1. Introduction
Melon (
Cucumis melo L.), a species of the
Cucurbitaceae family, is one of the most important vegetable crops since ancient times, and is cultivated in the warm season [
1,
2,
3].
Melon’s origin has been discussed for a long time and is still unclear. Due to the number of wild
Cucumis specimens, and followed by subsequent taxonomy and molecular data, Africa was thought to be the area where melon was domesticated [
2,
4,
5]. In the year of 2020, 28.5 M tons of melons were grown in the world. China is the world’s first melon producer with 13.8 million tons and followed by Türkiye with 1.7 M tons of melon [
6].
Grafting is a combination of two plant parts as a single plant by combining them with certain techniques. The use of grafted seedlings is spreading day by day and depends on the establishment of suitable methods and the development of strong rootstocks through breeding [
7,
8,
9]. The benefits of grafting in plants are based on economizing and facilitating agriculture including increase hybrid vigor, prevent soil-borne diseases and pests when there is a genetic deficiency for disease management [
10], increase efficiency, raise the yield and quality, increasing environment protection when using different chemicals like pesticides, expanding the production area (arid and saline areas, hot-cold areas etc.), broadening the production seasons including heat, cold, and etc [
11,
12,
13,
14,
15].
In melon,
Cucurbita interspecific hybrids (
Cucurbita maxima ×
Cucurbita moschata) are widely used as rootstock. Some rootstocks that have been studied but not yet used commercial including
Cucumis metuliferus,
Luffa cylindrica,
Benincasa hispida, and
Lageneria sicerena [
16]. The main purpose of grafting in melons is to provide resistance to soil-borne diseases (
Monosporascus cannonballus,
Fusarium oxysporum f. sp.
melonis (
Fom) and
Stagonosporopsis spp. and root-knot nematodes (
Meloidogyne incognita and
M. Javanica) [
12,
17,
18,
19] to increase the yield, quality, aroma and carotenoid contents, salinity and drought tolerance, nutrient intake and tolerance to nutrient deficiency [
20,
21,
22,
23]. According to the various studies also it has been determined that rootstock-scion combinations affect pH, flowering, sugar, color, carotenoid content and fruit surface [
24,
25,
26,
27,
28,
29,
30]. Melons like other cucurbit plants require relatively higher temperatures than other species, seedlings grafted onto
Cucurbita rootstocks were used for cultivation at low soil temperatures and earliness [
31]. Furthermore, grafted seedlings have increased in melons, and have been limited especially in cantaloupe melons due to low rootstock-scion compatibility [
32].
Production and fruit quality decreases as rootstock and scion can not be matched in grafting system. Therefore, the selection of combination should be done in the best way [
23]. For instance, in melons grafted onto
Cucurbita ficifolia, the transfer of photosynthetic substances from scion to the rootstock is prevented due to the incompatible rootstock-scion combination [
33,
34]. In the selection of the appropriate rootstocks, biotic and abiotic stress conditions of plants are considered [
30,
35], for instance, resistance/tolerance to soil-borne pathogens like
Fusarium oxysporum f. sp.
melonis [
35,
36,
37], salinity tolerance [
38,
39], tolerance to low and high soil temperatures [
40,
41,
42,
43]. It has been observed that RNA, protein, and small molecules can be transported from the rootstock to the scion and directly affect the scion physiology [
30].
Currently, in melons, the most common used grafting method is F1 hybrid which ensures healthy dominant genes in a single genotype [
44]. The F1 varieties are technically superior in case of arising yield, earliness, quality, uniformity, length of vegetation period, disease and pest tolerance, shorter time to produce new varieties, and high adaptability [
23]. The F1 hybrid seeds are obtained by the hand pollination of a female parent by male parent. It is a time consuming and expensive seed production method. In hybrid seed production, high quality and quantity is desired, however in some occasions low amount and empty seeds are acquired and it results in time and financial losses.
Though there have been several studies performed on grating in melons [
2,
17,
45,
46], there has been no study which carried out an the evaluation of the effects of grafted seedlings on hybrid seed production. The current research is based on the hypothesis that grafting melons onto different rootstocks affects plant growth, fruit quality, and seed quality. The present study, therefore, focused on the determination of the effects of grafting cantaloupe melons onto different rootstocks on plant growth, fruit, and seed quality.
4. Discussion
Previously, distinct studies have stated that rootstocks have a positive effect on plant length and leaf number, depending on the scion genotype, and that when the root structure of the rootstock was strong, the stem thickness and plant length increased [
49]. In the recent study, the result obtained from grafting 9 different rootstocks to the Kırkağaç 589 genotype have shown that the longest plants were ungrafted control plants and plants grafted onto interspecies hybrid “TZ 148” rootstock (457.8 and 456.3 cm, respectively), whereas the shortest plants have been determined as 301.8 cm in grafted plant on loofah rootstock with white seeds [
16]. Generally, plants were affected by the rootstock used in the experiment [
30,
50]. According to this study’s results, grafted plants had higher values compared to the control group. The Nun9075/H27 combination had the longest (270.93 cm) plants in the average graft combination.
Interspecific hybrid Cucurbita rootstocks have thick and long hypocotyledons that facilitate grafting, and although their emergence rates are high, they may cause a delay in flowering and maturation because of their vigor [
51]. Moreover, in a study performed, Galia melon cv. Arava was grafted onto hybrid squash rootstock Strong Tosa; the first female flower formation was delayed by 8–9 days, and it has been reported that the harvest times were 38 days in ungrafted plants and 39 days in self-grafted plants [
52]. Currently, in a study using a total of 9 different melon cultivars, consisting of 6 hybrid melon cultivar candidates and 3 control cultivars, the harvest time varied between 57 and 72 days, and it was stated that grafting had no effect on the harvest time, thus the only variation was observed based on the growing seasons [
53]. This means that the rootstocks and scions used in the study do not alter the harvest time because optimum harvest maturity is crucial for obtaining high-quality fruits during that period and is challenging to be determined between crops and even within melon species [
54]. Various studies have reported that fruit weight, fruit flesh thickness, fruit length, and diameter were positively correlated with rootstocks [
55,
56].
In general, the
Cucumis melo and
Cucurbita interspecific rootstocks have little or no effect on the scion’s fruit weight [
57]. The United Nation Economic Commission for Europe (UNECE) reported the size of Galia melon fruit ant its diameter [
58]. Due to the results of the study performed in 2018, the 9 different rootstocks were used for 589 melon cultivars and their fruit weights have been ranged between 1096 and 4375 (g) [
16]. Beside this, two pure lines were grafted onto Canay F1 melon cultivars and their fruit weights have been varied in range of 670 and 990 (g) [
59]. Furthermore, by using 7 different rootstocks for Falez and Galia melon cultivars, the fruit weights of the Galia melon cultivars remained between 1009 and 1241 (g). Kırkağaç type Sinem 45 Fı and Sürmeli F1 melon cultivars were grafted onto three
Cucurbita hybrid (
C. maxima Duch. ×
C. moschata Duch.) Ares Fı, Nun 9075 Fı and TZ 148 Fı rootstocks to investigate the effect of grafting on yield and quality. The results indicated that fruit weight was 4200 g and was not affected by grafting. Self-grafted plants had higher values that ranged from 6.5 to 7 kg [
52]. In the study carried out by Soteriou et al. [
51] using the interspecific hybrid “TZ 148” as rootstock in Galia melons, there was no effect observed on fruit weight. In this study, in accordance with other studies, fruit seed cavity and diameter were not affected by grafting, nevertheless, fruit weights variation were determined in a combination of H4 scion to Nun9075 rootstock (Nun9075/H4).
According to Karabulut et al. [
16], grafted plants had lower values (21.5 cm) in fruit length than ungrafted plants (23.1 cm). Although rootstocks did not have any effect on fruit diameter when using TZ 148 as rootstocks. Moreover, based on melon studies, fruit length ranged between 20.30 cm and 29.9 cm, and fruit diameter ranged between 15.69 cm and 16.68 cm, and it was found that grafting was not effective on these parameters [
22]. Compare this to Namli et al. [
60], where fruit length varied between 34.98 cm and 21.33 cm and fruit diameter was in the range of 17.21 cm and 13.67 cm with Ares F1 and TZ 148 rootstocks. The present study, the results demonstrated that the longest fruit (12.88) cm and widest fruit (14.16 cm) were observed when TZ 148/H27 cultivar was used as rootstock in second year.
The big fruit cavity is an undesirable trait that reduces the fruit quality. For instance, the length of the cavity has been varied between 12.3 and 22.67 cm, while the cavity diameter showed similar results of 6.3 and 8.28 cm. Furthermore, the average thickness of fruit flesh was 2.5 cm in ungrafted plants and 2.4 cm in grafted plants, and it changed from 1.6 cm to 2.6 cm during the use of TZ 148 rootstock [
16]. On the other hand, fruit rind thickness was determined as 0.7 cm in ungrafted and 0.5 cm in grafted plants, between 0.5 and 0.9 in the use of other rootstocks [
61], and 5.17 and 4.05 mm in ref. [
53]. The thickness of the fruit flesh varied between 5.25 cm and 4.51 cm, and rind thickness between 2.40 and 3.22 mm [
59]. These differences in fruit flesh thickness may be due to rootstock and scion, location, and environmental differences [
16,
46,
53].
Total soluble solid is an important fruit quality criterion in melons. In a previous study, it was stated that TSS in melons is more affected by the prevailing temperature and planting dates [
62]. Although TSS analysis is a practical method for determining harvest maturity, it may not always correlate with the sensory quality and sugar ratio [
63]. According to UNECE standards, the juice taken from the middle of the melon fruit is good when it is in the range of 10 Brix and 8 brix, while the USDA reported that the minimum should be between 9 brix and 11 Brix for very good quality [
64]. The commercially acceptable brix should be 10 or more than 10 [
58]. In the study by Ünlü et al. [
59], the TSS ranged between 6.67 and 14.52 for graft combinations with lower values than ungrafted plants. The fact that during the summer season (the hottest months), the uptake of water and minerals in plants slows down and the rapid ripening of the fruits reduces the rate of TSS. In the study carried out by Lecholocholo et al. [
65],
Cucurbita maxima ×
Cucurbita moschata hybrid rootstocks were grafted to 4 different melon cultivars, and TSS was found to be higher than ungrafted plants, and TSS remained below 10% in both years. In our study, different rootstocks did not affect the TSS rate. The TSS rates varied between 10.3 and 12.1 in the first year and between 10.4 and 11.6 in the second year. In their study, Yarsi et al. [
46] demonstrated that TSS changed between 9.04 and 7.7 and remained between 8.7 and 7.7 in Galia melons. In melons grown in spring and autumn, higher TSS was detected compared to the summer season [
66]. Furthermore, Ohletz and Loy [
67] determined the TSS between 11.8 and 11.00 (%). Our results showed the TSS was not affected by grafting, and the TSS rates were in the range of 13.42 and 8.94 (%).
The germination of seeds varies depending on the plant species and variety, and environmental effects such as water, temperature, light and oxygen [
68]. In the study of Karabulut et al. [
16] the highest seed germination rate was found to be 100% in TZ 148 and pumpkin rootstocks, and the lowest germination rate (75%) was obtained in white seed of loofah rootstock. However, the results of our study indicated the highest germination rates that was observed from the H27 (87.16%), and self-grafted H27 (83.05%) genotypes used as rootstock. Based on the results from Edelstein and Nerson [
69]’ study, authors reported that fruit weight and size were positively correlated with seeds in watermelon. In the study conducted by Yetisir and Sarı, [
70] by using 5 Lagenaria rootstocks were grafted onto the Crimson Tide watermelon cultivar and their seed yield was examined. The number of seeds per plant varied between 858.8 and 556.1 seeds in the first year and between 738.6 and 489.2 seeds in the second year, and more seeds were obtained in the first year. The number of seeds increased with grafting. In results of this study, the year was not a factor on the number of full seed, but the genotype and year × genotype interaction increased the number of full seeds, and the highest number of seeds (514.38 seeds) was obtained by crossing the self-grafted H27 and self-grafted H4 genotypes. In the study performed on Crimson sweet, watermelon cultivars were grafted with three different rootstocks,
Cucurbita ‘Nun9075′,
Lagenaria ‘Argentario’, and
Citron watermelon ‘PI296341’, to investigate their fruit flesh to seed ratio. The number of seeds varied between 558.11 and 805.00 (seeds) in the first year, and between 241.25 and 483.00 (seeds) in the second year. The 1000 seed weight was not affected by grafting and was ranged from 36.14 to 40.74 g in the first year and between 28.15 and 32.52 g in the second year.
Seed germination and emergence rates were much higher (39.3%) than the control group (30.5%), while in the graft combinations, the germination and emergence rates varied between 68.5% and 61.0% and were mentioned as low values. The highest values of germination and emergence rates observed were 97.5%, 91.0%, respectively, from watermelon varieties [
71]. Seed germination day varied between 2.6 and 7.3 days; the latest germination was in the control group. Seed emergence days ranged from 10.8 to 7.1 days. Since the rootstocks improved root development, seed yield, plant strength, fruit size, and seed number increased, and the use of grafting techniques can affect the harvest day and weak lines in hybrid seed production [
70]. In our study, the 1000 seed weight increased, and the heaviest seed average was obtained in 2019 (28.21 g). The TZ 148/H27 × H4 graft combination produced 29.21 g and 31.32 g in the first year. Our results also highlighted that the year × genotype interaction increased seed characteristics such as the number of full seeds, seed weight, germination, and emergence rates. Additionally, the high seeds were obtained from self-grafted hybrids.