rate, and yield. The sugar content was measured using a Brix meter (Master-53T, ATAGO Co., Ltd., Tokyo, Japan) by pressing and extracting the flesh. The hardness was measured by cutting the watermelon into fruit stalk and blossom end and cutting the central cross section into 5 × 5 × 5 cm in size. The pericarp was cut into 5 × 5 × pericarp thickness cm (width × length × height cm) and measured by penetrating a 5 mm probe (flat) from the center to a depth of 5 mm at a rate of 2 mm·s^-1 utilizing a hardness meter (TA-XT2, Stable Micro Systems Co. Ltd., Godalming, England).

4. Statistical analysis

The experimental design was a completely randomized block design with 4 blocks including 128 plants per treatment. In terms of the characteristics of the analysis of growth, 18 plants per treatment were examined. Among the fruit characteristics, fruiting rate, fruit weight, commodity fruit rate, and productivity were enumerated, and fruit characteristics such as fruit length and fruit width were investigated in five repetitions based on the average fruit weight of all enumerations. Statistical analysis of the results was performed utilizing a SAS (Statistical Analysis System, 9.2 Version, SAS Institute, Cary, NC, USA) to confirm statistical significance by comparing average values of Duncan’s multiple range test in the treatment section.

Results

1. Water content and root zone temperature changes of mixed substrates

In the semi-forcing and retarding cultures, the substrate water content was the lowest at 10:0, followed by the highest water content at 4:6 and 0:10 (Fig. 3). In contrast to the semi-forcing culture, the retarding culture indicated a larger and clearer range of the change in water content between substrates by growth stage. In semi-forcing culture, the 10:0 substrate had a higher temperature than the 0:10 substrate during the vegetative growth period (27/16℃ of day/night (D/N) temperature) when the greenhouse temperature was low. In the fruit enlargement period (28/21℃ (D/N)) the 10:0 substrate indicated a lower temperature than the 0:10 substrate when the greenhouse temperature increased. During retarding culture period (30/24℃ (D/N)), the 10:0 substrate indicated a lower temperature than the 0:10 substrate during the entire cultivation period. During the whole period of semi-forcing culture with relatively low temperature, the accumulated temperature of the perlite substrate was 11°C higher than that of coir. In contrast, during the entire growth period of the high-temperature retarding culture, the accumulated temperature of the perlite substrate was 41°C lower than that of coir.

Fig. 3

Changes in (A) substrate water content during semi-forcing culture, (B) substrate water content during retarding culture, (C) substrate temperature during semi-forcing culture, and (D) substrate temperature during retarding culture according to the mixing ratio of perlite to coir of Royal-Black watermelon.

2. Plant growth characteristics

The plant height was higher in the mixed ratio treatments during the semi-forcing culture of Royal-Black, particularly in contrast to the single substrates of 10:0 and 0:10 (Table 1). Watermelon stem diameter, which has a high correlation with weight during retarding culture, indicated significantly higher values in 10:0 and 8:2 treatments than in other treatments. Dalkomi-Mini, a small watermelon, indicated a significantly higher watermelon stem diameter, leaf length, and leaf width in coir and 8:2 treatments than other treatments (Table 2).

3. Fruit and yield characteristics

During semi-forcing culture, Royal-Black showed no significant difference between the mixed substrate and the single substrate, but it demonstrated a numerically higher fruit weight in the mixed substrate (Table 3). The highest fruit weight and sugar content were presented in the 4:6 mixing ratio. A single substrate, 10:0, indicated significantly higher values in the fruit weight and positively correlated items such as fruit circumference length, fruit length, and fruit width than other treatments, and these values presented 6%, 15%, and 7% improvement, respectively, in contrast to 0:10 which indicated the lowest value. Here, 0:10 indicated the lowest value among all the treatment groups in all fruit characteristics such as fruit weight, fruit circumference length, fruit length, and sugar content. In the Royal-Black retarding culture, the fruit weight tended to increase as the perlite mixing ratio in the substrate increased (Table 3). The 10:0 treatment indicated the highest fruit weight among all the treatments, which was significantly higher by 51% in contrast to the lowest fruit weight, 0:10. Fruit circumference length, fruit length, and fruit width also indicated similar trends to fruit weight, and the 10:0 treatment indicated significantly higher values than the 2:8 and 0:10, which had a high coir ratio.

Dalkomi-Mini, a small watermelon tended to increase the fruit weight as the mixing ratio of coir increased which is the opposite direction of the small and medium-sized watermelon, Royal-Black. The fruit weight of 0:10 was 2.3 kg which is the highest weight, in contrast to all treatment groups (Table 4) and this was 35% higher weight than 10:0, which indicated the smallest fruit weight at 1.7 kg. The fruit length and fruit width were also significantly higher than all treatment groups at 0:10. Generally, small watermelons have the highest quality with a fruit weight of 1.5－2.0 kg rather than large ones. However, if the fruits are smaller or larger than that, they may be inappropriately priced. Therefore, the fruit setting rate per plant and several times of harvest with the hand size of uniform fruits (fruit weight 1.0 －2.5 kg) helps improve income. Table 5 presents the product quantity of small watermelons reflecting the fruit setting rate. In the 10:0 substrate, the fruit setting rate per plant was 18% and the fruit setting rate of two fruits per plant at 82% was the highest. For the mixing substrate (8:2－2:8), the fruit setting rate of two fruits per plant was 39－55%, the fruit setting rate of one fruit per plant was 65% which is the highest among all the treatment groups, and the fruit setting rate of two fruits per plant was the lowest at 35% at the 0:10 (coir). The 10:0 substrate indicated the smallest difference in contrast to all other treatments, and the proportion of uniform fruit was high as for the fruit weight difference between the two fruits in the fruit setting of two fruits per plant. The substrate 0:10 indicated the largest fruit weight difference. In contrast to all the other treatments, it increased six times to the 10:0 substrate, which intensified the nutrient bias. Among the harvested fruits excluding the ones less than 1.0 kg and more than 2.5 kg, the 10:0 substrate was the highest at 89%, and the 0:10 was the lowest at 58%. As a result, the product quantity increased by 97% from the 10:0 substrate to the 0:10.

Discussion

During the cultivation of the fruit vegetables, the amount of irrigation should be appropriately controlled as the growth progresses. In terms of the Mudeungsan watermelon, approximately 0.3 to 0.5 L of nutrient solution is supplied per plant during the seedling stage, but as the growth becomes vigorous, up to 3 to 4 L of water could be supplied per plant (Park et al., 1998a). This may be interpreted as a gradual increase in transpiration due to an increase in leaf area. After all, water uptake increases six to ten times during the peak growth period compared to the seedling period. When using perlite or coir alone, the rhizosphere’s over-humidifying or over-drying environmental conditions are easily created, depending on the weather conditions. However, using a medium with a proper mixture of perlite and coir can protect the plant from such adverse environments (Park et al., 1999).

In this study, the water content of the substrate treatments was the lowest at 10:0, and the highest water content in the order of 4:6 and 0:10, regardless of the cultivation period (Fig. 3). In an experiment to study the water content of the mixed substrate of peat moss and perlite, most of the pores were filled with water in single peat moss, while the pores were filled with air in a single perlite. As the peat moss and perlite are mixed, the composition ratio of the liquid phase and gas phase are formed appropriately (Heiskanen, 1995; Verdonck and Demeyer, 2001). Excessive fluctuations in substrate water content can induce physiological disorders, such as flower rot (Sezen et al., 2010). Therefore, maintaining adequate water supply in the substrate during the flowering and fruit formation periods is essential for maximizing yield (Blum, 2005). In this study, the trend of increased fruit weight for medium-sized watermelons (one fruit per plant) and small watermelons (two fruits per plant) showed difference according to the substrate used. Because the water requirements vary significantly based on the genetic traits and physiological responses of the varieties, it is concluded that the differences in fruit weight between the two varieties are attributed to the absorption tendencies of water and nutrients for each variety.

The root zone temperature change by the substrate of mixing ratio was that 10:0 of substrate indicated a higher temperature than 0:10 of the substrate in the greenhouses with a low temperature (27/16℃ (D/N)), and 10:0 of substrate indicated a lower temperature than 0:10 of the substrate in the greenhouses with a high-temperature (28－30/21－24℃ (D/N)) (Fig. 3). This coincided with the research results demonstrating that the temperature of the root zone of Gynura procumbens in a high-temperature period (May－June) was the highest for coir, followed by vermiculite and perlite (Lee et al., 2020).

Root zone temperature is a major environmental factor that may delay or promote growth and maturation in fruit cultivation (Adams et al., 2001). In the substrate, temperature change of the substrate has an effect than soil cultivation and the root zone temperature may rely on the characteristics of the container (Fretz, 1971; Ingram, 1981; Verma, 1979). Therefore, the smaller the volume of the container surrounding the substrate, the larger the temperature change (Giuffrida, 2000). In the condition where the root zone temperature is too high or too low, it is contrary to the optimum temperature for growth which adversely affects photosynthesis (Gosselin and Trudel, 1984) and growth (Lambers et al., 2008). In tomatoes, sustaining the optimal root zone temperature (20－26℃) throughout the growing season improved growth and yield and reduced the incidence of disease (Díaz-Pérez et al., 2007; Jones, 2007; Kenndy et al., 1993). In cucumbers, the root weight, leaf area, leaf biomass, and mineral absorption decreased as the root zone temperature was higher than 35℃. On the contrary, the beneficial root growth and the contents of sugar, malic acid, and fumaric acid are increased at 25－35℃ (Du and Tachibana, 1994). In previous studies on the investigation of the nutrient absorption rate conforming to the root zone temperature, boron, iron, and molybdenum were irrelevantly affected by the tomato root zone temperature. Nevertheless, nitrogen, which is intimately related to crop growth, recorded its maximum absorption at 26.7℃ (Tindall et al., 1990).

The growth characteristics of small and medium-sized watermelons during a low-temperature period (semi-forcing culture) indicated significantly higher plant growth in all mixed ratio treatments in comparison with the single substrates, 10:0 and 0:10 (excluding 8:2 processing) (Table 1). In the previous substrate cultivation experiment of the Mudeungsan watermelon, growth characteristics such as plant height, fresh weight, and dry weight were excellent in the coir and perlite mixed substrate, and the leaf area was high in the order of coir and perlite mixed substrate > perlite (single) > coir (single) (Park et al., 1999). Similar to the results of the vegetative growth, the fruit characteristics of small and medium-sized watermelons (Royal-Black) demonstrated no meaningful difference in the mixed substrate treatment in contrast with the single substrate treatment. However, numerically high fruit weight was recorded, and the highest fruit weight and sweetness were observed at a 4:6 (perlite:coir) mixing ratio (Table 3). We consider that the root zone was not over-moistened or dried through the high physicochemical stability of the mixed substrate in the semi-forcing culture. The research result was consistent with the fact that the mixed substrate of coir and perlite exhibited a higher yield than a single substrate during watermelon hydroponic cultivation (Park et al., 1999).

Stem diameter has a high correlation with fruit weight during retarding culture, indicating significantly higher values in 10:0 and 8:2 treatments than in other treatments (Table 1). This is considered as a result of the tendency for fruit weight to increase as the mixing ratio of perlite in the substrate increases and is considered to be connected (Table 3). A significant increase in fruit weight was noted in perlite substrate, which had lower water content and temperature compared to coir substrate during retarding culture, but no significant difference in fruit weight was observed during semi-forcing culture. This indicates that reduced root vitality during high temperatures negatively impacted water absorption, thereby affecting watermelon fruit weight. The primary reason for the increase in fruit weight in the case of retarding culture takes into account the substrate with a high rate of perlite maintaining the root zone temperature relatively low despite the higher temperature than the effect of the physicochemical stability of the substrate.

The fruit weight of the Dalkomi-Mini tended to increase as the mixing ratio of coir increased that is manifesting the highest fruit weight at 0:10 with the highest moisture content of the substrate (Table 4). Coir substrate has high water retention capacity, but it is composed of negatively charged molecules, which can lead to the adsorption of cations and affect nutrient absorption in crops (Resh, 2016). In small-sized watermelons, the rapid increase in fruit size during the formation period is less pronounced compared to medium and large-sized watermelons. Therefore, the impact of nutrient imbalance during the early stages of fruit development in coir substrate was minimal. However, as the fruits grew larger, this impact increased. The fruit setting rate of two fruits per plant was the lowest at 35%, and the fruit weight of two fruits set in the same individual presented the largest fruit weight difference in comparison with all treatment groups, which was six times more severe than 10:0 substrate in the nutrient bias (Table 5). Among the harvested fruits, the yield rate of small watermelon fruit (more than 1.0 kg to less than 2.5 kg) increased by 97% in the 10:0 substrate in contrast to the 0:10. Consequently, it is anticipated that small and uniform fruit-weight watermelons may be produced without nutrient bias on the perlite with low water retention rather than small watermelons with higher fruit weight through increasing substrate moisture content, which helps in increasing yield.