CPPU treatment of oriental melons. T1, T2, and T3 represent the timings of CPPU treatment: day before flowering, day of flowering, and day after flowering, respectively.
Appearance comparison between (A) normal and abnormal melon fruit and (B) fertilized and unfertilized seeds from left to right, respectively.
Effects of CPPU treatment timing on the fruit setting (A) and abnormal fruit rate (B) of artificially pollinated and nonpollinated oriental melons. AP = artificial pollination; NP = no pollination; T1 = CPPU treatment a day before flowering; T2 = CPPU treatment on the day of flowering; T3 = CPPU treatment a day after flowering.
Cross-sections of CPPU-treated oriental melons. CPPU = N-(2-chloro-4-pyridyl)-N′-phenylurea; T1 = CPPU treatment a day before flowering; T2 = CPPU treatment on the day of flowering; T3 = CPPU treatment a day after flowering.
Appearance and cross-sections of pollinated and nonpollinated oriental melons treated with different concentrations CPPU a day before flowering and a day after flowering.
Changes in cucurbitacin B content in oriental melon fruits through artificial pollination without CPPU treatment and non-pollination with CPPU = N-(2-chloro-4-pyridyl)-N′-phenylurea treatment, verified by supplementation or no supplementation.
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The production of melons (Cucumis melo L.) in greenhouses relies on pollination. Extreme temperature and insufficient light reduce not only flower visitation by pollinators but also pollen viability, resulting in inefficient pollination. In this study, we investigated the effects of forchlorfenuron [(N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU)] on the fruit setting and growth of oriental melons (C. melo L. var. makuwa Makino). The primary objective was to devise a new strategy for the management of oriental melons. Treatment with 5 mg·L−1 CPPU a day before flowering (T1), on the day of flowering (T2), and a day after flowering (T3) increased the fruit setting rate (by 20.1% to 30%) in melons subjected to artificial pollination (AP) or no pollination (NP) compared with the rate in those subjected to only artificial pollination without CPPU (CK). CPPU treatment induced unfertilized seeds; in addition, a tendency toward parthenocarpy was noted. The highest fruit setting rate (∼95%) was noted in plants subjected to the following treatments: AP+T1 and NP+T3. The rates of abnormal fruit formation decreased from 45.2% of CK group to 9.4% in plants subjected to AP+T1 and to 19.4% in those subjected to NP+T3. Elevated exogenous concentrations of CPPU markedly increased fruit weight. Plants subjected to NP+10 mg·L−1 CPPU bore the heaviest fruits (541.0 g), which were heavier than those borne by plants subjected to AP+10 mg·L−1 CPPU. CPPU treatment reduced the fruit cavity ratio in a concentration-dependent manner from 47.3% to 33.6% and increased the pulp thickness from 1.5 to 2.5 cm. Notably, supplementary CPPU treatment exerted no significant effects on fruit traits. Regarding taste, inconsistent results were obtained for sugar accumulation. Although the content of cucurbitacin B increased immediately after the initial CPPU treatment, it markedly decreased after 15 days of CPPU treatment. Therefore, mature melons did not have a bitter taste.
Melon (Cucumis melo L.), a diploid crop belonging to the Cucurbitaceae family, is a vegetable of commercial importance (Grumet et al. 2021). These melons are sweet, aromatic, and nutritious (Cui et al. 2022), and therefore their production has steadily increased worldwide (FAOSTAT 2022). Melons exhibit considerable diversity. They can be categorized into various horticultural groups, including agrestis, kachri, chito, tibish, acidulus, momordica, conomon, makuwa, chinensis, flexuosus, chate, dudaim, chandalak, indicus, ameri, cassaba, ibericus, inodorus, and cantalupensi. Among these intraspecific groups, inodorus (e.g., Honeydew type) and cantalupensis (e.g., Saccharinus type) are the two most dominant commercially (Wehner et al. 2020; Zhao et al. 2019). Notably, the oriental melon (C. melo L. var. makuwa Makino) belong to makuwa group, is a prominent variety in East Asian countries, such as Korea, Japan, China, and Taiwan (Shin et al. 2017). Typically, this variety is cultivated in greenhouses because of its high productive value.
Oriental melons are andromonoecious plants. They exhibit both male and hermaphrodite flowers on the same plants. Male flowers have five stamens and a nonfunctional style surrounded by nectaries; by contrast, hermaphrodite flowers have a three-lobed stigma, three peripheral anthers on the outer side to prevent self-pollination, and inferior ovaries (Delaplane and Mayer 2000; Kiill et al. 2016). Therefore, the successful reproduction of melons relies on cross-pollination by biotic pollinators, particularly in protected environments (Klein et al. 2007). Honeybees (Apis mellifera) and bumblebees (Bombus impatiens) are recognized as key pollinators because of their widespread availability and cost-effectiveness (Choi and Jung 2015; Kalev et al. 2002). They help enhance the yield and nutritional quality of crops (Ghosh and Jung 2018). In horticulture, the foraging behavior of bees is highly dependent on greenhouse temperature. A minimum temperature of 14 °C is essential for the activity of honeybees; the level of activity reaches its maximum at 33 °C, and no foraging is observed at temperatures above 39 °C. The temperature-dependent activity patterns may be attributable to the reduction in honeybee colony productivity under high temperature conditions (Abou-Shaara 2014; Begna et al. 2020; Gérard et al. 2022). Temperature stress also leads to fruit abortion or deformity in melons, primarily because of the reduction in pollen viability at high temperatures. For melons, day and night temperatures of 24 to 28 °C and 16 to 24 °C, respectively, improve fruit quality in terms of desirable reticulation, thick pulp, high sugar content, and prolonged shelf life (Baker and Reddy 2001; Hikosaka et al. 2008). Solar radiation serves the direct source of heat in greenhouses located in low-latitude areas, increasing the internal temperature beyond the maximum levels tolerated by most pollinators and plants, often exceeding 50 °C (Chen and Shen 2022). Various strategies have been adopted to improve fruit setting and development under temperature stress conditions; these strategies include environmental adjustments, tolerance breeding, parthenocarpy, and growth regulator application (Kawasaki 2015).
Forchlorfenuron—N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU)—is a synthetic cytokinin of the phenyl urea family. It regulates fruit setting, fruit size, fruit shape, and inflorescence morphology. CPPU has been demonstrated to promote fruit growth in grapes (Nickell 1986; Ogata et al. 1989), pears (Banno et al. 1986), kiwifruits (Iwahori et al. 1988), and litchis (Liu et al. 2022). In melons, CPPU increases fruit setting rates and sucrose contents (Hayata et al. 2000; Kano 2005). However, it may exert certain undesirable effects, such as reduced anthocyanin contents, reduced vitamin C contents, and increased bitterness levels (Luo et al. 2020; Qian et al. 2018). The application of CPPU in melon fruit has been reported to result in the development of bitter fruit, attributed to the accumulation of cucurbitacins (Luo et al. 2020). Cucurbitacins are a group of tetracyclic triterpenes with a cucurbitane skeleton that were initially identified in the Cucurbitaceae family and are commonly associated with a bitter taste (Chen et al. 2005). Cucurbitacin (Cu) can be categorized into various types according to the distinct molecular formulas, among which Cu B is mainly in melon, Cu C in cucumber and Cu E in watermelon (Rehm et al. 1957; Shang et al. 2020).
Few studies have explored the effects of CPPU on oriental melons grown in greenhouses, particularly in those located in subtropical and tropical regions. Therefore, in the present study, we conducted a comprehensive trial to optimize CPPU treatment and investigate the effects of treatment timing, CPPU concentration, and supplementary treatment on the fruit setting, growth, and quality of greenhouse-grown oriental melons. Our findings aid the development of effective management strategies for oriental melons.
The experiments were conducted in a plastic greenhouse under natural ventilation. The greenhouse is located at Taichung District Agricultural Research and Extension Station, Taiwan (23°55′ N, 120°27′ E). The study spanned three cropping sessions between 2022 and 2023. The experimental compartment was divided into four sections (length, 36 m; width, 16 m; and height, 6 m). The roof had two-side window and was covered with a clear, 0.15-mm polyethylene film (Plastika Kritis Group, Heraklion, Greece). Three-week-old oriental melon ‘Silver Light’ (Known-You Seed, Taiwan) seedlings were transplanted into peatmoss substrate bags (size, 100 cm × 20 cm × 15 cm; King Root; Dayi Agritech, Pintung, Taiwan) where a between-row distance of 35 cm was maintained. Single-stem pruning was practiced, where only a single fruit located between nodes 10 and 15 was retained. Drip fertigation was performed and regulated using the fertilizer dosing system NutriFit (Priva, De Lier, Netherlands). The nutrient recipe was based on the findings of van der Lugt et al. (2020); this recipe was adjusted in accordance with the stage of plant growth.
To evaluate the effects of CPPU treatment timing on the fruit setting and growth of oriental melons, three sets of plants were treated with CPPU at three time points (Fig. 1): a day before flowering (T1), on the day of flowering (T2), and a day after flowering (T3). The plants were subjected to either artificial pollination (AP) or no pollination (NP). Untreated plants that were subjected only to AP served as control (CK) plants. A freshly prepared 5 mg·L−1 CPPU solution was evenly sprayed onto each ovary between nodes 10 and 15; the treatment dose was ∼2 mL per ovary. Only one fruit was retained per plant. The experiment was conducted between Jul and Aug 2022. Inside the greenhouse, the average ambient temperature was 30.7 °C, and the daily accumulated radiation value [daily light integral (DLI)] was 1343.6 J·m−2·d−1.
Citation: HortScience 59, 3; 10.21273/HORTSCI17594-23
The effects of four CPPU concentrations, 1.25, 2.5, 5, and 10 mg·L−1, were evaluated. On the basis of the results of Expt. 1, the following treatment conditions were selected: AP+T3 and NP+T1. Expt. 2 was conducted following the same method as that used for Expt. 1. The experiment was conducted between Oct and Dec 2022. The average ambient temperature was 26.6 °C, and the DLI was 1346.0 J·m−2·d−1.
To investigate whether a supplementary CPPU treatment can accelerate fruit growth, nonpollinated plants were subjected to an additional CPPU treatment (5 mg·L−1). The initial treatment was performed a day after flowering (NP+T3), whereas the supplementary treatment was performed when the fruit diameter reached 2 cm (∼5 d after the initial treatment). The methods used for CPPU spraying and fruit thinning were the same as those used for the previous experiments. The experiment was conducted between Apr and Jun 2023. The average ambient temperature was 28.2 °C, and the DLI was 1235.6 J·m−2·d−1.
Fruit setting and the rate of abnormal fruit formation were evaluated after 5 and 10 d of treatment; lack of rounded shape and cracked fruits were defined as abnormalities (Fig. 2A). After data investigation, thinning was implemented. At the harvest stage, mature fruits were sampled for assessing weight and flesh thickness. The fruits were cut into cross-sections, and the fruit cavity area was measured using the following formula: 0.25 × (length × width × π). In addition, the seeds were extracted, washed, and dried at 40 °C for 3 d. Total seed count and weight were evaluated. The seeds were divided into two categories: well-developed seeds and unfertilized seeds. The evaluation of distinctions between unfertilized seeds and fertilized seeds was performed by applying pressure to assess the development of the embryo. In contrast to fertilized ones, exhibit unfertilized seeds only develop seedcoat and with unfulfilled even nonexistent endosperm as well as embryo (Fig. 2B). Then the proportion of each category of seeds was calculated. Fruit length, width, and thickness were measured to estimate the elliptical volume.
Citation: HortScience 59, 3; 10.21273/HORTSCI17594-23
After the removal of seeds and fruit peel, ∼50% of the total fruit pulp was homogenized. Subsequently, the homogenized mixture was filtered through a No.1 filter paper. The filtrate was used for measuring the levels of total soluble solids (TSSs) and titratable acidity (TA). These two quality indices were measured using a refractometer (PR-101α; ATAGO, Tokyo, Japan; results were expressed in degrees Brix) and a digital titrator (Titronic 300; SI Analytics, Mainz, Germany; results were expressed in terms of percentage of citric acid content), respectively.
The accumulation Cu B within melons results in a bitter taste. The content of Cu B was measured using the modified version of a method described by Zhou et al. (2016). The pedicle pulp was collected from mature fruits and ground with liquid nitrogen. From this, 0.5 g of powder was taken and mixed with 1.5 mL of methanol. The mixture was vortexed for 30 min and then sonicated at 20 °C for 60 min. Subsequently, the extracted liquid was centrifuged at 10,000 gn for 10 min at 4 °C. The supernatant was collected and filtered through a 0.22-µm membrane filter. The filtrate was subjected to high-performance liquid chromatography (HPLC). For HPLC, we used a Chromaster 5110 pump equipped with a CM-5420 ultraviolet detector (Hitachi Hightech, Tokyo, Japan) and a C18 column (250 × 4.6 mm; 5 μm). The detector wavelength was 228 nm, and the column temperature was 30 °C. The mobile phase comprised 70% HPLC-grade methanol and 30% ultrapure water (v/v). The flow rate was 0.5 mL·min−1. Cu B was identified and quantified based on its retention time and peak height noted on a standard calibration curve.
A completely randomized design was adopted for the experiments. Three replicates (six plants per replicate) were used per experiment. Percentage data were converted into angular values using the Bliss (1938) table. SAS software (SAS Institute Inc., Cary, NC, USA) was conducted to analyze the data. The t test and Fisher’s least significant difference test was used to compare treatments when the one- or two-way analysis of variance showed significant difference between means at P ≤ 0.05.
CPPU treatment increased the fruit setting rate from 65.3% to 88.3% to 95.3% and 85.5% to 95.2% in pollinated and nonpollinated fruits, respectively. Moreover, the rate of abnormal fruit formation markedly decreased from 45.2% to 9.4% to 23.3% and 19.4% to 29.1% in pollinated and nonpollinated fruits, respectively. Notably, the highest fruit setting rate and the lowest deformity rate were observed under the following treatments (Fig. 3): AP+T1 and NP+T3.
Citation: HortScience 59, 3; 10.21273/HORTSCI17594-23
Regarding fruit quality, AP significantly affected fruit weight, fruit cavity ratio, pulp thickness, and Cu B content. Furthermore, CPPU treatment timing influenced fruit cavity ratio, TSS level, and Cu B content. Except for TA level, all other fruit traits were influenced by the interaction between AP and CPPU treatment. However, in pollinated and nonpollinated fruits, the effects of CPPU treatment timing on fruit weight and cavity ratio were inconsistent. Plants subjected to AP+T3 bore the heaviest and largest fruits. These fruits had an 83.3-g heavier weight and a smaller cavity (48.3%) compared with fruits borne by the control plants (459.0 g and 52.3%, respectively). However, plants subjected to NP+T1 bore fruits that were similar to the control fruits in terms of weight (447.2 g) but had a cavity that was smaller (42.7%) than that of the control fruits (Fig. 4; Table 1). TSS levels varied slightly but significantly among plants subjected to T1, those subjected to T2, and those subjected to T3. The acid-to-sugar ratio ranged from 41.9 (NP+T1) to 66.0 (AP+T1). The content of Cu B markedly increased from 7.4 to 8.7–10.7 mg·g−1 after CPPU treatment. Cu content was generally higher in pollinated fruits than in nonpollinated fruits and did not vary depending on treatment timing (Table 1).
Citation: HortScience 59, 3; 10.21273/HORTSCI17594-23
The seed performance metrics for CPPU-treated melons are presented in Table 2. Total seed count and unfertilized seed proportion were considerably affected by the timing of CPPU treatment. The proportion of unfertilized seeds was lower in melons subjected to AP+T2 (43.3%) than in those subjected to AP+T1 (82.0%). However, in nonpollinated fruits, no significant difference was noted among the three treatment timings; the proportion of unfertilized seeds ranged from 91.3% to 98.7%. Pollinated melons had ∼3-fold heavier and 2-fold larger seeds than did nonpollinated melons; these parameters varied negligibly with respect to CPPU treatment timing. Fruits borne by plants subjected to NP+T1 and those subjected to NP+T2 exhibited a pronounced deeper color around the placenta, and their seed color closely resembled those in pollinated fruits (Fig. 4).
The weight of both pollinated and nonpollinated fruits increased with CPPU concentration (Fig. 5); fruit weight was associated with CPPU concentration in a linear or quadratic manner (data not shown). Melons subjected to NP+10 mg·L−1 CPPU bore the heaviest fruits (541.0 g), which were heavier than fruits borne by plants subjected to AP+10 mg·L−1 CPPU (480.2 g). In both pollinated and nonpollinated melons, CPPU treatment concentration-dependently reduced the fruit cavity ratio from 47.3% to 33.6% and the increased the pulp thickness from 1.5 to 2.5 cm. However, no CPPU concentration-dependent variations were noted in TSS level, TA level, or Cu B. Nonetheless, a slight increase in Cu B content was detected in pollinated fruits treated with 5 and 10 mg·L−1 CPPU (Table 3).
Citation: HortScience 59, 3; 10.21273/HORTSCI17594-23
AP and its interaction with CPPU concentration were found to influence seed development. Among melons treated with different concentrations of CPPU, the rate of fertility was higher in pollinated fruits containing heavier and larger seeds (23.3% to 84%) than in nonpollinated fruits (5% to 15%). Although CPPU concentration alone did not affect total seed count and weight, it significantly influenced seed fertility and size. In pollinated melons, the proportion of unfertilized seeds increased after CPPU treatment but decreased when the concentration of CPPU exceeded 2.5 mg·L−1. Seed size exhibited no significant differences among plants treated with various concentrations of CPPU (1.25, 2.5, and 5 mg·L−1). Notably, the seeds were smaller in CPPU-treated plants than in untreated plants; however, a sharp increase was observed in the seed size of fruits treated with 10 mg·L−1 CPPU (Table 4).
The supplementary CPPU treatment did not considerably influence fruit weight, cavity ratio, or pulp thickness. Furthermore, no significant changes were noted in the level of TSS or TA (Table 5). However, the supplementary treatment influenced seed development: a slight, nonsignificant increase was observed in total seed count and the seed weight (100 seeds) increased from 0.33 to 0.83 g. The proportion of unfertilized seeds was reduced by 6.4% (Table 6).
We further investigated the effects of CPPU treatment—both the initial and supplementary treatments—on the content of Cu B in nonpollinated melons. Untreated pollinated fruits formed the control group. On day 0, the content of Cu was similar (98.9–121.5 mg·g−1) among the aforementioned three treatment conditions. Cu B content was slightly higher in the CPPU-treated melons than in the control melons. On day 5, Cu content was substantially higher (to 305.8 mg·g−1) in melons that were treated twice with CPPU, remained unchanged (132.9 mg·g−1) in melons that did not receive any supplementary treatment, and was substantially lower (6.2 mg·g−1) in the control melons. On day 15, Cu B content was close to 0 in all melons (Fig. 6); this level was maintained until the harvest stage.
Citation: HortScience 59, 3; 10.21273/HORTSCI17594-23
Efficient pollination, whether performed manually or through pollinators, is a conventional approach essential for the production of melons in greenhouses. Both manual pollination and pollinator-mediated pollination can improve the fruit setting efficiency, yield, and quality (sucrose and amino acid contents) of melons (Huang et al. 2017). AP is primarily adopted to enhance fruit setting and quality while preventing any potential monetary loss (Wurz et al. 2021). Manual pollination has its unique advantages; for example, it enables growers to have precise control over pollen source and quantity (Oronje et al. 2012), regulate pollination timing and frequency (Gonzalez et al. 1998), and reduce yield susceptibility to environmental fluctuations (Partap and Ya 2012). However, small-scale farms or home gardens find it challenging to keep up with the escalating labor costs associated with AP. In commercial settings, pollination is commonly mediated by honeybees and bumblebees. Fertilized seeds resulting from bee-mediated pollination exhibit efficient growth and fruit development in the future. However, insect-mediated pollination is dependent on environmental conditions, such as suitable habitat (Steffan-Dewenter and Tscharntke 1999), light intensity (Ricketts et al. 2008), and weather conditions (Kouonon et al. 2009).
Melons should be pollinated in the early morning, between 6:00 AM and 9:00 AM. This recommendation is related to the elevated level of humidity in the early morning. During this time, the pistils tend to release mucus, which can influence the adhesion of pollens to the stigma (Jett 2006; Respatijarti and Adiredjo 2019). Best pollination timing is suggested at flowering moment to a day when the highest receptivity of stigma and the maximum viability of pollen are (Respatijarti and Adiredjo 2019). Temperature-related limitations can reduce the effectiveness of both manual and bee-mediated pollination; for example, pollen germination exhibits an irregular trend at temperatures above 40 °C (Maestro and Alvarez 1988). Considering the aforementioned problems with pollination, plant growth regulators must be explored as an alternative strategy.
Successful fruit setting is dependent on the intricate interactions of multiple hormones (Ozga and Reinecke 2003; Srivastava and Handa 2005). Phytohormones such as auxin, gibberellin, and cytokinin can, either individually or in combination, induce parthenocarpy, which improves fruit quality and extends shelf life. Gibberellin in combination with 4-chloro-phenoxyacetic acid are commonly used for artificial parthenocarpy in melons during unfavorable climate conditions (Pariasca et al. 2005, Shin et al. 2007). The present study observed that CPPU treatments performed at different time points significantly increased the fruit setting rate and qualified rate of pollinated oriental melons by 23% to 26.1% and 21.9% to 35.8%, respectively. In nonpollinated plants, CPPU treatment increased the fruit setting rate to 80.8% to 95.1%, which was higher than that noted in hand-pollinated plants and similar to that noted in plants subjected to the AP+CPPU treatment (Fig. 3). In addition, CPPU treatment substantially increased the proportion of unfertilized seeds (Tables 2 and 4).
Both pollination and cytokinin treatment increase the level of indole acetic acid (IAA) and reduce that of abscisic acid (ABA) within the placenta and mesocarp of carpel-bearing pistillate flowers. Changes in IAA and ABA levels enhance fruit setting and growth during the early stage of melon development (Grumet et al. 2007; Hayata et al. 2002). CPPU, an exogenous cytokinin growth regulator, has been demonstrated to increase the levels of gibberellin and auxin but reduce that of ABA during fruit setting. Liu et al. (2023) reported that CPPU induces gibberellin biosynthesis by activating the gibberellin-related pathway in melons during parthenocarpy. The researchers further indicated that CPPU upregulated the expression of the gene encoding the key synthase enzyme gibberellin 20-oxidase 1 (CmGA20ox1). An analysis of 13 cDNAs derived from melon genes associated with fertilization and fruit setting revealed similar expression patterns in pollinated and CPPU-treated fruits, suggesting that the mechanisms underlying parthenocarpy-mediated fruit setting and pollination-mediated fruit setting are largely similar (Grumet et al. 2007; Nagasawa et al. 2005). On the basis of our findings and those of previous studies that compared genetic and hormonal effects between pollination and CPPU, we believe that CPPU treatment can serve as an alternative approach to AP.
CPPU treatment timing and concentration partially affected the fruit setting efficiency, traits, and quality of oriental melons. Fruit weight was the highest and the cavity ratio was the lowest (42.7%) in melons subjected to NP+T1. Fruits borne under the NP+T1 treatment condition had the largest edible portions. However, opposite results were obtained under the AP condition (Table 1; Fig. 4). In both pollinated and nonpollinated melons, fruit weight increased with increasing CPPU concentration; however, no apparent CPPU concentration-dependent variation was noted in TSS level, TA level, or Cu B content. Plants subjected to NP+10 mg·L−1 CPPU bore heavier fruits (541.0 g) with a smaller cavity ratio (33.6%) than did the other plants. Notably, fruits borne under the NP+10 mg·L−1 CPPU treatment condition were heavier (by 60.8 g) than those borne under the AP+10 mg·L−1 CPPU treatment condition (Table 3; Fig. 5). Fruit size has been reported to be associated with CPPU concentration; this relationship plateaus at a CPPU concentration of ∼10 mg·L−1. For crops such as Japanese pears (Hosui nashi; Pyrus pyrifolia), Spadona and Costia pears (Pyrus communis L.), green kiwifruits (Actinidia chinensis var. deliciosa Hayward), and hardy kiwifruits (Actinidia arguta), the recommended concentration of CPPU is 10 mg·L−1 (Flaishman et al. 2001; Hayata et al. 2000; Huitrón et al. 2007; Kim et al. 2006; Nardozza et al. 2017; Sakamoto et al. 2016).
Our results regarding the effects of CPPU on fruit quality are inconsistent with those of Li et al. (2021); this disparity may be attributable to differences in the type of melon species or cultivar used in the two studies. Some studies have reported that CPPU treatment reduced the levels of sucrose and glucose in oriental melons (Luo et al. 2020; Wang et al. 2023). However, CPPU treatment was demonstrated to increase the level of sucrose in muskmelons markedly (Hayata et al. 2000, 2001; Hou et al. 2011). Studies have suggested no significant differences between CPPU and AP in terms of the level of sugar accumulation or TSSs in parthenocarpic melons (Huang et al. 2017; Li et al. 2000).
Considering findings in the literature, we present a histological explanation for the reduced size and increased cell density of CPPU-induced fruits compared with pollinated fruits. CPPU-induced fruit setting primarily promotes cell division, whereas pollination-induced fruit setting promotes cell expansion in addition to cell division (Liu et al. 2023). The small vacuoles present in meristematic cells increase in size and gradually coalesce as the cells enlarge and age (Kano 2005), and most of the imported sugars accumulate in the vacuoles of sink-tissue storage cells (Leigh et al. 1979; Yamaki and Ino 1992). Furthermore, CPPU accelerates and maintains the accumulation of gibberellin; the disparity in gibberellin levels between CPPU-induced and pollination-induced fruits may explain the observed difference in cell size and the inconsistent results pertaining to TSS levels in CPPU-induced oriental melons.
High concentrations of exogenous CPPU may negatively affect fruit quality; in particular, the biosynthesis and accumulation of Cu B within melons may increase the level of bitterness (Hayata et al. 2000; Li et al. 2021). Transcriptional analyses revealed that the expression of genes involved in the biosynthesis of Cu B (CmBi, Cm710, and CmACT) and a fruit-specific transcription factor (CmBt) was significantly induced by 20 mg·L−1 CPPU (Luo et al. 2020). We observed a slight increase in the content of Cu B in pollinated oriental melons that were treated with 5 and 10 mg·L−1 CPPU (Table 3). Furthermore, the variations in Cu B content during the 5-day interval between the initial and supplementary treatments suggest that the initial treatment promotes the accumulation of Cu. Nevertheless, Cu content returns to a relatively low level within 15 d of treatment; therefore, CPPU treatment does not lead to poorer tasting melons (Fig. 6). These changes are consistent with the fact that dissipation half-life of CPPU is 1.20 to 1.33 d in melons treated with its recommended dose (Wang et al. 2021). The half-life of CPPU varies across crops; for example, it is 1.20 to 1.67 d in watermelon (Chen et al. 2006), 3.4 to 5.0 d in grapes (Sharma and Awasthi 2003; Ugare et al. 2013), and 15.8 to 23.0 d in citrus fruits (Chen et al. 2013).
In conclusion, exogenous CPPU treatment effectively induces fruit setting in both pollinated and parthenocarpic oriental melons, without compromising the quality of fruits. Our findings provide insights into the optimal timing, concentration, and session for CPPU treatment. The use of CPPU will not only reduce the AP-associated labor costs but also remedy weaknesses in conventional pollination methods.
CPPU treatment of oriental melons. T1, T2, and T3 represent the timings of CPPU treatment: day before flowering, day of flowering, and day after flowering, respectively.
Appearance comparison between (A) normal and abnormal melon fruit and (B) fertilized and unfertilized seeds from left to right, respectively.
Effects of CPPU treatment timing on the fruit setting (A) and abnormal fruit rate (B) of artificially pollinated and nonpollinated oriental melons. AP = artificial pollination; NP = no pollination; T1 = CPPU treatment a day before flowering; T2 = CPPU treatment on the day of flowering; T3 = CPPU treatment a day after flowering.
Cross-sections of CPPU-treated oriental melons. CPPU = N-(2-chloro-4-pyridyl)-N′-phenylurea; T1 = CPPU treatment a day before flowering; T2 = CPPU treatment on the day of flowering; T3 = CPPU treatment a day after flowering.
Appearance and cross-sections of pollinated and nonpollinated oriental melons treated with different concentrations CPPU a day before flowering and a day after flowering.
Changes in cucurbitacin B content in oriental melon fruits through artificial pollination without CPPU treatment and non-pollination with CPPU = N-(2-chloro-4-pyridyl)-N′-phenylurea treatment, verified by supplementation or no supplementation.
Contributor Notes
W.L.C. is the corresponding author. E-mail: chenwl@tcdares.gov.tw.
CPPU treatment of oriental melons. T1, T2, and T3 represent the timings of CPPU treatment: day before flowering, day of flowering, and day after flowering, respectively.
Appearance comparison between (A) normal and abnormal melon fruit and (B) fertilized and unfertilized seeds from left to right, respectively.
Effects of CPPU treatment timing on the fruit setting (A) and abnormal fruit rate (B) of artificially pollinated and nonpollinated oriental melons. AP = artificial pollination; NP = no pollination; T1 = CPPU treatment a day before flowering; T2 = CPPU treatment on the day of flowering; T3 = CPPU treatment a day after flowering.
Cross-sections of CPPU-treated oriental melons. CPPU = N-(2-chloro-4-pyridyl)-N′-phenylurea; T1 = CPPU treatment a day before flowering; T2 = CPPU treatment on the day of flowering; T3 = CPPU treatment a day after flowering.
Appearance and cross-sections of pollinated and nonpollinated oriental melons treated with different concentrations CPPU a day before flowering and a day after flowering.
Changes in cucurbitacin B content in oriental melon fruits through artificial pollination without CPPU treatment and non-pollination with CPPU = N-(2-chloro-4-pyridyl)-N′-phenylurea treatment, verified by supplementation or no supplementation.
