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Article

Influence of Rootstock on Growth, Yield, and Fruit Quality of the ‘Femminello’ Bergamot (Citrus bergamia Risso & Poit.)

by
Rocco Mafrica
1,
Antonio Gattuso
1,2,*,
Davide Mafrica
1,
Alessandra De Bruno
3,* and
Marco Poiana
1
1
Department of AGRARIA, University Mediterranea of Reggio Calabria, 89124 Reggio Calabria, Italy
2
Experimental Station for the Industry of the Essential Oils and Citrus Products SSEA, 89127 Reggio Calabria, Italy
3
Department of Human Sciences and Promotion of the Quality of Life, San Raffaele University, 00166 Rome, Italy
*
Authors to whom correspondence should be addressed.
Agriculture 2026, 16(4), 405; https://doi.org/10.3390/agriculture16040405
Submission received: 26 December 2025 / Revised: 5 February 2026 / Accepted: 7 February 2026 / Published: 10 February 2026
(This article belongs to the Section Crop Production)
Versions Notes

Abstract

To identify the most suitable rootstocks for bergamot production in Italy, vegetative growth, yield performance, and fruit quality were assed in “Femminello” bergamot trees grafted onto eight different rootstocks under the Mediterranean edaphoclimatic conditions of Reggio Calabria (Southern Italy). Rootstock selection significantly affected tree vigor, productivity, and fruit quality. Alemow induced the greatest vegetative growth, producing trees with canopy volumes up to 60% larger than those grafted onto Sour Orange, whereas Flying Dragon caused a strong dwarfing effect, reducing canopy volume by approximately 80%. Carrizo Citrange and Swingle Citrumelo exhibited the highest yield efficiency (9.7 and 9.5 kg m−3, respectively), about 30% higher than Sour Orange, while Alemow showed the lowest efficiency (1.8 kg m−3). Cumulative yield over seven cropping seasons was highest on Carrizo Citrange (196 kg tree−1), with comparable values recorded for Sour Orange, Swingle Citrumelo, and Trifoliate Orange. In contrast, Alemow and Flying Dragon yielded 55% and 85% less, respectively. Rootstock selection significantly influenced fruit size, peel characteristics, and juice quality. Rootstock selection had a marked effect on fruit size, peel characteristics, and juice quality. Fruit weight ranged from under 170 g on Sour Orange, Volkameriana, and Alemow to approximately 196 g on Trifoliate Orange, while at full maturation, most rootstocks produced fruits weighing between 213 and 223 g, except for Alemow (<200 g). Trifoliate Orange and its hybrids promoted thinner peel and higher juice content, whereas Alemow and Volkameriana produced fruits with thicker peel and up to 15% lower juice content than Carrizo Citrange. Juice titratable acidity decreased during maturation, ranging from over 50 g L−1 on Sour Orange and Alemow to around 39–41 g L−1 on Trifoliate Orange, Carrizo Citrange, Troyer Citrange, and Flying Dragon at harvest. Overall, Trifoliate Orange, Carrizo Citrange, and Swingle Citrumelo emerged as promising alternatives to Sour Orange, combining high yield efficiency, satisfactory fruit quality, and improved yield precocity.

1. Introduction

Bergamot (Citrus bergamia Risso & Poit.) is a minor citrus species. According to recent phylogenetic and biomolecular evidence, it is considered a hybrid between citron (Citrus medica L.) and sour orange (Citrus aurantium L.) [1]. Its cultivation is almost entirely limited to a narrow coastal area in the province of Reggio Calabria, Southern Italy [2], where it has traditionally been grown primarily for the extraction of rind oil, which is widely used in the cosmetic, pharmaceutical, and food industries [3,4]. In contrast, juice extracted from the endocarp is not commonly consumed as a beverage due to its bitter taste [4,5].
In recent years, bergamot derivatives have attracted increasing scientific interest due to their beneficial effects on human health [6,7,8,9,10,11]. In particular, bergamot juice has been reported to exhibit antioxidant [12,13], anti-inflammatory [14,15,16], anticancer [17,18,19], and hypolipidemic [20,21] properties. This growing body of evidence has stimulated consumer interest in bergamot juice and fresh fruits. As a result, a new market for fresh bergamot fruit has emerged alongside the traditional essential oil industry, contributing to a gradual expansion of cultivated areas.
The introduction of bergamot into the fresh fruit market represents an important opportunity but also necessitates ensuring that fruits meet high quality standards. This, in turn, highlights the need to identify the most suitable cultivation areas [22], optimal harvest time [23], and strategies to maximize crop performance through appropriate rootstock selection and rational agronomic management. Unfortunately, current knowledge in these areas remains limited, particularly regarding rootstock choice.
To date, sour orange (Citrus aurantium L.) has been the only rootstock used in the bergamot industry, as it has long been considered the most suitable in various bergamot growing regions due to its good productivity under different environmental and pedological conditions, including calcareous soils [24]. Its tolerance to several fungal diseases, such as Phytophthora spp., and to viroids such as exocortis (CEVd) and xyloporosis (CCaVd), has further contributed to its widespread use [25]. The satisfactory production performance of sour orange in orchards dedicated to essential oil extraction has historically limited the exploration of alternative rootstocks.
Nevertheless, the increasing need to improve yields and fruit quality for the fresh-fruit market makes the identification of new rootstock options essential, as has occurred in other citrus species. Rootstock plays a key role not only in enhancing plant tolerance to biotic and abiotic stresses [26,27,28,29], but also in influencing tree physiology [30], mineral uptake [31], canopy size, fruit yield efficiency [32], maturity and quality [33,34,35,36], and the production of primary and secondary metabolites [37,38,39]. Overall, citrus yield performance, growth habit, precocity, and fruit quality are strongly influenced by the rootstock–scion interaction [31,40,41,42,43]. Accordingly, the objective of the present study was to evaluate the response of the “Femminello” bergamot cultivar grafted onto eight different rootstocks under the environmental conditions of the traditional bergamot-growing area in the province of Reggio Calabria.

2. Materials and Methods

2.1. Plant Material and Experimental Conditions

In April 2015, scions of the “Femminello” bergamot cultivar were grafted onto the eight rootstocks listed in Table 1. All nursery operations were performed in an aphid-proof greenhouse equipped with an evaporative cooling system, where temperatures ranged from 18 to 27 °C and relative humidity was maintained at approximately 80%. In April 2016, one year after budding, the nursery trees were used to establish an experimental citrus grove. The orchard was planted using a randomized complete block design with three blocks and three trees per rootstock in each block, for a total of 72 trees. Tree spacing was 5 × 5 m. Border rows surrounded the experimental area on all four sides.
The experimental citrus grove was located in Melito Porto Salvo (15 m a.s.l., 15°48′24″ E longitude, 37°55′24″ N latitude), approximately 30 km from Reggio Calabria (Southern Italy). The area has maximum and minimum temperatures of 26.8 and 11.3 °C, respectively, and an average annual rainfall of 596 mm. The soil (0–90 cm depth) is a clay loam with pH 7.0, CaCO3 3%, active calcium carbonate 2.2%, and electrical conductivity of 3.97 mS·cm−1 (25 °C). Irrigation water had a pH of 7.0 and electrical conductivity ranging from 0.15 to 0.25 mS·cm−1.
Standard cultural practices were applied in managing the experimental grove. Trees were irrigated every three days from April to October via a drip system with six pressure compensating drippers delivering 8 L/hour per tree. The irrigation volume varied according to tree age and time of year. Fertilization was applied from the second year onwards, with rates increasing annually. Between the second and fifth years, application rates increased from 30 to 200 kg ha−1 for N, from 8 to 50 kg ha−1 for P2O5, and from 15 to 120 kg ha−1 for K2O. From the sixth year onwards, annual application rates were 250 kg N ha−1, 76 kg P2O5 ha−1 and 200 K2O ha−1. Nitrogen was applied in split doses each year (two-thirds in mid-March and one-third in summer), while P2O5 and K2O were applied in a single dose in mid-December. After three years of cultivation, trees were hand-pruned annually after harvest to promote regular and balanced production. Pruning consisted of removing unproductive, dry, misplaced or excessively long branches, and water shoots present in the internal parts of the canopy to prevent them from becoming too dominant. Pest and disease control was carried out according to integrated pest management (IPM) principles.

2.2. Vegetative and Yield Determinations

Tree height, canopy diameter and trunk diameter (measured 10 cm above and below the bud union) were recorded annually at the end of January from 2017 to 2024. Canopy volume (V) and the scion–rootstock ratio were calculated each year.
Canopy volume was calculated using the following equation:
V = 0.524 × height × width2
The tree shape was assumed to be one-half a prolate spheroid [44].
The trunk diameter index was determined as the ratio between the trunk diameter above (scion) and below (rootstock) the graft union.
Yield data were recorded starting from the first harvest (2017/2018). All fruits from each tree, including recently dropped fruits collected weekly, were weighed to determine total yield (kg·tree−1). Cumulative yield and yield efficiency were also calculated. Cumulative yield was calculated over the seven cropping years (from 2017/2018 up to 2023/2024). Yield efficiency (kg m−3) was calculated as the ratio between yield (kg tree−1) and canopy volume (m3) determined in the 2022/2023 and 2023/2024 cropping years.

2.3. Physical Parameters Measurements

Fruit quality was determined during 2019/20, 2020/21, 2021/22 and 2022/23 crop years. For each season, two harvests were performed at 230 and 260 days after full bloom (DAFB). Full bloom was defined as the stage when approximately 50% of flowers were open and petal fall had begun. The two harvest periods, 230 and 260 DAFB, which in the bergamot growing area of the province of Reggio Calabria approximately correspond to mid-December and mid-January, respectively, represent the traditional harvest period for “Femminello” bergamot. In the absence of specific maturity indices for bergamot, fruits in this area are generally harvested when the peel color changes from light green to yellow. During this period Femminello bergamot fruits typically have a juice yield close to or greater than 40%, and the juice is no longer excessively sour for consumption.
For each harvest date, thirty-six representative fruits (12 fruits per block × 3 blocks) per rootstock–scion combination were collected, transported to the laboratory, and individually weighed and measured. Fruit weight was determined using a digital balance (BC 2200C; ORMA, Milan, Italy). Fruit height, fruit width and peel thickness were measured with an electronic digital slide gauge (mod. 1651DGT, Beta Utensili S.p.a., Sovico, Italy) and the fruit shape index (height/width) was calculated.
Bergamot peel color was assessed at four equidistant points in the equatorial region of individual fruits using a Minolta CM-700d spectrophotometer (Minolta, Osaka, Japan), according to the Commission Internationale de l’Éclairage (CIE) and expressed as L*, a* and b* color values [45,46]. Mean values for L*, a* and b* were calculated for each fruit. From these, chromaticity or chroma (C* = (a*2 + b*2)1/2) and hue angle (H0 = arctan (b*/a*)) were calculated, and the corresponding citrus color index (CCI) was determined using the equation proposed by Jimenez-Cuesta et al. [47], CCI = 1000 a*/L* b*.

2.4. Juice Quality Parameters Determination

In addition to the measurement of physical parameters, juice content and other qualitative juice traits were assessed over four consecutive crop years (from 2019/20 up to 2022/23) and for two harvest times (at 230 and 260 DAFB). Fruit juice was extracted using a commercial juice extractor (Citrus Juicer mod. Apollo 150W, SIRMAN SpA, Curtarolo, Italy). For each rootstock, three juice samples were prepared by pooling the juice from 12 fruits per block (3 blocks). The extracted juice was weighed and recorded in grams. Juice content was determined according to the following equation and expressed as a percentage:
JC = (juice weight/fruits weight) × 100.
Total soluble solids (TSS) were measured using a digital refractometer (DBR 047 SALT, Giorgio Bormac Srl, Modena, Italy) and expressed as °Brix. Titratable acidity (TA) was determined via potentiometric titration (TitraLab AT1000 Series, Hach Lange S.r.l Lainate, Milano, Italy)) using 0.1 N NaOH to pH 8.1 [48] and expressed as g L−1 of citric acid equivalent. Maturity Index (MI) was calculated as the ratio between TSS and TA.
Ascorbic acid was evaluated by liquid chromatography [49] using a HPLC-DAD system (Knauer HPLC Smartline Pump 1000; Knauer Smartline UV Detector 2600, KNAUER, Berlin, Germany) equipped with a SYNERGI HYDRO-RP column (250 mm × 4.6 mm i.d., 4 μm) at 22 °C. Filtered samples (20 μL) were injected and eluted isocratically at 0.7 mL·min−1 using a 20 mM potassium phosphate mobile phase, acidified to pH 2.9. Concentrations were calculated using external calibration curves and expressed as mg L−1 juice.

2.5. Statistical Analysis

Data were subjected to analysis of variance (ANOVA), and differences between means were evaluated using Tukey’s test at p < 0.05 with the Systat 13 statistical program (SYSTAT Software Inc., Chicago, IL, USA).

3. Results

3.1. Vegetative Growth and Fruit Yield

The different rootstocks significantly influenced several growth parameters of the bergamot cultivar Femminello (Table 2). The effects of the rootstocks on tree growth became evident at an early stage. At the end of the second growing season (twenty-two months after planting), trees budded onto Alemow showed a significantly larger canopy volume than those grafted onto the other rootstocks. Trees budded onto Sour Orange and Volkameriana exhibited canopy volumes approximately half of those observed on Alemow. Even smaller canopies (around 0.4 m3) were observed in trees grafted onto Trifoliate Orange and its hybrids, while the shortest trees were those grafted onto Flying Dragon. The differences in canopy growth induced by the rootstocks in the Femminello bergamot cultivar remained evident in the following years and throughout the duration of the study. At the end of the eighth growing season, trees budded onto Alemow consistently showed the highest canopy volume (11.6 m3). Compared with trees grafted onto Alemow, those grafted onto Sour Orange and Volkameriana exhibited a canopy volume approximately 40% smaller. Trees budded onto Trifoliate Orange and its hybrids showed reduced canopy volumes, with values ranging between 4.9 and 5.3 m3. The smallest canopy volume was observed in combination with Flying Dragon (1.5 m3). Rootstock trunk diameter values ranged from 7.5 cm (in Flying Dragon) to 12.8 cm (in Alemow) and appeared to be associated with canopy volume (Table 2). Significant differences in the trunk diameter index were recorded (Table 2). This parameter measures relative growth rate between scion and rootstock [50] and is commonly used as an indicator of scion/rootstock affinity, with values close to 1 indicating very good affinity [51]. Sour Orange, Alemow and Volkameriana tended to grow more rapidly than the scion. Conversely, in trees budded onto Flying Dragon, Trifoliate Orange and its hybrids, the rootstock grew more than the scion. However, the growth differences between scion and rootstock were very small for all rootstocks.
Trees on all rootstocks started bearing in the 2017/2018 cropping year, one year after planting (Table 3). The first harvest of Femminello bergamot trees on all eight rootstocks was limited, although the fruits were generally commercially acceptable. Despite the very low yields, statistically significant differences were recorded among the rootstocks. In this first harvest, trees grafted onto Trifoliate Orange and Carrizo Citrange exhibited higher yields than those on all other rootstocks, indicating earlier fruiting. Trees grafted onto Flying Dragon were the least productive.
Yields gradually increased in subsequent cropping years as tree growth progressed. In the first four crop years, trees budded onto Carrizo Citrange and Trifoliate Orange showed the highest yields, reaching a production close to 30 kg·tree−1 in the 2020/2021 cropping year. Oven this period (from 2017/2018 to 2020/2021), high yields were also recorded in trees budded onto Troyer Citrange and Swingle Citrumelo, with significantly higher values than those of the other remaining rootstocks. From the 2020/2021 cropping year onward, yields were similarly high for trees budded on Sour Orange and Volkameriana. In the following two crop years, trees grafted onto these two rootstocks, together with those budded onto Carrizo Citrange, showed the highest yields. In the 2023/2024 cropping year, the highest yields were recorded in trees grafted onto Sour Orange, Carrizo Citrange, and Swingle Citrumelo. However, yields of trees grafted onto the latter two rootstocks did not differ statistically from those of trees grafted onto Trifoliate Orange and Volkameriana. The minimal yield differences observed between the last two crop years (2022/2023 and 2023/2024) across all grafting combinations indicated that, eight years after planting, Femminello bergamot trees had reached, or were close to reaching, full production. In this cropping year, trees budded on Sour Orange showed the highest yields, with a production of around 50.7 kg·tree−1. However, this yield was not statistically different from that of trees budded onto Carrizo Citrange and Swingle Citrumelo, supporting the excellent performance of these rootstocks throughout the duration of the study. Conversely, trees on Alemow and Flying Dragon were the least productive.
Trees on Carrizo Citrange showed the highest cumulative yield, exceeding 195 kg·tree−1 after seven crop years; however, this value did not differ significantly from those recorded on Sour Orange, Swingle Citrumelo, and Trifoliate Orange (Table 3). The cumulative yields of trees grafted onto Sour Orange, Swingle Citrumelo, Trifoliate Orange and Volkameriana were not significantly different from one another. Among the Trifoliate Orange hybrids evaluated, trees grafted onto Troyer Citrange showed the lowest cumulative yields. Trees grafted onto Alemow and Flying Dragon were the least productive, with cumulative yields of 83 and 30 kg·tree−1, respectively.
Significant differences in yield efficiency were observed among rootstocks (Table 3). The highest yield efficiency was recorded in trees grafted onto Carrizo Citrange and Swingle Citrumelo (9.7 and 9.5 kg m−3 respectively), with values significantly higher than those of the other rootstocks. Yield efficiency was also relatively high in trees grafted onto Trifoliate Orange (8.8 kg·m−3), and Troyer Citrange (7.7 kg m−3). Intermediate values were observed in trees on Sour Orange and Volkameriana (6.1 and 5.9 kg m−3, respectively). The lowest yield efficiency was observed in trees grafted onto Alemow (1.4 kg m−3).

3.2. Physical Parameters of the Fruits

The main physical and qualitative fruit parameters were evaluated over four consecutive cropping years, starting from the 2019/2020 cropping year, when the trees had already reached a satisfactory level of fruiting. For each crop year, fruit evaluation was carried out at two harvest times (230 and 260 DAFB). The results obtained indicate that the rootstock affected most of the parameters evaluated (Table 4). At both harvest times, fruit weight was significantly influenced by the rootstocks (Table 4). At the first harvest time (230 DAFB), the highest fruit weights were recorded in trees grafted onto Trifoliate Orange and its hybrids, as well as in its dwarfing mutant Flying Dragon. Fruits from trees grafted onto Sour Orange, Volkameriana, and Alemow showed the lowest fruit weights. However, no statistically significant differences were observed between fruits from trees budded onto Sour Orange and Troyer Citrange.
At the second harvest time (260 DAFB), differences in fruit weight among rootstocks, although still statistically significant, were less pronounced. Indeed, the greater increase in fruit weight (around 30%) observed between 230 and 260 DAFB in trees grafted onto Sour Orange and Volkameriana allowed these fruits to reach weight values comparable to those observed for the other rootstocks. Only fruits from trees grafted onto Alemow continued to show the lowest weight, although this value was not statistically different from that recorded for Volkameriana.
Similar trends observed for fruit weight were also recorded for fruit height and width (Table 4). For both harvest times, no significant effect of rootstock was observed on the fruit height/width ratio (Table 4).

3.3. Color and Thickness of the Peel

Significant differences among rootstocks were recorded in bergamot fruit peel color parameters (Table 5). As observed for fruit size, differences among rootstocks were more evident in fruit harvested at 230 DAFB. At the first harvest, peel lightness (L* value) ranged from 63.1 in fruits from trees grafted onto Alemow to 68.8 in those grafted onto Trifoliate Orange (Table 5). Notably, L* values of fruits from trees grafted onto Trifoliate Orange, Flying Dragon, and Carrizo Citrange were significantly higher than those recorded for the other rootstocks. Values recorded for Troyer Citrange were not significantly different from those of the previously mentioned rootstocks, nor from those of Sour Orange and Volkameriana. The lowest L* values were observed in fruits from trees grafted onto Swingle Citrumelo and Alemow; however, these values were not significantly different from those of Sour Orange. Peel lightness increased as fruit ripening progressed. During the second harvest, L* values were significantly higher than those at the first harvest (Table 5). Differences among rootstocks remained broadly unchanged, with fruits from trees grafted onto Carrizo Citrange, Troyer Citrange, and Trifoliate Orange showing a lighter-colored peel (higher L* values).
Substantial differences were also observed for the colorimetric coordinate a* (Table 5). At the first harvest (230 DAFB), a* values of fruits from trees grafted onto Alemow and Swingle Citrumelo were particularly low (close to zero or even negative), indicating that a certain amount of chlorophyll was still present in the peel. Differences among rootstocks, although still significant, became narrower at the second harvest (260 DAFB), with a* values ranging from 3.9 (Swingle Citrumelo) to 6.7 (Trifoliate Orange).
At the first harvest, b* values ranged from 34.1 in fruits from trees grafted onto Swingle Citrumelo to 44.1 in those grafted onto Flying Dragon (Table 5). Although fruits from all graft combinations exhibited yellow peels, those harvested from trees grafted onto Flying Dragon, Trifoliate Orange, Carrizo Citrange, and Troyer Citrange showed a more intense color tone than fruits from the other rootstocks. Significantly higher b* values were recorded at the second harvest, ranging from 41.5 (Swingle Citrumelo) to 47.8 (Carrizo Citrange).
At the first harvest, peel hue angle (h°) values of fruits from trees grafted onto Carrizo and Troyer Citrange, Flying Dragon, and Trifoliate Orange were similar to those measured in fruits from trees grafted onto Volkameriana, whereas the remaining rootstocks induced higher hue values, which are associated with a greener peel color. At the second harvest, hue values were lower and the range of variation among rootstocks was narrower (from 82 to 85) than at the first harvest.
In fruits harvested at 230 DAFB, C* values ranged from 34.1 to 44.3 (Table 5). Six of the eight rootstocks studied showed C* values significantly higher than those recorded in fruits grafted onto Swingle Citrumelo. This indicated that, compared to Swingle Citrumelo and Alemow (which was not statistically different), trees budded onto the remaining rootstocks produced fruits with a more vivid and saturated peel color. Despite a significant increase in C* values and overall color saturation, differences among rootstocks remained evident at the second harvest. Fruit from trees grafted onto Swingle Citrumelo, Volkameriana, and Sour Orange exhibited less vivid peel color than those from the other rootstocks.
The tendency of fruits from trees grafted onto Carrizo and Troyer Citrange, Flying Dragon, and Trifoliate Orange to degrade chlorophyll more rapidly than those from other rootstocks was confirmed by the Citrus Color Index (CCI) results (Table 4). At the first harvest, fruits from these rootstocks showed significantly higher peel CCI values. Among the remaining rootstocks, the values recorded for Swingle Citrumelo (−0.5) and Alemow (0.1) were particularly low, clearly indicating that chlorophyll in the peel of these fruits had not yet been completely degraded. Subsequently, although differences among rootstocks remained statistically significant, they tended to decrease. At 260 DAFB, fruit peel from all graft combinations showed significantly positive CCI values, confirming that chlorophyll had been completely degraded by that time.
Peel thickness decreased as ripening progressed in all graft combinations. However, this decrease was significant only in fruits from trees grafted onto Sour Orange, Swingle Citrumelo, Volkameriana, and Alemow. Differences among rootstocks in peel thickness, although significant, were minimal (Table 5). Fruits obtained from trees budded onto Alemow, Volkameriana, and Sour Orange were characterized by the thickest peel at both harvest times.

3.4. Juice Quality and Chemical Parameters

Analysis of fruit quality data highlighted significant rootstock effects on many of the parameters evaluated. The juice content of Femminello bergamot fruits exceeded 35% across all rootstock combinations and at both harvest times (Table 6), indicating a substantial rootstock effect on this important quality parameter.
At the first harvest, very high juice contents (above or close to 50%) were recorded in fruits harvested from trees grafted onto Carrizo Citrange, Trifoliate Orange, Flying Dragon, and Troyer Citrange. High juice contents (around 45%), not statistically different from those of the previously mentioned rootstocks, were also recorded in fruits from trees grafted onto Swingle Citrumelo and Sour Orange. Lower juice contents (<40%) were observed in fruits from trees grafted onto Volkameriana and Alemow.
Compared to the first harvest, an increase in juice content was recorded across all rootstock combinations. However, this increase was significant only in fruits from trees grafted onto Sour Orange, Swingle Citrumelo, Volkameriana and Alemow, with an average increase of 16%. Smaller and non-significant increases (approximately 7%) were observed in the remaining four rootstocks.
Only minor differences among rootstocks were observed for juice total soluble solids (TSS). At the first harvest, TSS values showed a very narrow range of variation (8.1 and 8.4 °Brix). Greater variability and statistically significant differences among rootstocks were observed in fruits harvested at 260 DAFB. Fruits from trees grafted onto Swingle Citrumelo showed the highest TSS value (8.0 °Brix), which was significantly higher than those recorded for Carrizo and Troyer Citrange, Trifoliate Orange, Volkameriana, and Alemow (7.6–7.7 °Brix), which showed the lowest values.
More pronounced differences among rootstocks were observed for juice titratable acidity (TA). At the first harvest, juices from fruits of trees grafted onto Alemow (51.7 g·L−1), Swingle Citrumelo (50.7 g·L−1), and Sour Orange (50.1 g·L−1) showed the highest TA values, which were significantly higher than those recorded for Volkameriana, Carrizo and Troyer Citrange, Flying Dragon, and Trifoliate Orange (43.6 and 47.2 g·L−1). At the second harvest, characterized by a significant decrease in TA values compared with the first harvest, the highest TA contents were observed in fruits from trees grafted onto Swingle Citrumelo and Sour Orange (46.0 and 44.9 g·L−1, respectively).
The TSS/TA ratio, conventionally referred to as sugar/acid ratio, was significantly affected by rootstocks. At the first harvest, fruits from trees grafted onto Trifoliate Orange, Flying Dragon, Carrizo Citrange, and Troyer Citrange showed the highest TSS/TA values, although fruits from Carrizo Citrange did not differ statistically from those from Volkameriana. At the second harvest, TSS/TA values increased; however, this increase was statistically significant only in fruits from trees grafted onto Sour Orange, Alemow, Swingle Citrumelo, and Volkameriana. The increase was particularly marked in fruits from trees grafted onto Volkameriana (+10.5%) and Alemow (+19.3%). This pronounced increase was mainly due to the significant reduction in TA values recorded between the first and second harvests in these two graft combinations.
Fruits harvested at the second harvest showed higher ascorbic acid contents than those harvested at the first harvest, regardless of the rootstock. A significant rootstock effect on ascorbic acid content was also observed. Although some variability in rootstock behavior was detected between the two harvest times, fruits from trees grafted onto Sour Orange and Swingle Citrumelo consistently showed significantly higher ascorbic acid contents than those from Trifoliate Orange and Flying Dragon at the first harvest, and than those from Trifoliate Orange, Flying Dragon, Volkameriana, and Alemow at the second harvest.

4. Discussion

4.1. Rootstock Effects on Tree Vigor and Graft Compatibility

The results of this study clearly demonstrate that rootstock strongly influences the vegetative and productive behavior of the bergamot cultivar Femminello, affecting tree growth, yield, and the carpometric and qualitative characteristics of the fruits. Regarding tree growth, compared with Sour Orange, the traditional bergamot rootstock, Alemow was distinguished by promoting increased tree vigor, while Volkameriana induced vigor levels similar to those observed for Sour Orange. Overall, Alemow, Sour Orange, and Volkameriana proved to be rootstocks leading to larger trees, characterized by greater leaf mass and canopy volume in bergamot, as has already been widely reported for other citrus species [52,53,54,55,56]. This behavior may be related to the presence of larger xylem vessels, which facilitates more efficient water and nutrient transport [57,58,59,60]. Differences in tree growth are also likely associated with an enhanced ability of plants to assimilate CO2 during photosynthesis [61]. Trees grafted onto rootstocks that enhance photosynthetic capacity tend to grow more vigorously than those grafted onto other genotypes [62]. In particular, a higher leaf mass combined with increased net CO2 assimilation rates likely result in greater translocation of carbon compounds (mainly sucrose and starch) from shoots to roots [62]. The increased availability of carbohydrates in the roots allows for enhanced root system development and, consequently, greater water and nutrient uptake capacity [63]. In contrast, Trifoliate Orange and its hybrids, exhibited more limited tree growth, in agreement with results reported for other citrus species [64,65]. Flying Dragon strongly reduced tree growth, confirming its role as the most dwarfing citrus rootstock currently available on the market [60,66,67,68].
Growth reduction induced by dwarfing rootstocks has been associated with lower leaf and stem water potentials in the scions grafted onto these rootstocks compared with those grafted onto vigorous rootstocks, probably due to high hydraulic resistance. This condition may lead to water deficits in leaves during periods of high evaporative demand, resulting in stomatal closure [60]. Consequently, dwarfing rootstocks are less efficient in transporting water from the soil to the stem [69]. Although statistically significant, the small differences observed between scion and rootstock growth indicate that all the rootstocks studied exhibit good graft compatibility with the bergamot cultivar Femminello.
Indeed, in all cases, despite some differences in the relative growth of the scion compared with the rootstock, trunk diameter index values were close to 1, indicating a high level of graft compatibility [51]. Graft compatibility with the bergamot cultivar Femminello was also good for rootstocks such as Swingle Citrumelo, which are often considered problematic in this respect. Previous studies have in fact reported high incompatibility, based on trunk diameter indices, between Swingle Citrumelo and several citrus cultivars, including Lane Late, Delta, and Shamouti sweet oranges [35,52], as well as Nova and Clementine mandarins [70,71]. Swingle Citrumelo typically shows trunk overgrowth with most scion cultivars and has also been reported to be incompatible with other scions such as Eureka lemon, Tomango and Pera oranges, and Murcott tangor [63].

4.2. Yield Performance and Yield Efficiency

Regarding tree productivity, the results highlight significant differences in the yield performance of Femminello bergamot trees induced by the different rootstocks. Trifoliate Orange and its hybrids, particularly Carrizo Citrange, were shown to induce not only high overall fruit production but also earlier bearing. Although from the 2020/2021 cropping year onward yields of trees grafted onto these rootstocks became similar to those of trees grafted onto Volkameriana and Sour Orange, during the earlier cropping years trees grafted onto Trifoliate Orange and its hybrids exhibited higher yields. This behavior was particularly evident in trees grafted onto Carrizo Citrange, which allowed trees grafted onto this rootstock to achieve significantly higher cumulative yields during the first three cropping years than trees grafted onto the other rootstocks. The high yields recorded for trees grafted onto Sour Orange and Volkameriana starting from the 2020/2021 cropping year enabled these rootstocks to compensate for their lower productivity in the earlier years. As a result, the cumulative yield values of trees budded onto Sour Orange were statistically similar to those recorded for Carrizo Citrange, Swingle Citrumelo, and Trifoliate Orange. Marked differences among the rootstocks studied were also observed with respect to yield efficiency. Trees budded onto Trifoliate Orange and its hybrids showed much higher yield efficiencies than those grafted onto the other rootstocks. Among these, the highest yield efficiency values were recorded for trees budded onto Carrizo Citrange and Swingle Citrumelo, in agreement with previous findings reported for other citrus cultivars [70,72,73,74]. In contrast, yield efficiency values observed in trees grafted onto Flying Dragon and Alemow were relatively low. However, the underlying reasons for this behavior appear to differ between the two rootstocks. In the case of Alemow, the low yield efficiency is likely associated with vigorous vegetative growth of trees grafted onto this rootstock. A large proportion of photosynthates, mainly high-energy monosaccharides, is likely allocated to vegetative growth at the expense of fruiting [75,76]. Conversely, for Flying Dragon, a rootstock known to induce high production efficiency in many citrus cultivars [67,77,78,79,80], the reduced performance observed in this study may be related to its anatomical and physiological peculiarities.
In fruit trees, growth reduction induced by dwarfing rootstocks has been associated with lower leaf and stem water potentials in scions grafted onto them compared with those grafted onto vigorous rootstocks, likely due to high hydraulic resistance [81,82,83]. This condition may result in water deficits in leaves during periods of high evaporative demand [69,84,85]. Reduced stomatal conductance, regulated by plant water status [86], consequently, leads to a decrease in net photosynthetic CO2 assimilation rates, with adverse effects on canopy growth [87]. According to Martínez-Alcántara et al. [60], in dwarfing rootstocks such as Flying Dragon, reduced carbohydrate transport across the bud union may limit root system development, ultimately resulting in lower yield efficiency.
The joint analysis of vegetative and reproductive performance of the different grafting combinations evaluated in this study highlights that a reduction in tree vigor induced by the rootstock provides significant benefits to the productive behavior of the Femminello bergamot cultivar, particularly in terms of yield precocity and yield efficiency. Owing to these characteristics, the Trifoliate Orange and its hybrids, particularly Carrizo Citrange and Swingle Citrumelo, despite producing significantly smaller canopies, are able to ensure yield levels in mature trees comparable to those obtained with Sour Orange, which has a larger canopy. Moreover, the reduced tree size induced by Trifoliate Orange and its hybrids could allow an increase in planting density compared with Sour Orange, resulting in increased yield per hectare. Reduced tree size would also improve labor efficiency, particularly for pruning and harvesting operations, thereby contributing to lower production costs. However, highly dwarfing rootstocks such as Flying Dragon, which theoretically could significantly increase planting density (with more than double the number of trees per hectare compared with Sour Orange), do not appear particularly suited to the Femminello bergamot cultivar, as they not only reduce tree vigor but also reduce reproductive activity.

4.3. Rootstock Influence on Fruit Size and Peel Parameters

The results related to the carpometric and qualitative characteristics of fruits indicate that the rootstock has a profound impact on these parameters. Rootstock effects on fruit quality can be attributed to several factors, including nutrient uptake and transport, graft compatibility, hormonal signaling, and gene expression [88]. Despite some differences between the two harvest times, fruits from almost all graft combinations were of good size and consistent with the typical values of the Femminello bergamot cultivar [23]. This aspect is particularly important for the fresh market, which requires fruits of adequate size. Only fruits from trees grafted onto Alemow showed smaller fruit size; however, even in this case, fruits still met commercial standards. Significant differences among rootstocks were also observed for peel thickness, a parameter that can influence bergamot fruit quality, especially for fresh consumption [89]. The thinnest peels were observed in fruits from trees grafted onto Trifoliate Orange, its dwarfing mutant Flying Dragon, and its hybrids, whereas significantly thicker peels were found in fruits produced by trees grafted onto Volkameriana and Alemow.
These results are in agreement with previous studies carried out on other citrus species, which reported that these two rootstocks produced fruits with thicker peels [89,90,91,92,93]. Citrus peel thickness is known to be strongly influenced by mineral nutrition. Although it is well established that rootstocks affect nutrient uptake [94,95], only a limited number of studies have examined the relationship between rootstock effects on peel thickness and fruit nutritional composition. Moreover, the results reported in the literature are often conflicting [31,70]. More recently, the influence of rootstocks on endogenous hormone metabolism regulating peel thickness has been investigated [96]. In particular, a study conducted on Kiyomi tangor showed that the greater peel thickness observed in fruits from trees grafted onto Citrus junos, compared with those grafted onto Trifoliate Orange, coincided with higher levels of the hormones IAA, GA3, and ZT in the Citrus junos rootstock [96].

4.4. Juice Yield and Internal Fruit Quality

The results show that rootstock also influences juice yield. This aspect is of considerable importance because juice content represents a key quality parameter for citrus fruits, particularly when they are intended for fresh consumption [55]. The lowest juice yields were observed in fruits from trees grafted onto Volkameriana and Alemow, in agreement with previous studies conducted on other citrus species [92]. Although lower than those recorded for the other rootstocks, the juice yields of bergamot fruits from trees grafted onto Volkameriana and Alemow were still above the minimum acceptable threshold (35%) for citrus fruits intended for fresh consumption.
Rootstock effects on juice yield are generally associated with differences in water uptake capacity. Some studies on sweet orange trees have related the lower juice yield of trees grafted onto rough lemon to a lower juice osmotic potential [97,98]. In most studies, thicker peel has been correlated with lower juice yield [35,72,99]. Although a thicker rind may favor better postharvest performance, it also tends to negatively affect juice content and may therefore be considered detrimental to overall fruit quality. The data obtained in the present study fully confirm this trend, as Volkameriana and Alemow were characterized by lower juice yield and thicker peel. Carrizo and Troyer Citranges, Trifoliate Orange, and Flying Dragon produced fruits with lower acidity levels than the other rootstocks, whereas Sour Orange and Swingle Citrumelo recorded higher acidity values. The tendency of Swingle Citrumelo and Sour Orange to produce fruits with higher acidity content has also been reported in previous studies conducted on other citrus species [100,101,102,103,104]. The behavior observed in fruits from trees grafted onto Volkameriana and Alemow was quite peculiar, showing a sharp decline in acidity content between the first and second harvests. This finding is consistent with observations reported for other citrus species [42,92,93,102]. The results also indicate that the ascorbic acid content of fruits of the bergamot cultivar Femminello is influenced by the rootstock, as previously reported for other citrus species [105]. However, rootstock effects on the ascorbic acid content appear to be highly dependent on the species and cultivar [106,107,108], as well as on the growing environment and tree condition [109]. In the present study, across both harvest times, the highest ascorbic acid contents were found in fruits from trees grafted onto Sour Orange and Swingle Citrumelo. The tendency of cultivars grafted onto Sour Orange to produce fruits with high ascorbic acid contents has been widely reported [104,110,111]. Fewer studies, however, have highlighted the ability of Swingle Citrumelo to induce high ascorbic acid contents in citrus fruits [112].

4.5. Rootstock-Mediated Effects on Ripening Dynamics

The results also demonstrate a clear rootstock effect on the fruit ripening dynamics. The relatively low increases in fruit weight recorded between the first and second harvests in fruits from trees grafted onto Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon, compared with those observed for other rootstocks, suggest that fruit growth and ripening occur earlier in these grafting combinations. It is well established that as citrus fruits approach ripening, their growth rate decreases markedly [113,114,115]. At 230 DAFB, fruits from trees grafted onto these rootstocks had already reached, or were very close to, their final weight and were likely in phase III of fruit growth. In contrast, fruits from trees grafted onto the other rootstocks were still in a phase of rapid growth at this time. The results indicate that these latter fruits require an additional 3–4 weeks to reach phase III. Accordingly, at 260 DAFB, except for fruits from trees grafted onto Alemow, which still showed slightly lower values, fruits from all other graft combinations were characterized by statistically similar weight values.
The hypothesis that Carrizo and Troyer Citranges, Trifoliate Orange and Flying Dragon can induce earlier fruit ripening in the Femminello bergamot cultivar is further supported by the results related to peel color. Indeed, analysis of the color parameter data clearly indicates that, at 230 DAFB, fruits from trees grafted onto Sour Orange, Swingle Citrumelo, Volkameriana, and Alemow still contained chlorophyll, or had not yet reached the intensity and vividness of the yellow coloration observed in fruits from the other rootstocks, which is characteristic of ripe bergamot fruits. Changes in fruit peel color occur because of chlorophyll degradation, responsible for the green coloration, and the accumulation of carotenoid pigments, which confer the characteristic yellow to orange hues [116,117]. The results of the present study suggest that fruits from trees budded onto Sour Orange, Swingle Citrumelo, Volkameriana, and Alemow require at least an additional 3–4 weeks to reach a peel color comparable to that already achieved at 230 DAFB by fruits from trees grafted onto Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon. In subtropical areas, color break generally occurs in mid-autumn, when temperatures decrease and day length shortens. The decline in rind chlorophyll proceeds over several months, and the onset of carotenoid accumulation almost coincides with the disappearance of chlorophyll [118]. Air temperature exerts the greatest influence on the external coloration of citrus fruits during maturation. Peel color development generally occur earlier in fruits grown in cooler regions, whereas in warmer regions color changes are delayed and, in some cases, external coloration may not develop satisfactorily [117,119,120]. However, it is well established that the rootstock on which a citrus cultivar is grown can influence biochemical compounds, resulting in differences in both external and internal fruit color [121,122]. Differences in citrus peel color may therefore be related to rootstock effects on pigment accumulation. In Owari mandarin grafted onto two rootstocks, fruits budded onto Cleopatra mandarin showed higher levels of the carotenoids β-cryptoxanthin and violaxanthin (in both flavedo and albedo tissues) than those grafted onto Troyer Citrange [123]. Other studies have suggested that differences in nitrogen uptake and storage capacity may also promote color changes in the flavedo of citrus fruits [124]. Haas [125] reported different nitrogen contents in the dry matter of Valencia orange peel depending on the rootstock used. Fruits from scions grafted onto Sour Orange and Rough Lemon showed the highest total nitrogen content, whereas those grafted onto Trifoliate Orange showed the lowest. However, to date, no studies have directly correlated rootstock-related variations in nitrogen content with changes in fruit color.
Rootstock effects on citrus peel color may also be indirect and linked to broader and more complex processes such as fruit ripening. Emmanouilidou and Kyriacou [35] reported that differences observed in the peel color of the Lane Late and Delta orange cultivars grafted onto five different rootstocks were more closely associated with fruit maturity status than with a direct rootstock effect. Potential indirect effects of rootstock on peel coloration may also involve canopy volume. Canopy size, a direct consequence of rootstock vigor, affects the light microclimate, generating zones with differing light intensity and quality. In citrus fruits, as in many fruit trees species characterized by dense foliage, large canopies are associated with greater internal shading than smaller ones. These differences in light microclimate inevitably influence several physical and chemical parameters, including peel coloration. Although some results are conflicting, numerous studies have highlighted a positive effect of light on peel coloration in various citrus species [126,127,128,129]. Fruits located in the most exposed canopy zones generally color earlier and show higher color intensity [130]. Light appears to stimulate carotenoid biosynthesis and accumulation in citrus peel, although light-independent regulation mechanisms may also be involved [131]. Nevertheless, the interaction between rootstock and scion should be considered, as it can modulate the influence of the rootstock on external fruit color. For example, the dwarfing hybrid rootstock Forner-Alcaide 418 was reported to induce lower color index values in Navel orange compared with Carrizo Citrange and another dwarfing hybrid rootstock, Forner-Alcaide 517 [132]. Further evidence supporting the hypothesis of earlier ripening induced by Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon also derives from the results obtained for some internal fruit parameters. In this context, one of the most indicative parameters is juice yield, particularly the differences observed among rootstocks between the two harvest times. During fruit growth, following the accumulation of solutes and water, the vacuoles of juice sac cells become greatly enlarged, accounting for more than 90% of the total cell volume, and eventually release their contents as juice [133].
A recent study carried out by Mafrica et al. [23] on the Femminello bergamot cultivar indicates that the juice yield increases significantly up to 260 DAFB, then remains relatively stable for about two months, before progressively decreasing from 350 DAFB onward. Fruits from trees grafted onto Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon already showed significantly higher juice contents at the first harvest (230 DAFB), with values around 50%, compared with those recorded for the other rootstocks. Furthermore, while the increase in juice content between the two harvest times was very limited and not statistically significant in fruits from trees grafted onto Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon, much larger increases were observed in fruits from the other rootstocks. This marked increase allowed fruits from trees grafted onto Sour Orange and Swingle Citrumelo to reach juice yields at 260 DAFB that were statistically similar to those of fruits from trees grafted onto Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon. However, these increases were not sufficient to align the juice yield of fruits from trees grafted onto Volkameriana and Alemow with that of the other rootstocks. Consequently, Volkameriana and Alemow were the rootstocks associated with the lowest juice yields. The poor juice yield performance of these two rootstocks is consistent with previous studies conducted on other citrus species [70,71,92,134,135].
The early ripening induced by Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon is further supported by the results obtained for juice titratable acidity. Data clearly show that, at the first harvest, titratable acidity levels in fruits from these rootstocks were significantly lower than those recorded for the other rootstocks. During the first half of phase II, citrus fruits accumulate a considerable amount of organic acids in the vacuoles of juice sac cells, which are progressively catabolized from the second half of phase II through phase III [118,136,137]. The low acidity values observed in fruits from trees grafted onto Carrizo and Troyer Citrange, Trifoliate Orange, and Flying Dragon indicate that, in these graft combinations, organic acid catabolism had likely already begun by 230 DAFB, whereas in fruits from trees grafted onto the other rootstocks this process had not yet started.
The early ripening induced by Carrizo and Troyer Citrange, Trifoliate Orange and Flying Dragon could be attributed to the greater supply of carbohydrates to the fruits ensured by these rootstocks. Morinaga and Ikeda [138] linked increased carbohydrate supply to fruits to rootstock-induced differences in scion leaf photosynthetic rates and in the distribution of photosynthetic products. In particular, sugar accumulation in fruit has been related to vascular resistance to sucrose transport at the rootstock’s budding union. Indeed, although resistance at the bud union to water transport and xylem anatomical characteristics, particularly vessel number and diameter, may limit overall plant growth, carbohydrate distribution also represent an important constraint involved in tree response [64,124]. Reduced translocation of photoassimilates from leaves to roots limits root development and contributes to greater carbohydrate availability in the scion, resulting in increased carbon allocation to fruits [75,132]. This mechanism explains both the high yield efficiency, the good fruit quality, and the early ripening process induced by these rootstocks. Consistent with these findings, a study by Mafrica et al. [139] on Femminello Zagara Bianca lemon highlighted that greater carbohydrate availability to fruits during the initial period of the cell enlargement phase allows anticipation of peel degreening and the overall ripening process.

5. Conclusions

This study demonstrated that rootstock selection significantly influences tree vigor, yield performance, yield efficiency, fruit quality, and ripening dynamics of ‘Femminello’ bergamot. Alemow induced the highest vegetative vigor, whereas Flying Dragon resulted in the smallest trees. Volkameriana produced growth comparable to that of Sour Orange, while Trifoliate Orange and its hybrids induced more moderate canopy development. All tested rootstocks showed good graft compatibility with ‘Femminello’ bergamot, as no abnormalities at the graft union or tree decline were observed eight years after planting. Despite their reduced canopy size, trees grafted onto Carrizo Citrange achieved cumulative yields comparable to those Sour Orange, resulting in superior yield efficiency. High yield efficiency was also recorded for Trifoliate Orange and Swingle Citrumelo, whereas trees grafted onto Alemow showed low productivity and poor yield efficiency.
With the exception of Volkameriana and Alemow, all tested rootstocks allowed the production of fruits with satisfactory quality traits. Moreover, Carrizo and Troyer Citranges, Trifoliate Orange, and Flying Dragon promoted earlier fruit maturation compared with Sour Orange, anticipating fruit growth and peel color development and reducing juice acidity. This characteristic represents a relevant advantage for extending the harvest window and improving the availability of bergamot fruit for the fresh market.
Overall, Trifoliate Orange, Carrizo Citrange, and Swingle Citrumelo emerged as promising alternatives to Sour Orange for the cultivation of ‘Femminello’ bergamot. These findings provide useful guidance for growers and support the development of more sustainable and optimized bergamot production systems. Nevertheless, soil characteristics and local environmental conditions should be carefully considered when selecting alternative rootstocks, and further studies are needed to evaluate their long-term performance under different agronomic conditions.

Author Contributions

Conceptualization, R.M.; methodology, R.M., A.G. and D.M.; software, R.M. and A.G.; validation, M.P. and A.D.B.; formal analysis, R.M., A.G. and D.M.; investigation, R.M., A.G. and D.M.; resources, R.M. and M.P.; data curation, R.M., A.G. and A.D.B.; writing—original draft preparation, R.M.; writing—review and editing, A.D.B., A.G. and M.P.; visualization, M.P. and A.D.B.; supervision, M.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors thank Fratelli Catanoso Farm for making the land available for experimental citrus grove establishment and for agronomic management of the citrus orchard; and the Experimental Station for the Industry of the Essential Oils and Citrus Products (SSEA) of Reggio Calabria for research support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Table 1. Rootstocks used for grafting the “Femminello” cultivar.
Table 1. Rootstocks used for grafting the “Femminello” cultivar.
RootstockFull Scientific Name
Carrizo CitrangeCitrus sinensis (L.) Osb. × Poncirus trifoliate (L.) Raf.
Flying DragonPoncirus trifoliate (L.) Raf. var. monstrosa
AlemowCitrus macrophylla Wester
Sour OrangeCitrus aurantium L.
Swingle CitrumeloCitrus paradisi Macf. × Poncirus trifoliata (L.) Raf.
Trifoliate OrangePoncirus trifoliata L. Raf.
Troyer CitrangeCitrus sinensis (L.) Osb. × Poncirus trifoliate (L.) Raf.
VolkamerianaCitrus volkameriana Ten. and Pasq.
Table 2. Canopy volume from crop year 2017/2018 up to 2023/2024, rootstock trunk diameter and trunk diameter index in the crop year 2023/2024 of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy.
Table 2. Canopy volume from crop year 2017/2018 up to 2023/2024, rootstock trunk diameter and trunk diameter index in the crop year 2023/2024 of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy.
Rootstocks
ParameterCrop YearSour
Orange		Trifoliate
Orange		Flying
Dragon		Carrizo
Citrange		Troyer
Citrange		Swingle
Citrumelo		VolkamerianaAlemowSign.
Canopy Volume (m3)2017/20180.5 ± 0.0 b0.4 ± 0.0 c0.1 ± 0.0 d0.4 ± 0.0 c0.4 ± 0.0 c0.4 ± 0.0 c0.5 ± 0.0 b1.2 ± 0.0 a**
2018/20191.9 ± 0.1 b1.1 ± 0.0 d0.3 ± 0.0 e1.2 ± 0.0 cd1.2 ± 0.0 cd1.3 ± 0.0 c2.0 ± 0.0 b2.8 ± 0.0 a**
2019/20202.9 ± 0.1 b1.8 ± 0.1 d0.5 ± 0.0 e1.8 ± 0.1 d1.8 ± 0.0 d2.3 ± 0.1 c2.8 ± 0.1 b4.7 ± 0.1 a**
2020/20213.6 ± 0.1 b3.1 ± 0.1 cd0.9 ± 0.0 e3.1 ± 0.1 cd3.2 ± 0.1 bc2.7 ± 0.1 d3.5 ± 0.1 b5.7 ± 0.1 a**
2021/20225.2 ± 0.1 b3.8 ± 0.1 c1.1 ± 0.0 d3.8 ± 0.0 c3.9 ± 0.1 c3.8 ± 0.1 c5.1 ± 0.1 b8.6 ± 0.1 a**
2022/20236.7 ± 0.2 b4.7 ± 0.1 c1.4 ± 0.1 d4.8 ± 0.2 c4.8 ± 0.1 c4.7 ± 0.1 c6.2 ± 0.1 b10.9 ± 0.1 a**
2023/20247.2 ± 0.3 b5.1 ± 0.1 c1.5 ± 0.1 d5.0 ± 0.2 c5.3 ± 0.1 c4.9 ± 0.1 c6.6 ± 0.2 b11.6 ± 0.1 a**
Rootstock Trunk Diameter 1 (cm)10.8 ± 0.1 b9.9 ± 0.2 c7.5 ± 0.1 d10.3 ± 0.1 bc10.1 ± 0.1 c10.2 ± 0.1 c10.8 ± 0.1 b12.8 ± 0.1 a**
Trunk Diameter Index 21.05 ± 0.01 a0.95 ± 0.0 b0.96 ± 0.0 b0.96 ± 0.0 b0.97 ± 0.0 b0.97 ± 0.0 b1.04 ± 0.0 a1.04 ± 0.0 a**
1 Rootstock trunk diameters were based on trunk diameter measurements 10 cm below the graft union determined in the crop year 2023/2024. 2 Trunk diameter index was expressed as the ratio trunk diameter 10 cm above (scion) and below (rootstock) the graft union determined in the crop year 2023/2024. Values are means ± standard error. Means followed by same letter in same row do not significantly differ according to Tukey’s test (p ≤ 0.05). Significance level: ** p ≤ 0.01.
Table 3. Yield and cumulative yield from crop year 2017/2018 up to 2023/2024 and yield efficiency determined in the crop years 2022/2023 and 2023/2024 of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy (mean value ± standard error).
Table 3. Yield and cumulative yield from crop year 2017/2018 up to 2023/2024 and yield efficiency determined in the crop years 2022/2023 and 2023/2024 of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy (mean value ± standard error).
ParameterCrop YearRootstocks
Sour
Orange		Trifoliate
Orange		Flying
Dragon		Carrizo
Citrange		Troyer
Citrange		Swingle
Citrumelo		VolkamerianaAlemowSign.
Yield (kg·tree−1)2017/20181.9 ± 0.1 c3.0 ± 0.1 a0.5 ± 0.0 d3.0 ± 0.1 a2.4 ± 0.1 b2.7 ± 0.1 ab1.5 ± 0.1 c1.8 ± 0.1 c**
2018/20196.8 ± 0.2 c10.0 ± 0.3 a1.6 ± 0.1 f10.7 ± 0.4 a8.1 ± 0.1 b8.4 ± 0.2 b5.7 ± 0.1 d4.7 ± 0.1 e**
2019/202016.1 ± 0.3 c17.8 ± 0.4 b3.1 ± 0.1 e19.9 ± 0.5 a16.5 ± 0.4 bc17.3 ± 0.5 bc15.9 ± 0.4 c8.2 ± 0.1 d**
2020/202126.0 ± 0.7 b27.8 ± 0.8 ab4.4 ± 0.1 d29.5 ± 0.6 a25.4 ± 0.2 b26.3 ± 0.8 b26.1 ± 0.6 b12.3 ± 0.2 c**
2021/202239.9 ± 0.3 ab36.0 ± 0.4 cd5.5 ± 0.2 f38.1 ± 0.4 abc34.2 ± 0.4 d37.7 ± 0.9 c40.2 ± 0.5 a16.4 ± 0.4 e**
2022/202348.4 ± 1.0 a42.0 ± 0.6 cd6.8 ± 0.3 f47.4 ± 1.1 abc38.6 ± 0.6 d44.5 ± 0.7 bc46.5 ± 0.8 a19.6 ± 0.9 e**
2023/202450.7 ± 1.0 a44.1 ± 0.9 b5.2 ± 0.3 e47.7 ± 1.4 ab39.4 ± 0.7 c47.0 ± 0.9 ab43.9 ± 0.7 b20.0 ± 1.0 d**
Cumulative Yield
(kg·tree−1)		189.7 ±3.3 ab180.8 ± 3.2 ab26.9 ± 0.9 e196.3 ± 4.3 a164.7 ± 1.9 c183.9 ± 3.8 ab179.8 ± 3.0 b82.9 ± 2.6 d**
Yield Efficiency 1
(kg·m−3)		7.2 ± 0.1 d8.8 ± 0.1 b4.1 ± 0.2 e9.7 ± 0.1 a7.7 ± 0.1 c9.5 ± 0.1 a7.1 ± 0.1 d1.8 ± 0.1 f**
1 Yield efficiency was calculated as the ratio between fruit yield (kg tree−1) and canopy volume (m3) determined in the crop years 2022/2023 and 2023/2024. Values are means ± standard error. Means followed by same letter in same row do not significantly differ according to Tukey’s test (p ≤ 0.05). Significance level: ** p ≤ 0.01.
Table 4. Fruit weight, fruit height, fruit width and ratio height/width of fruits of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy (mean value ± standard error over seven years).
Table 4. Fruit weight, fruit height, fruit width and ratio height/width of fruits of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy (mean value ± standard error over seven years).
ParameterHarvest TimeRootstocks
Sour
Orange		Trifoliate
Orange		Flying
Dragon		Carrizo
Citrange		Troyer
Citrange		Swingle
Citrumelo		VolkamerianaAlemowSign.
Weight fruit (g)230 DAFB169.0 ± 2.7 bc195.8 ± 4.1 a191.5 ± 3.0 a188.5 ± 2.8 a184.0 ± 3.6 ab186.7 ± 2.4 a167.5 ± 3.0 c159.6 ± 4.6 c**
260 DAFB219.7 ± 4.0 a217.9 ± 3.5 a216.5 ± 3.3 a218.0 ± 2.0 a215.2 ± 3.0 a222.8 ± 4.7 a212.5 ± 3.9 ab197.6 ± 2.5 b**
Sign.****************
Heigth fruit (mm)230 DAFB68.5 ± 0.7 bc72.1 ± 0.4 a71.9 ± 0.7 a71.0 ± 0.4 ab71.6 ± 0.5 a71.0 ± 0.4 a68.2 ± 0.6 c66.2 ± 0.8 c**
260 DAFB76.1 ± 0.4 a75.4 ± 0.3 a76.3 ± 0.5 a76.0 ± 0.5 a75.3 ± 0.3 a76.5 ± 0.7 a74.9 ± 0.5 a72.3 ± 0.5 b**
Sign.****************
Width fruit (mm)230 DAFB68.6 ± 0.5 bcd72.6 ± 0.6 a72.4 ± 0.6 a71.5 ± 0.3 ab71.7 ± 0.5 a71.3 ± 0.4 abc68.4 ± 0.7 cd67.4 ± 1.1 d**
260 DAFB76.3 ± 0.6 a75.8 ± 0.5 a76.4 ± 0.4 a76.1 ± 0.4 a75.3 ± 0.4 a76.6 ± 0.6 a75.0 ± 0.4 ab72.9 ± 0.4 b**
Sign.***************
Ratio height/width230 DAFB1.00 ± 0.000.99 ± 0.010.99 ± 0.000.99 ± 0.001.00 ± 0.000.99 ± 0.001.00 ± 0.000.98 ± 0.01ns
260 DAFB1.00 ± 0.000.99 ± 0.001.00 ± 0.001.00 ± 0.001.00 ± 0.001.00 ± 0.001.00 ± 0.000.99 ± 0.00ns
Sign.nsnsnsnsnsnsnsns
Values are means ± standard error. Means followed by same letter in same row do not significantly differ according to Tukey’s test (p ≤ 0.05). Significance level: ** p ≤ 0.01; * p ≤ 0.05; ns, not significant.
Table 5. Main peel characteristics of fruits of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy.
Table 5. Main peel characteristics of fruits of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy.
ParameterHarvest TimeRootstocks
Sour
Orange		Trifoliate
Orange		Flying
Dragon		Carrizo
Citrange		Troyer
Citrange		Swingle
Citrumelo		VolkamerianaAlemowSign.
L*230 DAFB64.8 ± 0.7 bc68.8 ± 0.4 a68.6 ± 0.3 a68.1 ± 0.2 a66.9 ± 0.4 ab63.5 ± 0.4 c65.8 ± 0.3 b63.1 ± 0.4 c**
260 DAFB69.4 ± 0.2 bc70.7 ± 0.3 a70.2 ± 0.3 ab71.1 ± 0.1 a70.9 ± 0.2 a67.7 ± 0.3 e68.8 ± 0.2 cd67.8 ± 0.2 de**
Sign.**************
a*230 DAFB1.2 ± 0.5 cd4.5 ± 0.3 a4.1 ± 0.2 ab3.5 ± 0.3 ab3.8 ± 0.3 ab−0.9 ± 0.5 e2.4 ± 0.37 bc0.2 ± 0.6 de**
260 DAFB5.2 ± 0.2 c6.7 ± 0.4 a6.5 ± 0.3 ab6.2 ± 0.3 ab6.3 ± 0.3 ab3.9 ± 0.2 c5.2 ± 0.24 bc4.1 ± 0.2 c**
Sign.****************
b*230 DAFB39.0 ± 0.4 b43.9 ± 0.5 a44.1 ± 0.36 a43.6 ± 1.01 a42.2 ± 0.3 ab34.1 ± 1.1 c39.9 ± 0.7 b36.1 ± 0.8 bc**
260 DAFB44.5 ± 0.4 abcd47.2 ± 0.3 ab47.0 ± 0.81 ab47.8 ± 0.67 a46.5 ± 0.8 ab41.5 ± 0.4 d44.7 ± 0.8 bc41.7 ± 0.6 cd**
Sign.*************
h°230 DAFB88.3 ± 0.7 bc84.2 ± 0.3 d84.7 ± 0.2 d85.4 ± 0.4 d84.8 ± 0.3 d91.6 ± 0.9 a86.6 ± 0.5 cd89.6 ± 0.9 ab**
260 DAFB83.3 ± 0.8 a82.0 ± 0.4 b82.2 ± 0.3 b82.6 ± 0.4 a82.3 ± 0.3 a84.6 ± 0.5 a83.4 ± 0.8 a84.4 ± 0.2 a*
Sign.**************
C*230 DAFB39.0 ± 0.4 bc44.1 ± 0.5 a44.3 ± 0.3 a43.8 ± 1.0 a42.4 ± 0.3 ab34.1 ± 1.1 d40.0 ± 0.7 c36.1 ± 0.8 cd**
260 DAFB44.8 ± 0.4 bc47.7 ± 0.4 ab47.4 ± 0.8 ab48.2 ± 0.7 a46.9 ± 0.8 ab41.7 ± 0.4 c45.0 ± 0.8 b41.9 ± 0.6 c**
Sign.**************
Citrus color index230 DAFB0.5 ± 0.2 b1.5 ± 0.1 a1.4 ± 0.1 a1.2 ± 0.1 a1.4 ± 0.1 a−0.5 ± 0.3 c0.9 ± 0.1 ab0.1 ± 0.3 b**
260 DAFB1.7 ± 0.1 ab2.0 ± 0.2 a2.0 ± 0.1 a1.8 ± 0.1 ab1.9 ± 0.1 ab1.4 ± 0.1 b1.7 ± 0.1 ab1.4 ± 0.1 b*
Sign.************
Peel
thickness (mm)		230 DAFB4.1 ± 0.1 abc3.3 ± 0.1 cd3.4 ± 0.1 cd3.2 ± 0.1 d3.5 ± 0.2 cd3.8 ± 0.1 b4.3 ± 0.1 ab4.5 ± 0.1 a**
260 DAFB3.4 ± 0.1 b3.1 ± 0.1 c3.2 ± 0.1 c3.0 ± 0.1 c3.2 ± 0.1 c3.3 ± 0.1 c3.8 ± 0.1 ab3.9 ± 0.0 a**
Sign.**nsnsnsns***
Values are means ± standard error. Means followed by same letter in same row do not significantly differ according to Tukey’s test (p ≤ 0.05). Significance level: ** p ≤ 0.01; * p ≤ 0.05; ns, not significant.
Table 6. Juice content and the main qualitative characteristics of juice of fruits of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy (mean value ± standard error over seven years).
Table 6. Juice content and the main qualitative characteristics of juice of fruits of ‘Femminello’ bergamot trees grafted on eight different rootstocks grown in Melito Porto Salvo, Southern Italy (mean value ± standard error over seven years).
ParameterHarvest TimeRootstocks
Sour
Orange		Trifoliate
Orange		Flying
Dragon		Carrizo
Citrange		Troyer
Citrange		Swingle
Citrumelo		VolkamerianaAlemowSign.
Juice content (%)230 DAFB44.0 ± 0.7 abc50.1 ± 1.7 a49.3 ± 1.5 a50.3 ± 1.5 a48.2 ± 1.7 a45.7 ± 0.7 ab39.9 ± 1.3 bc37.8 ± 1.9 c**
260 DAFB50.7 ± 0.3 a53.7 ± 0.6 a52.9 ± 0.7 a53.7 ± 1.1 a51.8 ± 1.3 a52.7 ± 0.3 a45.9 ± 1.0 b45.0 ± 0.6 b**
Sign.**nsnsnsns****
Total soluble solids (Brix)230 DAFB8.3 ± 0.18.1 ± 0.18.2 ± 0.18.2 ± 0.18.1 ± 0.18.4 ± 0.18.3 ± 0.18.4 ± 0.1ns
260 DAFB7.9 ± 0.1 ab7.7 ± 0.1 bc7.8 ± 0.0 abc7.7 ± 0.0 bc7.7 ± 0.1 bc8.0 ± 0.0 a7.6 ± 0.1 c7.6 ± 0.1 c**
Sign.*************
Titratable acidity (g L−1)230 DAFB50.1 ± 0.9 ab43.6 ± 0.8 d44.2 ± 0.4 cd44.8 ± 0.5 cd44.2 ± 0.9 cd50.7 ± 0.3 a47.2 ± 0.3 bc51.7 ± 0.9 a**
260 DAFB44.9 ± 0.7 a40.1 ± 0.8 b40.8 ± 0.4 b41.0 ± 0.5 b40.4 ± 0.7 b46.0 ± 0.3 a39.1 ± 0.8 b39.6 ± 0.8 b**
Sign.**************
TSS/TA ratio230 DAFB1.65 ± 0.02 cd1.85 ± 0.02 a1.85 ± 0.03 a1.82 ± 0.01 ab1.84 ± 0.02 a1.65 ± 0.02 cd1.75 ± 0.01 bc1.62 ± 0.01 d**
260 DAFB1.77 ± 0.01 a1.92 ± 0.05 a1.90 ± 0.01 ab1.88 ± 0.03 ab1.92 ± 0.04 a1.74 ± 0.01 b1.93 ± 0.06 a1.93 ± 0.04 a**
Sign.**nsnsnsns****
Ascorbic acid (mg 100 mL−1)230 DAFB0.67 ± 0.01 a0.60 ± 0.01 c0.63 ± 0.01 bc0.65 ± 0.01 ab0.66 ± 0.01 ab0.69 ± 0.01 a0.66 ± 0.00 ab0.69 ± 0.01 a**
260 DAFB0.50 ± 0.01 a0.40 ± 0.01 c0.42 ± 0.01 bc0.47 ± 0.01 ab0.46 ± 0.01 ab0.52 ± 0.01 a0.42 ± 0.01 bc0.44 ± 0.01 bc**
Sign.****************
Values are means ± standard error. Means followed by same letter in same row do not significantly differ according to Tukey’s test (p ≤ 0.05). Significance level: ** p ≤ 0.01; * p ≤ 0.05; ns, not significant.
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MDPI and ACS Style

Mafrica, R.; Gattuso, A.; Mafrica, D.; De Bruno, A.; Poiana, M. Influence of Rootstock on Growth, Yield, and Fruit Quality of the ‘Femminello’ Bergamot (Citrus bergamia Risso & Poit.). Agriculture 2026, 16, 405. https://doi.org/10.3390/agriculture16040405

AMA Style

Mafrica R, Gattuso A, Mafrica D, De Bruno A, Poiana M. Influence of Rootstock on Growth, Yield, and Fruit Quality of the ‘Femminello’ Bergamot (Citrus bergamia Risso & Poit.). Agriculture. 2026; 16(4):405. https://doi.org/10.3390/agriculture16040405

Chicago/Turabian Style

Mafrica, Rocco, Antonio Gattuso, Davide Mafrica, Alessandra De Bruno, and Marco Poiana. 2026. "Influence of Rootstock on Growth, Yield, and Fruit Quality of the ‘Femminello’ Bergamot (Citrus bergamia Risso & Poit.)" Agriculture 16, no. 4: 405. https://doi.org/10.3390/agriculture16040405

APA Style

Mafrica, R., Gattuso, A., Mafrica, D., De Bruno, A., & Poiana, M. (2026). Influence of Rootstock on Growth, Yield, and Fruit Quality of the ‘Femminello’ Bergamot (Citrus bergamia Risso & Poit.). Agriculture, 16(4), 405. https://doi.org/10.3390/agriculture16040405

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