Cultivation of Watermelon (Citrullus lanatus (Tunb.)) in a Temperate Climate: Agronomic Strategies and Phytochemical Composition

Abstract

This study investigates the phytochemical composition and cultivation strategies for watermelon (Citrullus lanatus (Thunb.)) in Lithuania’s temperate climate, focusing on its biological activity. Employing innovative grafting techniques and clear plastic film mulching, we successfully countered fusarium wilt while promoting growth and bioactive compound accumulation. Our analysis showed significant cultivar-dependent variations in total phenolic content (ranging from 94.34 ± 8.12 to 327.42 ± 9.14 mg GAE/kg fw in pulps and from 120.46 ± 7.52 to 364.27 ± 6.85 mg GAE/kg fw in rinds), lycopene (ranging from 1.15 ± 0.42 to 103.60 ± 1.69 mg/kg fw in pulps), sugar, and nitrate levels, revealing the influence of genetics and environment on the fruit’s phytochemical profile. Moreover, several Lithuanian watermelon cultivars exhibited comparable or superior levels of key bioactive compounds relative to imported varieties. These findings underscore the potential of watermelon rind and pulp as valuable sources of antioxidants and other bioactive phytochemicals, relevant for nutritional enhancement and medicinal applications. The results contribute to a deeper understanding of watermelon cultivation in Lithuania and highlight opportunities for optimizing agricultural practices to enhance the health benefits associated with this important fruit.

1. Introduction

Fruits and vegetables are considered natural and valuable sources of various bioactive compounds [1]. These substances play an important role in maintaining functions of the human body and improving health [2,3]. In general, consumption of fruits and their products has been shown to reduce the risk of many chronic diseases, such as cardiovascular disorders [4], type II diabetes [5], cancer [6], and various neurodegenerative and ageing-related diseases [7].

Watermelon (Citrullus lanatus (Thunb.)) is a popular dessert fruit originating from Africa and is grown in favorable climates from the tropics to Mediterranean regions of the world for its large edible fruit, which is a berry, botanically called a pepo, with a thick skin (exocarp) and a fleshy center (mesocarp and endocarp) [8]. Watermelon is a scrambling, prostrate, annual vine-clad member of the Cucurbitaceae (Juss.) family, which contains more than 800 different species [9]. Watermelon is a tropical fruit, well adapted to warm, seasonally dry habitats. In line with these assumptions, cultivating watermelons in temperate climates, like the one in Lithuania, is challenging. The fluctuation of temperature and humid climate favors numerous foliar diseases. Soilborne diseases such as fusarium wilt are one of the major issues, challenging growers in Lithuania and worldwide. Fusarium wilt of watermelon is caused by the fungal pathogen Fusarium oxyporum f. [10]. Commercially available chemical fungicides are ineffective against fusarium wilt. Due to its persistence in the soil and the ability of this pathogen to mutate into new races, fusarium wilt control is complicated [11]. Grafting watermelons onto squash or gourd rootstocks that are resistant to Fusarium oxyporum f. has gained great importance due to its efficacy against biotic and abiotic stressors [12]. Even though it is a time- and labor-consuming technique, grafting can increase the survival rate of grafted C. lanatus seedlings remarkably [13]. Mulching the soil with various types of films is often applied in watermelon cultivation [14]. Keeping in mind the origin and biology of C. lanatus, mulching is an important tool to increase soil temperature and protect transplants from excessive rainwater. In addition, selecting the optimal film might be the key factor for higher yields, respectively.

Despite the predominant view that watermelon is essentially only water and sugar, scientific research shows that watermelon is really a plant matrix, containing a body of various bioactive compounds that can enrich our daily diet [9]. Bioactive compounds identified in watermelon fruit flesh (endocarp) include carotenoids (lycopene, β-carotene) [15], phenolic compounds [16], flavonoids [17], citrulline and other amino acids [18], curcubitacins (oxygenated steroidal triterpenes) [9,19], vitamins (B, A and C) [20], and minerals (K, Mg, Ca, Fe) [21]. In this context, including watermelon in our diet has been associated with positive biological effects, such as antibacterial properties [22], anti-atherosclerotic activity and protection against cardiovascular diseases [23], and antioxidant [3] and anti-inflammatory activity [24]. Therefore, watermelon is an increasingly attractive crop from both nutritional and economic perspectives. Consequently, consumer demand for high-quality, nutrient-rich watermelons is growing, particularly in temperate regions where cultivation is challenging due to suboptimal climatic conditions.

Since watermelon is a new horticultural crop in Lithuania, there are no comprehensive studies that would be performed on the qualitative and quantitative composition of bioactive compounds in watermelon fruits, cultivated in the Lithuanian climatic conditions. Moreover, the results of scientific research show that the amounts of phenolic compounds, lycopene and citrulline, depend on the variety of watermelon fruit, so it is relevant to perform research on the diversification of the phytochemical composition of different varieties and hybrids [17,25]. In addition, nowadays the consumers are demanding high quality and healthy fruits and vegetables. The sweetness of watermelon fruits is the vital characteristic in defining fruit organoleptic quality and contributes to the popularity of this dessert fruit [26]. The sweet taste of watermelon is attributed to the stimulation of taste buds on the tongue by the monosaccharides glucose and fructose, as well as the disaccharide sucrose [21]. However, it remains unknown whether watermelon fruits cultivated in Lithuania accumulate enough sugar to fulfil quality requirements important for consumers.

C. lanatus fruits, like other fruits, are known to accumulate nitrates as well. As a result, the presence of excessive nitrite and nitrate in fruits can have toxic and carcinogenic effects on the human body [27]. Moreover, the cancer risks of these two ions have been observed since they potentially convert into carcinogenic nitrosamines [28]. However, to the best of our knowledge, only a few studies focused on nitrate content in watermelon fruits. Therefore, it is important to determine the variation in nitrate content in watermelon cultivars due to the potential health threat to humans when present in high concentration.

Therefore, this study aims to develop a cultivation strategy for watermelons adapted to Lithuania’s natural conditions and assess the quantitative composition of phenolic compounds, lycopene, sugar content, and nitrates in locally cultivated fruits. By comparing these parameters with watermelons grown in other countries, we seek to determine whether Lithuania’s climatic conditions allow for producing high-quality watermelons that meet consumer expectations and market demands. The findings will provide valuable insights for the optimization of cultivation techniques in temperate regions.

2. Materials and Methods

2.1. Watermelon Cultivation

2.1.1. Grafting Technique and Post-Grafting Management

The rootstock and scion plants were 14 days old when grafting was performed. The first one or two true leaves appeared, but had not yet reached full development both in the rootstock and scion seedlings. Gourd (Lagenaria siceraria) seedlings were employed as rootstock (Figure 1A). A relatively simple side grafting technique has been chosen, as introduced by Hassell et al. [12]. The technique has been slightly modified. The top portion of the rootstock was cut off a day prior to grafting. A two-edge sharp razor blade was used to cut a slit (1–1.5 cm) all the way through the stem of the rootstock below the cotyledons. The slit was long enough to insert the scion. The scion was cut below the cotyledons at a 45° angle on two sides to form a wedge (Figure 1B), following the insertion of the scion into the slit of the rootstock as displayed in Figure 1C. Finally, the scion was held in place with a grafting clip (Figure 1D).

The grafted plants were placed in the healing chamber for 7–10 days and kept in a relative humid environment (60–80%) at 18–20 °C, protected from direct sunlight. Although vascular connection was established between scion and rootstock at approximately 7–10 days after grafting, it took another 7–10 days for the graft union to fully heal. The grafted plants were slowly acclimatized without causing permanent wilting. Then, the transplants were transferred to the greenhouse and kept for an additional 7 days so they could harden off before transplanting outside.

2.1.2. Preparation of Plant Growth Beds and Soil Mulching

Plant growth beds were prepared at a 1.5 m distance from each other. For the selection of mulching films, the soil was covered with four different types of films: black polyethylene (PE) film (0.5 μm), clear polyethylene (PE) film (0.5 μm), polypropylene (PP) black film (20 g/m²), and polypropylene (PP) white film (20 g/m²), and was maintained for 24 h. The soil temperature was measured under each film using a non-contact thermometer (‘Microlife MC 200’, Microlife corporation, Baar, Switzerland). Mulching film was laid down on the beds, and holes were opened at 1 m × 1 m for the planting of grafted watermelon seedlings. Approximately thirty-day-old seedlings were transplanted in the first week of June.

2.2. Raw Material

The object of this study is the pulp and rind of watermelons. The fruits of different cultivars and hybrids of C. lanatus grown under the natural conditions of Lithuania are analyzed. The watermelons were cultivated in the southern part of Lithuania, in the village of Obelija, Alytus district (54°18′17.16″ N, 23°52′0.05″ E). The raw materials for this study were collected in August, when the fruits were fully ripe. Prior to the analysis, the raw materials were frozen and stored in the freezer at −18 °C. Data on the collected plant herbal raw materials are provided in

Table 1. Varieties and hybrids of C. lanatus grown under Lithuania’s natural conditions and their characteristics.

No.	Name	Type	Flesh Color	Days to Harvest	Fruit Weight
1.	‘Farao’	Hybrid	Red	75–85	8.0 kg
2.	‘Producer’	Cultivar	Red	75–80	8.0 kg
3.	‘Melia’	Hybrid	Red	55–60	5.2 kg
4.	‘Demre’	Hybrid	Red	70–75	7.3 kg
5.	‘Great Pekin’s Joy’	Hybrid	Red	80–85	3.3 kg
6.	‘Orange King’	Hybrid	Orange	60–65	8.2 kg
7.	‘Huelva’	Hybrid	Red	55–60	6.0 kg
8.	‘Karolina’	Hybrid	Red	65–69	12.0 kg
9.	‘Padarak Severu’	Hybrid	Red	75–85	10.4 kg
10.	‘Moon Star’	Cultivar	Red	90–95	11.8 kg
11.	‘Ice Cream’	Cultivar	Red	81–90	5.5 kg
12.	‘Janosik’	Cultivar	Yellow	75–90	3.4 kg
13.	‘SRD-2’	Cultivar	Red	55–60	7.5 kg

. The watermelons imported into Lithuania from foreign countries (Ukraine, Greece, Spain, Hungary, and Russia) were purchased at a supermarket for a comparative phytochemical composition analysis.

2.3. Preparation of Watermelon Extracts

The thawed pulp of 3–4 individual watermelons was homogenized using Philips daily collection blender (Philips, Amsterdam, The Netherlands), to obtain a uniform mass, with seeds removed from the raw material if present. The flesh from various cultivars of C. lanatus was employed for the production of hydro-ethanolic extracts. A raw material-to-ethanol ratio of 1:5 was selected, as described by Tan Mei Chin et al. [29]. A quantity of 2 g (±0.01 g) of the plant raw material was weighed and subsequently placed into 25 mL amber glass vials. Then, 10 mL (±0.1 mL) of 70% (v/v) ethanol was added. The vial was carefully shaken to ensure that the raw material was mixed with the solvent and distributed evenly. An ultrasound-assisted extraction (operating frequency—40 kHz, ambient temperature) was performed for 15 min. The resulting extracts were filtered through a filter moistened with distilled water into amber glass vials, which were then labeled. Until further analysis, the extracts were stored in tightly sealed containers at ambient temperature, protected from light. Three extracts were prepared from each herbal plant raw material sample. Hydroethanolic extracts from the rinds of watermelon fruits were prepared analogously to those from the pulp, with the exception that instead of the homogenized flesh, a uniform mass obtained by blending the rind of watermelons with Philips daily collection blender (Philips, Amsterdam, The Netherlands), was used.

2.4. Spectrophotometric Assays

2.4.1. Determination of Total Phenolic Content (TPC)

The spectrophotometric Folin–Ciocalteu method, based on color reaction, was used to determine the total phenolic compound content in ethanol extracts of the pulp and rind of C. lanatus. The assay was performed according to the method described by Babbar et al. [30]. Briefly, the sample solution was prepared by mixing 1 mL of the extract with 4 mL of a 7.5% sodium carbonate solution and 5 mL of the prepared Folin–Ciocalteu reagent. The resulting solution was kept in the dark at an ambient temperature for 2 h. The reference solution was prepared in the same way as the sample solution, except that 1 mL of watermelon pulp or rind extract was replaced with 1 mL of 70% (v/v) ethanol. The absorbance of the reaction mixture was measured at 765 nm using a spectrophotometer (‘CamSpec-M550’, Cambridge, UK). The results were expressed as mg gallic acid equivalent per kg of fresh weight (mg GAE/kg fw). The TPC content was determined by comparing the obtained absorbance value to the gallic acid standard calibration curve (y = 9.3172x − 0.0148; R² = 0.9987), which was generated from standard gallic acid solutions (0.0125 mg/mL–0.1 mg/mL).

2.4.2. Determination of Lycopene

The lycopene content was assayed by performing extraction with a mixture of hexane, ethanol, and acetone (2:1:1), according to the method described by Khamisa et al. [31]. The thawed pulp of watermelons was blended Philips daily collection blender (Philips, Amsterdam, The Netherlands), until a homogeneous mass was obtained, after removing the seeds if present. A weight of 0.4 g (±0.01 g) of the crushed watermelon pulp was added to a measuring flask, followed by 20 mL of the extractant mixture. The flasks with the obtained solutions were covered with “Parafilm” and shaken in a vortex shaker for 15 min. Then, 3 mL of distilled water was added, and the mixture was shaken for another 5 min. The resulting solutions were left to stand at room temperature for 5 min to allow phase separation. As a reference solution, a mixture of extraction solvents—hexane, ethanol, and acetone (2:1:1)—was used. The absorbance of the upper layer of the solution was measured with a spectrophotometer at 503 nm. The lycopene content in the extracts was expressed as mg/kg fw.

2.4.3. Determination of Nitrate

The nitrate content was measured according to the method described by Gaya et al. [32]. The pulp of C. lanatus was blended with Philips daily collection blender (Philips, Amsterdam, The Netherlands), until a homogeneous mass was obtained, after removing the seeds if present. A weight of 10 g (±0.1 g) of the resulting mass was added to a 250 mL flask, followed by 70 mL of distilled water and 2.5 mL of 4% sodium hydroxide solution. The contents of the flask were heated for 25 min at a temperature of 80 °C, with occasional shaking. The resulting solution was filtered through a filter paper moistened with distilled water into a 100 mL volumetric flask and diluted with distilled water up to the mark. A 4 mL sample was taken from the obtained solution, and the tube containing it was cooled. Sequentially, 1 mL of 5% silver sulfate solution, 7 mL of 98% sulfuric acid, and 0.1 mL of 5% phenol solution were added to the test tube. The resulting solution was left to stand for 20 min, shaking occasionally. Then, the solution was extracted in a separatory funnel by adding toluene and shaking for 5–10 min. The lower aqueous layer was discarded. The organic phase was washed twice with 10 mL of distilled water, discharging it from the separatory funnel each time. The organic phase was then extracted again with 10 mL of 10% sodium carbonate solution and collected into a test tube. The absorbance of the obtained solution was measured with a spectrophotometer at 407 nm. The nitrate content was determined by comparing the obtained absorbance value to the calibration curve (y = 0.0165x + 0.1698, R² = 0.9962) which was generated of the standard nitrate solutions (0.5 µg/mL–20 µg/mL).

2.5. Determination of Sugar Content Using a Refractometric Method

Approximately 10 g of true watermelon pulp was blended into a homogeneous mass using Philips daily collection blender (Philips, Amsterdam, The Netherlands), with seeds removed if present. The resulting mass was strained through a filter cloth into an amber glass vial. The juices extracted from the watermelon flesh were subsequently utilized for analysis and stored at room temperature in tightly sealed vials. Before conducting the analysis with the test sample, it is essential to calibrate the refractometer using distilled water. All measurements were performed on a sugar Brix refractometer, measuring the °Brix. The operational theory used for refractometry is presented by Serpen [33].

2.6. Reagents

The following reagents were used in this study: ethanol 96% (v/v) (AB Stumbras, Kaunas, Lithuania); sodium carbonate (Carl Roth GmbH & Co, Karlsruhe, Germany); Folin–Ciocalteu reagent (Sigma Aldrich, St. Louis, MO, USA); gallic acid monohydrate (Sigma Aldrich, St. Louis, MO, USA); hexane (Sigma Aldrich, St. Louis, MO, USA); acetone (Sigma Aldrich, St. Louis, MO, USA); sodium hydroxide (Sigma Aldrich, St. Louis, MO, USA); silver sulfate (Sigma Aldrich, St. Louis, MO, USA); sulfuric acid (Sigma Aldrich, St. Louis, MO, USA); phenolic crystalline powder (Sigma Aldrich, St. Louis, MO, USA); toluene (Sigma Aldrich, St. Louis, MO, USA); and nitrate standard solution 1000 mg/L (Sigma Aldrich, St. Louis, MO, USA). The purified deionized water used in the experiments was prepared by a Milli-Q^® (Millipore, Bedford, MA, USA) water purification system.

2.7. Statistical Analysis

Data analysis was performed with computer software programs Microsoft Excel 2016 (Microsoft, Redmond, DC, USA) and SPSS Statistics 30 (IBM, Armonk, NY, USA). The experiments were repeated three times, and arithmetic means were calculated from the obtained data, as well as standard deviations (SDs). Statistical significance (p < 0.05) was assessed using one-way ANOVA, applying Tukey’s multiple comparison test. Differences at p < 0.05 were considered significant. The correlation was determined by Pearson’s analysis. Additionally, samples of fruits from different varieties/hybrids of watermelons were compared based on the determined quantities of phenolic compounds in the pulp and rind, the lycopene content in the flesh, and the total sugar content in the juices, using hierarchical cluster analysis (HCA). The farthest neighbor clustering algorithm was applied for data fusion, using Euclidean distance as a similarity measure.

3. Results and Discussion

3.1. Selection of Suitable Cultivation Techniques for the Natural Climatic Conditions of Lithuania

Watermelon is traditionally considered a tropical fruit, thriving in hot, sunny environments [8]. Local climatic conditions necessitate the implementation of specific cultivation techniques to enhance the viability and productivity of watermelon crops. The primary challenge in cultivating watermelon in Lithuania is adapting this heat-loving plant to a cooler, temperate climate. The short growing season limits the time available for the plants to mature, leading to potential yield reductions. Furthermore, watermelon plants are vulnerable to various diseases, particularly fusarium wilt, caused by the fungal pathogen Fusarium oxyporum f., compounded by environmental stressors from inconsistent moisture levels and soil temperatures [11]. Effective management strategies are essential to mitigate these issues and enable the successful production of watermelons in Lithuania.

One effective method to combat the susceptibility of watermelon to diseases like fusarium wilt is grafting. This technique involves joining a scion (the desired watermelon variety) onto a rootstock that is resistant to disease. By utilizing resistant rootstocks, growers can significantly reduce the incidence of soil-borne pathogens and improve overall plant health [34]. The side grafting technique was employed due to its advantages in seamless integration and vigorous rootstock growth [12]. Our study revealed that grafted plants showed the first symptoms of Fusarium wilt 3 weeks later than those with natural roots, demonstrating that grafting significantly delays the onset of Fusarium wilt symptoms. This period is crucial for the full watermelon fruit development. Grafted plants exhibit stronger and more extensive root systems compared to their non-grafted counterparts as well. This enhanced root structure facilitates better nutrient and water uptake, crucial for watermelon growth in challenging climatic conditions (Figure 2).

In addition to grafting, soil mulching is another vital technique to optimize watermelon cultivation in Lithuania. The application of various types of films for mulching serves multiple purposes, primarily to increase soil temperatures and provide protection against adverse weather conditions [14]. We evaluated the effects of various mulching films, including black PE film (0.5 μm), clear PE (0.5 μm), black PP film (20 g/m²), and white PP film (20 g/m²), on soil temperature regulation and plant growth in watermelon cultivation. The following Figure 3 presents the mean soil temperatures (°C) recorded under different mulching treatments. As the data demonstrate, the use of PE clear film resulted in the highest mean soil temperature (22.73 °C), significantly higher (p < 0.05) than the uncovered soil (14.7 °C). This highlights the potential of mulching to create a more favorable thermal environment for watermelon growth. The black PE film also showed a significantly positive effect, resulting in a higher temperature than the uncovered soil (19.6 °C versus 14.7 °C). It is important to note that the PP films, both black and white, did not significantly increase the temperature compared to the uncovered control. While a direct quantitative comparison with the findings of Rao et al. is limited by methodological differences (e.g., specific mulch types and climatic conditions), our results generally support their observation that mulching enhances soil temperature. Similar to their findings, we discovered that black PE film and, most significantly, clear PE film (0.5 μm) increased the mean soil temperature compared to the uncovered control [35]. The data further illustrate that plants planted with PE mulch exhibited shoots that were 25–33% (p < 0.05) longer after three weeks of outdoor vegetation compared to those grown without mulch, supporting optimal growth conditions.

The selected side grafting and mulching techniques described in this study were applied to all 13 watermelon varieties and hybrids used in the phytochemical analysis. All varieties were grafted onto Lagenaria siceraria rootstock to improve disease resistance and enhance nutrient uptake. Following grafting, plants were subjected to clear PE film mulching to ensure optimal growth conditions. Further research is needed to optimize mulching techniques and explore alternative disease management strategies to ensure sustainable watermelon production in Lithuania’s climate.

3.2. Total Phenolic Content (TPC) in Watermelon Pulps and Rinds

3.2.1. Determination of TPC in Pulps

Beyond its organoleptic qualities, watermelon possesses a rich profile of bioactive compounds, including polyphenols, which have garnered significant attention for their potential health benefits [36]. Polyphenols, a diverse group of secondary metabolites, exhibit a wide range of biological activities, including antioxidant, anti-inflammatory, and antimicrobial properties [37,38]. While the nutritional composition of watermelon flesh (pulp) has been extensively studied, the phytochemical profile of the rind remains relatively less explored despite its potential contribution to the overall nutritional and bioactive value of the fruit [39]. This study investigates the TPC in both the pulp and rind of various watermelon varieties and hybrids grown under natural conditions in Lithuania. Analysis of watermelon pulp from 13 cultivars/hybrids grown in Lithuania revealed a significant range in TPC, ranging from 94.34 ± 8.12 to 327.42 ± 9.14 mg (GAE)/kg fw (Figure 4). The highest TPC was determined in sample of ‘Padarak severu’ hybrid (327.42 ± 9.14 mg GAE/kg fw), significantly exceeding the lowest concentrations detected in ‘Janosik’ (94.34 ± 8.12 mg GAE/kg FW) and ‘Great Pekin’s joy’ (104.54 ± 9.31 mg GAE/kg FW) (p < 0.05). No statistically significant difference (p ≥ 0.05) was identified between ‘Padarak severu’ and ‘SRD-2’ (312.04 ± 7.00 mg GAE/kg FW) samples, which also exhibited high TPC accumulation. The sample of cultivar ‘Janosik’ displayed a 3.47-fold lower concentration of TPC than detected in hybrid ‘Padarak severu’. The mean polyphenol content across all Lithuanian cultivars was 202.80 ± 75.76 mg GAE/kg fw.

The hierarchical cluster analysis (Figure 5) revealed two distinct clusters (I and II) based on pulp TPC, each further subdivided into two sub-clusters (A and B). Cluster I, characterized by lower TPC levels, contained sub-cluster A (‘Moon star’—174.66 mg GAE/kg fw, ‘Ice cream’—175.55 mg GAE/kg fw, ‘Orange king’—127.08 mg GAE/kg fw, ‘Huelva’ 129.94 mg GAE/kg fw) and sub-cluster B (‘Great Pekin’s joy’—104.54 mg GAE/kg fw, ‘Janosik’—94.34 mg GAE/kg fw). Cluster II, exhibiting higher TPC levels, comprised sub-cluster A (‘Padarak severu’—321.34 mg GAE/kg fw, ‘SRD-2’—312.04 mg GAE/kg fw) and sub-cluster B (‘Farao’—254.26 mg GAE/kg fw, ‘Producer’—272.69 mg GAE/kg fw, ‘Melia’—235.12 mg GAE/kg fw, ‘Demre’—219.38 mg GAE/kg fw, ‘Karolina’—209.36 mg GAE/kg fw).”

Recent studies suggest that the biosynthesis of carotenoids and phenolic compounds may follow distinct, and sometimes inversely related, metabolic pathways during fruit development. For instance, in Rubus chingii fruits, carotenoid accumulation increases while phenolic content declines as the fruit ripens, indicating a potential metabolic relationship between these two classes of bioactive compounds [40].

Further research is needed to explore the underlying genetic and metabolic factors contributing to potential correlations between the biosynthesis of polyphenolic compounds and fruit pigmentation. Higher TPC levels are often associated with enhanced antioxidant activity and potential health benefits [36,37]. This suggests that cultivars within Cluster II might be more beneficial in diets rich in polyphenols, which are linked to reduced risk of various diseases. By exploring the links between fruit pigmentation and polyphenol biosynthesis, there is a significant opportunity to improve the nutritional quality of fruits through targeted agricultural practices, and understanding the relationship between fruit flesh pigmentation and TPC can guide breeding programs aimed at enhancing polyphenol content.

3.2.2. Determination of TPC in Rinds

While the pulp and juice of watermelon are commonly consumed, the rind is often discarded as agricultural waste, despite accounting for approximately 30–50% of the total fruit weight [36,41]. This practice is particularly concerning, given that the rind is a rich source of bioactive compounds, notably polyphenols [42,43]. The fruit by-products commonly contain higher levels of bioactive compounds than their respective pulps, indicating that these often-discarded parts are rich sources of natural antioxidants [44]. The considerable variability in TPC across different watermelon cultivars [17] further emphasizes the need for research focused on this underutilized by-product. This study addresses this gap by investigating the TPC in watermelon rinds from various cultivars grown under natural conditions in Lithuania. The analysis of TPC, coupled with a comparative analysis of Lithuanian cultivars, and an exploration of potential industrial and nutraceutical applications of the rind, contributes towards a more sustainable utilization of watermelon and enhances the understanding of the health benefits associated with its consumption. Statistical analysis revealed a significant positive correlation (r = 0.722, p < 0.05) between the total phenolic content in watermelon pulp and rind samples, indicating that cultivars and hybrids that tend to accumulate greater amounts TPC in the pulp also exhibit higher levels of TPC in the rind. The biochemical underpinnings of this correlation may be attributed to shared metabolic pathways involved in the biosynthesis of phenolic compounds [45]. Fruit rinds and peels often accumulate higher concentrations of bioactive compounds than pulps due to their role as protective barriers, where they synthesize secondary metabolites like polyphenols and flavonoids to defend against environmental stressors, such as UV radiation, pests, and pathogens. These outer tissues are metabolically active zones where the production of antioxidants and defense-related compounds is enhanced to ensure fruit integrity and survival during development and storage [46].

In addition, environmental factors such as light exposure, soil conditions, and water availability can affect the overall health of the plant and influence the accumulation of phenolic compounds in both fruit tissues. Researchers have shown that stress responses in plants can lead to enhanced production of polyphenols, and this may manifest similarly in both the pulp and rind due to their interconnected physiological processes [47].

The analysis of watermelon rind from 13 cultivars/hybrids grown in Lithuania indicated a total polyphenol content ranging from 120.46 ± 7.52 to 364.27 ± 6.85 mg GAE/kg fw (Figure 6). The highest concentration was observed in ‘Padarak severu’ (364.27 ± 6.85 mg GAE/kg fw) (p < 0.05), significantly exceeding the lowest value in ‘Huelva’ (120.46 ± 7.52 mg GAE/kg fw). However, Huelva’ did not differ significantly from ‘Janosik’ and ‘Great Pekin’s joy’ (p ≥ 0.05). The mean polyphenol content across all the Lithuanian rind samples was 224.34 ± 76.64 mg GAE/kg fw.

The hierarchical cluster analysis (Figure 7) revealed three distinct clusters (I, II, and III) based on TPC in the rinds of various watermelon cultivars and hybrids. Cluster I is characterized by lower-than-average TPC levels, including cultivars such as ‘Great Pekin’s joy’ (133.16 ± 5.38 mg GAE/kg fw), ‘Janosik’ (133.52 ± 5.60 mg GAE/kg fw), ‘Huelva’ (120.45 ± 7.52 mg GAE/kg fw), and ‘Ice cream’ (15.77 ± 8.39 mg GAE/kg fw). These samples exhibit similarly low levels of TPC, suggesting potential common genetic or environmental influences affecting their polyphenolic accumulation. Cluster II demonstrates moderate TPC levels and consists of ‘Farao’ (233.22 ± 9.12 mg GAE/kg fw), ‘Karolina’ (229.93 ± 8.06 mg GAE/kg fw), ‘Demre’ (223.31 ± 4.03 mg GAE/kg fw), ‘Melia’ (250.86 ± 7.29 mg GAE/kg fw), and ‘SRD-2’ (240.48 ± 17.75 mg GAE/kg fw), ‘Orange king’ (208.64 ± 11.95 mg GAE/kg fw), and ‘Producer’ (287.3 5± 6.40 mg GAE/kg fw) samples. The members of this cluster show variability in TPC. Cluster III comprises hybrid ‘Padrak severu’ (359.26 ± 14.52 mg GAE/kg fw) and cultivar ‘Moon star’ (339.41 ± 3.28 mg GAE/kg fw), which exhibit the highest TPC levels among the groups. While the separation from other clusters highlights differences in phenolic profiles among the watermelon cultivars, further research is needed to elucidate cultivar-specific phytochemical compositions and link them to the potential health benefits associated with these variations.

Research by Tarazona-Diaz et al. has demonstrated that watermelon rind contains a higher total phenolic content (TPC), with values ranging from 385 to 507 mg chlorogenic acid equivalents (CAE)/kg fresh weight (FW), compared to the pulp, which ranged from 354 to 431 mg CAE/kg FW [48]. In our study, we observed that the total phenolic content (TPC) in Lithuanian watermelon rinds averaged 224.34 ± 76.64 mg GAE/kg fw, while the mean TPC in the pulp measured 202.80 ± 75.76 mg/kg fw. These findings align with the general trend noted in the literature, specifically in the work of Neglo et al., who reported a higher TPC in watermelon peel (0.087 ± 0.002 mg GAE/g dw) and rind (0.026 ± 0.003 mg GAE/g dw) compared to the pulp (0.010 ± 0.001 mg GAE/g dw) [39]. Although Zia et al. did not provide specific TPC data for direct numerical comparison, their review supports the significant polyphenol content present in watermelon rind [43]. These findings consistently highlight the rind as a valuable, underutilized source of polyphenolic compounds.

3.2.3. Determination of TPC in the Pulps and Rinds of Imported Watermelons

This study investigated the TPC accumulation in the pulp samples of watermelons imported to Lithuania from other countries as well (Figure 8). The analysis revealed that watermelons cultivated in Greece exhibited the highest yield of TPC, with an average concentration of 229.22 ± 8.04 mg GAE/kg fw. Conversely, the lowest concentration was observed in watermelon pulp samples from Ukraine, measuring 100.60 ± 6.76 mg GAE/kg fw. Comparative data indicate that the average phenolic content in Lithuanian-grown watermelons (202.80 ± 75.76 mg GAE/kg fw) is comparable to that of Greek watermelons (229,21 ± 8.04). Moreover, the watermelon hybrid ‘Padarak severu’ cultivated in Lithuania, demonstrated a significantly higher phenolic concentration of 327.42 ± 9.14 mg GAE/kg fw (p < 0.05), which is 1.43 times that of Greek specimens and 3.25 times that of those from Ukraine. In terms of rind analysis, the highest TPC was recorded in samples from Spanish-grown watermelons (324.38 ± 2.65 mg GAE/kg fw), while the lowest was from watermelons imported from Russia, which contained 162.14 ± 5.36 mg GAE/kg fw. A comparative analysis showed that the TPC in the rind samples from fruits grown in Greece (293.79 ± 10.47 mg GAE/kg fw), Spain (324.38 ± 2.65 mg GAE/kg fw), Hungary (257.84 ± 3.23 mg GAE/kg fw), and Ukraine (225.46 ± 3.80 mg GAE/kg fw) exceeded the average concentration found in Lithuanian-grown watermelon rinds (224.34 ± 76.64 mg GAE/kg fw). However, rind samples from the hybrids ’Padarak severu’ (359.26 ± 14.52 mg GAE/kg fw) and cultivar ’Moon star’ (339.41 ± 3.28 mg GAE/kg fw) exhibited higher phenolic contents than those in samples from Spain. These results may be influenced by various factors including the climatic conditions of the respective countries, rain levels, and the genetic characteristics of the cultivars/hybrids utilized in this study.

3.3. Lycopene Content in Pulp

Lycopene is a key carotenoid that contributes to the characteristic red pigmentation in many watermelon cultivars [24]. Suwanaruang demonstrated that watermelon contains significantly higher levels of lycopene (144.27 mg/kg) compared to tomatoes (Solanum lycopersicum), which exhibited lower concentrations (104.70 mg/kg). This observation underscores the potential of red-fleshed watermelons as a rich natural source of lycopene and highlights their nutritional significance [49]. The lycopene content in watermelon varies considerably, influenced by factors such as genotype, ploidy level, maturity stage, and growing conditions [15]. While extensive research has investigated lycopene content in various commercial cultivars [24,50], data on lycopene levels in watermelons grown under specific geographical and climatic conditions remain limited. Our analysis of Lithuanian watermelon pulp revealed a significant range of lycopene concentrations (1.15 ± 0.42 to 103.60 ± 1.69 mg/kg fw), with the highest levels detected in ‘SRD-2’ (103.60 ± 1.69 mg/kg fw) and the lowest in ‘Janosik’ (1.15 ± 0.42 mg/kg fw) (p < 0.05) (Figure 9). Statistical analysis revealed a significant positive correlation (r = 0.765, p < 0.05) between the total phenolic content (TPC) in watermelon pulp and lycopene concentration. Specifically, the cultivars ‘Padarak severu’ and ‘SRD-2’ exhibited elevated levels of both total phenolic content (TPC) and lycopene, suggesting that these cultivars may possess inherent genetic traits that facilitate the enhanced biosynthesis of these beneficial bioactive compounds.

The dendrogram (Figure 10) illustrates the grouping of thirteen watermelon cultivars/hybrids based on their lycopene content, determined using a hierarchical cluster analysis. Two main clusters (I and II) and associated sub-clusters (A and B) are evident, suggesting distinct groupings based on lycopene accumulation. Sub-cluster IA comprises cultivars ‘Melia’, ‘Ice cream’, ‘Farao’, ‘Padarak severu’, ‘Moon star’, and ‘SRD-2’, characterized by moderate-to-high lycopene content (from 70.21 ± 1.92 to 103.60 ± 1.69 mg/kg fw). The clustering pattern suggests that these cultivars might possess differing genetic determinants of lycopene biosynthesis or have different responses to environmental factors during growth. Sub-cluster IB comprises cultivars (‘Great Pekin’s joy’, ‘Huelva’, ‘Producer’, ‘Karolina‘, and ‘Demre’) which display relatively low-to-moderate lycopene accumulation (from 61.32 ± 10.86 to 43.56 ± 1.51 mg/kg fw). Cluster II consists of ‘Orange king‘ (orange flesh) and ‘Janosik‘ (yellow flesh) cultivars with the lowest lycopene concentrations (14.24 ± 1.32–1.15 ± 0.42 mg/kg fw). Our findings corroborate the established observation that yellow- or orange-fleshed watermelon cultivars generally exhibit significantly lower lycopene accumulation compared to red-fleshed cultivars [48]. This is supported by the significantly lower lycopene content (p < 0.05) observed in the orange-fleshed hybrid ‘Orange King’ (14.244 ± 1.32 mg/kg fw) and the yellow-fleshed cultivar ‘Janosik’. The mean lycopene concentration in red-fleshed Lithuanian watermelons was 68.88 ± 1.86 mg/kg fw. These results align closely with those obtained in a study of 50 watermelon cultivars and hybrids from Oklahoma, USA, where lycopene content ranged from 33 to 99.8 mg/kg fw [25]. Another study reported a narrower range (36.5–71.2 mg/kg fw) [51], highlighting that lycopene content in watermelon varies greatly depending on the cultivar, cultivation practices, and environmental factors and/or methodological differences in lycopene extraction and quantification.

Lycopene, a potent carotenoid with established antioxidant properties, plays a significant role in protecting against oxidative damage and inflammation [36,52]. Its consumption has been linked to a reduced risk of various chronic diseases, including cardiovascular disease and certain cancers [53,54]. While the exact amount of lycopene required to achieve these health benefits is not definitively established, numerous studies suggest that a higher dietary intake is associated with a greater protective effect [55]. Importantly, research indicates that lycopene derived from watermelon may exhibit superior antioxidant and anti-inflammatory effects compared to lycopene from tomatoes [56]. Moreover, comparative studies have demonstrated that heat treatment is not necessary for the absorption of lycopene from watermelon juice when compared to tomatoes [57]. Therefore, fresh or frozen watermelon can serve as a bioavailable source of lycopene in the diet. By incorporating lycopene-rich foods like watermelon, individuals can enhance their dietary patterns while enjoying delicious and refreshing fruits.

An analysis of watermelons imported to Lithuania and sold in supermarkets revealed that the highest concentration of lycopene in the pulp was determined in fruits cultivated in Greece, measuring 73.60 ± 2.85 mg/kg fw (Figure 11). Watermelon samples from Spain exhibited slightly lower lycopene levels at 58.72 ± 3.39 mg/kg fw; however, this value was not statistically significantly different from that of samples imported from Ukraine, which contained 56.88 ± 1.72 mg/kg fw (p ≥ 0.05). Conversely, the lowest lycopene concentration was recorded in pulp samples from watermelons imported from Russia, at 39.48 ± 1.38 mg/kg (p < 0.05). The lycopene content in Greek-grown watermelons was marginally higher than the average lycopene concentration determined in watermelons cultivated under natural conditions in Lithuania (68.88 ± 1.86 mg/kg fw). Remarkably, when comparing the lycopene levels of Greek-grown watermelons with specific cultivars and hybrids from Lithuania, four cultivars demonstrated higher lycopene concentrations in their pulp: ‘Farao’ (79.00 ± 1.75 mg/kg fw), ‘Padarak severu’ (83.68 ± 1.85 mg/kg fw), ‘Moon Star’ (93.98 ± 2.21 mg/kg fw), and ‘SRD-2’ (103.60 ± 1.69 mg/kg fw). These results highlight that the climatic conditions in Lithuania are sufficiently conducive for watermelon cultivation, allowing for the successful growth of high-lycopene cultivars. The ability to achieve comparable, if not superior, lycopene levels in certain Lithuanian watermelon varieties underscores the potential of local agricultural practices and selected cultivars in optimizing the nutritional value of watermelons.

These results highlight that the climatic conditions in Lithuania, despite being a temperate region, are sufficiently conducive for watermelon cultivation, particularly enabling the successful growth of high-lycopene cultivars. This finding suggests that key environmental factors necessary for lycopene synthesis, such as adequate sunlight exposure during critical growth stages, are met within the Lithuanian growing season. While Lithuania may not have the consistently high temperatures of traditional watermelon-growing regions, the observed lycopene levels indicate that other factors (specific cultivar, temperature fluctuation, soil composition, and grafting technique) may compensate or even provide an advantage. These findings suggest that with careful cultivar selection and optimized agricultural practices, it is possible to overcome the limitations of a temperate climate and produce watermelons with a high nutritional value. This highlights the potential for other regions with similar climates to adopt these strategies and cultivate high-lycopene watermelons locally, reducing reliance on imported fruits.

3.4. Total Sugar Content in Pulp Juice

Sugar content is a critical quality parameter in watermelon, directly influencing its flavor, acceptability, and overall consumer preference. A previous analysis, performed by Shameena Beegum et al. [58], revealed the presence of fructose (31.1 mg/mL), glucose (24.5 mg/mL), and sucrose (11.1 mg/mL) in watermelon juice. Fructose was the predominant sugar, followed by glucose and sucrose. In Lithuania, where watermelon cultivation is expanding due to favorable adaptations to local climatic conditions, understanding the sugar profile of various cultivars becomes essential for growers, marketers, and consumers alike. Sugar content in fruits, including watermelons, is commonly measured using the Brix scale (°Brix), which quantifies the total soluble solids (TSS). A higher °Brix value indicates greater sweetness and sugar concentration. While °Brix measurements provide a useful estimation of total soluble solids and sweetness, they do not specifically quantify sugar content. More precise assays, such as the DNS assay or liquid chromatography assays, would be necessary to distinguish individual sugar components and confirm the total sugar concentration. Future studies should incorporate such techniques to deepen the understanding of sugar composition in different cultivars. The refractometric analysis of TSS in Lithuanian watermelon pulp juice (Figure 12) revealed a range of 11.13 ± 0.11 to 17.67 ± 0.06 °Brix. ‘Karolina’ exhibited the highest TSS (17.67 ± 0.06 °Brix), although this value was not statistically significantly different from that of ‘Orange King’ (17.63 ± 0.15 °Brix) (p ≥ 0.05). The lowest TSS was observed in ‘Great Pekin’s joy’ (11.13 ± 0.11 °Brix). The mean TSS across all the Lithuanian watermelon cultivars was 14.19 ± 2.11 °Brix.

The conducted hierarchical cluster analysis demonstrates that, based on TSS determined in the fruit pulp juice samples, the watermelon varieties and hybrids are categorized into two primary clusters (I and II), as presented in Figure 13. Cluster I consists of two subclusters (A and B) and includes varieties and hybrids such as ‘Orange king’, ‘Karolina’, ‘Huelva’, and ‘SRD-2’. These are characterized by a higher-than-average total sugar content. Subcluster A, including ‘Orange king’ (17.63 ± 0.15 °Brix) and ‘Karolina’ (17.67 ± 0.06 °Brix), is particularly notable for its exceptionally high sugar levels. It has been suggested that these varieties have a propensity to accumulate significant quantities of sugars, particularly when cultivated in the natural conditions of Lithuania. Cluster II encompasses a larger group, with varieties and hybrids like ‘Janosik’, ‘Producer’, ‘Ice cream’, ‘Farao’, ‘Melia’, ‘Padarak severu’, ‘Demere’, ‘Moon star’, and ‘Great Pekin’s joy’. This cluster is marked by a lower-than-average total sugar content. Furthermore, Cluster II is subdivided into subclusters A (12.27 ± 0.06–12.67 ± 0.06 °Brix), B (13.00 ± 0.10–13.57 ± 0.16 °Brix), and C, where subcluster C includes the hybrid ‘Great Pekin’s joy’ with the lowest sugar content overall. This classification provides valuable insights into the sugar accumulation tendencies of various watermelon varieties, which can be crucial for breeding and cultivation strategies to meet specific taste and nutritional preferences. While limited data exist on the TSS of watermelon pulp juice in the scientific literature, a TSS exceeding 14 °Brix is generally considered desirable for optimal consumer acceptance [59]. Based on this criterion, several Lithuanian watermelon cultivars and hybrids meet the desired TSS threshold: ‘Orange King’ (17.63 ± 0.15 °Brix), ‘Huelva’ (16.03 ± 0.06 °Brix), ‘Karolina’ (17.67 ± 0.06 °Brix), and ‘SRD-2’ (16.43 ± 0.21 °Brix). These cultivars, therefore, represent promising candidates for commercial cultivation in Lithuania, offering superior sweetness profiles appealing to consumer preferences. Further research is needed to determine the genetic and environmental factors influencing TSS in these high-performing cultivars, as well as to correlate TSS with other quality parameters, such as flavor profiles and overall fruit quality.

In examining the total sugar content in watermelon fruit pulp juice samples imported into Lithuania, a range was observed from 11.00 ± 0.1 °Brix in fruits sourced from Russia, to 14.97 ± 0.06 °Brix in those obtained from Spain (Figure 14). Moreover, this maximum value closely approaches the mean TTS determined in watermelons cultivated in Lithuania, which is 14.19 ± 2.11 °Brix. A comparative analysis of the individual TSS in hybrid and variety fruit pulp juices from fruits grown domestically reveals that the ‘Orange king’, ‘Huelva’, ‘Padarak severu’, and ‘SRD-2’ varieties exhibit elevated total sugar levels relative to the highest values recorded in imported fruits. These findings imply that Lithuanian-grown watermelons possess sugar concentrations comparable to those cultivated in Southern Europe. It is hypothesized that the climatic conditions in Lithuania, particularly the levels of heat and sunlight, are adequate to induce sugar synthesis in cultivated watermelons.

3.5. Nitrate Content in Pulps

As the dietary habits of consumers continue to evolve, the analysis of nitrate content in locally grown watermelons is essential for assessing their safety and compliance with established regulatory limits. However, it becomes a substance of concern when its levels exceed the Acceptable Daily Intake (ADI) limits for human consumption. High nitrate consumption can lead to adverse health effects, including methemoglobinemia, which in severe cases can result in toxicity and even death [27,60]. According to the Joint Expert Committee on Food and Agriculture (JECFA) and the European Commission’s Scientific Committee on Food (SCF), the ADI for dietary nitrates is set at 3.7 mg/kg body weight [60,61]. Watermelon is typically valued for its refreshing taste and hydrating properties, but it also requires careful monitoring of nitrate levels to ensure safety for consumers. Therefore, the analysis of nitrate content in Lithuanian watermelon pulp (Figure 15) revealed a considerable range (21.59–61.33 mg/kg fw), underscoring the influence of cultivar/hybrid on nitrate accumulation. ‘Great Pekin’s joy’ exhibited the highest nitrate concentration (61.33 ± 0.70 mg/kg fw), followed by ‘Orange King’ (58.13 ± 1.65 mg/kg fw) and ‘Melia’ (52.48 ± 0.70 mg/kg fw). The lowest nitrate levels were observed in ‘Janosik’ (21.59 ± 0.54 mg/kg fw) (p < 0.05). The mean nitrate concentration across all the Lithuanian cultivars was 40.95 ± 0.84 mg/kg fw.

The dendrogram (Figure 16), based on nitrate content in watermelon cultivars, revealed two main clusters (I and II), each further subdivided into sub-clusters (A and B), suggesting distinct groupings based on nitrate content accumulation. Cluster I is characterized by low nitrate content accumulation and is divided into two sub-clusters. Sub-cluster IA includes cultivars ‘Farao’, ‘Karolina’, ‘Huelva’, ‘Producer’, and ‘SRD-2’, which exhibit nitrate content accumulation ranging from 34.85 ± 1.16 mg/kg fw to 42.12 ± 0.17 mg/kg fw. Sub-cluster IB includes cultivars ‘Moon star’, ‘Janosik’, and ‘Padarok severu’, which show the lowest nitrate accumulation (21.59 ± 0.54–28.40 ± 0.95 mg/kg fw). Cluster II encompasses cultivars with higher nitrate accumulation, also divided into sub-clusters (A and B). Sub-cluster IIA groups cultivars ‘Orange king’ (58,13 ± 1.65) and ‘Great Pekin’s joy’, the highest nitrate accumulation compared to Cluster I. Sub-cluster IIB is distinguished by the moderately high nitrate accumulation among all the cultivars in this study, comprising ‘Demre’, ‘Ice cream’, and ‘Melia’ (46.64 ± 0.49–52.48 ± 0.70 mg/kg fw).

For instance, the study performed by Chaleshtori et al. with Iranian watermelons reported nitrate content varying from 51 ± 32 mg/kg in spring samples to 62 ± 55 mg/kg in winter samples, which are close to our observations of nitrate variability among Lithuanian cultivars [62]. Another study, published by Rezaei et al., investigated nitrate and nitrite levels in various food products from Arak, Iran. Rezaei et al. reported an average nitrate concentration in watermelons of 26.61 mg/kg, with a range of 13.95–37.7 mg/kg [27]. Potential reasons for any discrepancies between the studies may include differences in cultivars, environmental conditions, agricultural practices, and analytical methodologies. Our study shows significant variability in nitrate content in Lithuanian watermelons, highlighting the importance of both cultivar selection and appropriate agricultural practices for managing nitrate accumulation.

The analysis of watermelons imported to Lithuania and commercially available in supermarkets revealed a maximum nitrate concentration of 51.62 ± 0.54 mg/kg FW in samples from Greece. Conversely, the minimum concentration was observed in samples from Spain (34.50 ± 0.40 mg/kg FW) (Figure 17). A comparison of these findings with those from Lithuanian-grown watermelons indicates that cultivars from Greece, Ukraine, and Hungary exhibited higher mean nitrate levels than the average observed in Lithuanian watermelons. However, Lithuanian cultivars ‘Great Pekin’s joy’, ‘Orange King’, and ‘Melia’ demonstrated nitrate concentrations exceeding those found in the imported samples. Variability in nitrate accumulation among watermelons from different geographical origins may be attributed to variations in soil composition, fertilization regimes, and inherent genotypic differences in nitrate uptake and metabolism. Importantly, Lithuanian watermelons were cultivated under consistent soil and fertilization conditions, eliminating these factors as contributors to the observed inter-cultivar variation in nitrate content. The significant differences in nitrate levels among Lithuanian cultivars emphasize that genotypic predisposition is a major determinant influencing nitrate accumulation in watermelons. Therefore, the careful consideration of cultivar-specific nitrate accumulation is crucial when selecting watermelon varieties for cultivation.

3.6. Principal Component Analysis of Citrullus lanatus (Thunb.) Fruit Cultivars

A principal component analysis was applied using all the quantitative variables evaluated for tested samples, namely, the total phenolic content in pulps and rinds, lycopene, nitrate amounts, and total sugar content. The PCA model explained 71.89% of the total variance, with PC1 (46.92%) primarily linked to lycopene content and phenolic compounds in both pulp and rind. PC2 (24.92%) represented variations mainly driven by total sugar content, while nitrate levels were negatively correlated with both components (Figure 18).

Cultivars ‘Padarak severu’, ‘SRD-2’, ‘Moon star’, and ‘Farao’ are positioned at the top of the plot, indicating a high content of lycopene and phenolic compounds in both the pulp and rind. These cultivars are more antioxidant-rich, making them favorable for nutritional benefits. ‘Huelva’, ‘Orange King’, and ‘Janosik’ are positioned further along the positive PC2 axis, having a high sugar content. These cultivars may be sweeter and more palatable. On the other hand, they have lower lycopene levels due to the negative correlation of lycopene with PC2. ‘Great Pekin’s Joy’ and cultivars obtained from Greece (GR) appear in the lower left quadrant, indicating low levels of phenolics, lycopene, and sugar, and therefore lower nutritional and sensory appeal compared to others. Lithuanian (LT) cultivars are predominantly located in the upper half, indicating generally high phenolic and lycopene content. Cultivars in the negative area of PC1 and PC2 on the plot have higher nitrate levels.

4. Conclusions

This study represents the first comprehensive investigation into the cultivation and phytochemical characterization of watermelon under Lithuanian climatic conditions. The successful implementation of grafting and mulching techniques, specifically addressing the challenges of Fusarium wilt and suboptimal growing temperatures, provides valuable practical guidelines for growers in the region. Significant cultivar-dependent variations in total phenolic content (TPC), lycopene, sugar, and nitrate levels were observed, demonstrating the influence of both genotype and environment on the fruit’s nutritional and bioactive composition. Notably, several Lithuanian cultivars exhibited comparable or superior levels of key bioactive compounds—TPC and lycopene—compared to imported varieties, highlighting the potential for the local production of high-quality watermelons. The findings emphasize the potential of watermelon rind and pulp as valuable sources of antioxidants and other bioactive phytochemicals, suggesting opportunities for developing value-added products. The results are based on a specific set of cultivars grown under the environmental conditions of a single growing season. Further research is needed to evaluate a broader range of cultivars and assess the impact of inter-annual climate variability on fruit quality. Additionally, future studies should explore the genetic mechanisms underlying the observed variations in bioactive compound accumulation and investigate the potential of advanced cultivation techniques, such as precision fertilization and controlled-environment agriculture, to further enhance fruit quality and nutritional value. Finally, research into consumer preferences and market dynamics will be crucial for ensuring the long-term sustainability of local watermelon production in Lithuania and other temperate regions. By addressing these limitations and pursuing these future research directions, we can unlock the full potential of watermelon cultivation in non-traditional growing regions and provide consumers with access to nutritious, locally sourced produce.