Ripening Dynamics and Optimal Harvest Timing of ‘Fantastico’ and ‘Femminello’ Bergamot Fruit

Abstract

Bergamot was traditionally grown for its essential oil, but recently, the juice’s health benefits have increased consumer demand. The need to understand how fruit characteristics change during growth and ripening is essential for optimizing the yield and market attractiveness in order to select the best harvest time, understanding when the fruits have reached the best quality and carpometric characteristics. Currently, the knowledge on this topic is very limited. The aim of this study was to evaluate the ripening changes in Fantastico and Femminello bergamot cultivars in the traditional bergamot growing area in the province of Reggio Calabria (Southern Italy). Physico-chemical changes in fruits were evaluated from 200 to 410 days after full flowering (DAFB) through field observations and laboratory evaluations. The fruit drop remained low up to 290 DAFB, while the fruit weight increased to 350 DAFB. By mid-December, the peel of both cultivars had turned completely yellow. The juice yield progressively increased up to 260 DAFB, maintained levels higher than 50% for another two months and then decreased. To maximize quantitative production, the harvest should not occur before 260 DAFB for the Femminello cultivar and 290 DAFB for the Fantastico cultivar. However, delaying the harvest beyond 350 DAFB is not recommended, as it results in significantly reduced yields. The period between 260 and 320 DAFB also appears to be the ideal time for enhancing the qualitative characteristics of bergamot fruits.

1. Introduction

The bergamot (Citrus bergamia Risso and Poiteau) is a tree with evergreen leaves and yellow fruits from the family Rutaceae. Like many other citrus species, it has a hybrid origin. Recent studies using molecular markers indicate that bitter orange (Citrus aurantium L.) and citron (Citrus medica L.) are the possible parents [1,2]. Among citrus species, bergamot production represents a rare example of a crop monopoly. In fact, almost all of the world area under bergamot cultivation is located in the province of Reggio Calabria (Southern Italy). At present, the area under bergamot cultivation in this district is around 2000 hectares, with an average production of about 30,000 tons per year (about 90% of world production). In the past, bergamot grew almost exclusively for the extraction of the peel essential oil. Bergamot essential oil is very valuable and is in high demand by the cosmetic, pharmaceutical and food industries [3,4,5,6,7]. In recent years, the discovery of the remarkable health properties of bergamot juice [8,9,10,11,12,13] has made it possible to launch, in parallel with the traditional one of the essential oils, a new fresh fruit market. Growers have always considered the bergamot fruit a purely industrial product, aimed exclusively at the extraction of the essential oils present in the peel. For this reason, they have preserved few details about the aspects inherent to the harvest time. Generally, the fruits for industry are harvested when the skin color changes from dark green to yellowish green or yellow. No attention is paid to the internal fruit characteristics. However, the increasing interest in this fruit, not only for the extraction of the essential oils but also for its juice or to be eaten, means it is necessary to understand the changes in the external and internal fruit characteristics during growth and ripening. During ripening, a series of changes occur in the fruits regulated by a complex interaction of endogenous hormonal and nutritional signals and strongly influenced by the availability of carbohydrates [14], environmental factors [15,16,17] and agronomic practices [18]. The study of citrus ripening is very complex as it involves two quite different tissues and systems: internal changes occurring in the fruit flesh and external color modifications taking place in the fruit peel [19]. Evidence indicates that ripening of the peel and pulp are not fully coordinated and that the natural ripening of both tissues is an autonomous and independent process [20], making their understanding more complex. This independent physiological behavior of the peel and pulp has been related, among others, to the lack of vascular connections [21]. Moreover, citrus fruits show a non-climacteric ripening behavior and should be harvested when internal maturity has been achieved, since no further relevant changes occur in the fruit flesh composition after harvest [22]. Therefore, to encourage further development of the bergamot fruit market for consumption fresh and to give consumers high-quality fruits, it is particularly important to understand in depth the ripening process as well as to determine the optimal harvesting period. The aim of this study was to evaluate the changes that occur during the ripening of the fruits of two bergamot cultivars. Within the bergamot varietal heritage, which is very limited, these two cultivars are currently the most cultivated. Their fruits are used both for the extraction of essential oils and for the fresh market.

2. Materials and Methods

2.1. Plant Material and Site Location

This study was carried out during the crop years 2021 and 2022 on 6-year-old trees of the ‘Fantastico’ and ‘Femminello’ bergamot cultivars grafted on bitter orange (Citrus aurantium L.) rootstock and planted at 5 m × 5 m distances apart at an experimental grove (15 m a.s.l., 15°48′22″ E longitude, 37°55′22″ N latitude) located within the traditional bergamot production area in Calabria (Southern Italy) (Figure 1).

The soil texture was sandy clay loam (22% clay, 16% silt and 62% sand). The soil had a pH of 7 and contained 3% CaCO₃ at a depth of 0–90 cm. The area has maximum and minimum temperatures of 26.8 and 11.3 °C, respectively, and an average annual rainfall of 596 mm (Figure 2). The trees were submitted to identical cultural practices. The soil management included a shallow tillage, carried out at the end of winter to break up the soil, eliminating weeds that have grown in the autumn–winter period and burying the fertilizer scattered on the ground and periodic mowing of noxious weeds during spring and summer. Nitrogen (N) was applied at a rate of 250 kg N ha⁻¹ (2/3 in mid-March and 1/3 during the summer period), while phosphorus (P) and potassium (K) were applied (in mid-December) at a rate of 76 kg P₂O₅ ha⁻¹ and 200 K₂O ha⁻¹, respectively. The pruning, carried out at the end of winter, was very light and limited only to removing or shortening a few water shoots present in the internal parts of the canopy to prevent them from becoming too dominant. The trees were irrigated weekly from April to October using a drip irrigation system with 6 pressure-compensating drippers of 8 L/h per tree. The total seasonal amount of irrigation water was about 6000 cubic meters/ha. Before and during the study, all the trees exhibited good development and phytosanitary conditions. Each cultivar had a randomized complete block design with three blocks (replications) and three trees per block. In total, 9 trees per cultivar (3 blocks × 3 trees per block) were studied.

2.2. Climate Parameters

Weather data for 30 years (from 1991 to 2020) were obtained from ARPACAL (Regional Agency for the Protection of the Environment of Calabria) station in Capo Spartivento, located approximately 30 km from experimental site. The air temperature and rainfall during the experiment period were measured through a WatchDog 2425 weather station (SPECTRUM Technologies Inc., Thayer Court, Aurora, IL, USA) located inside the bergamot grove.

2.3. Phenological Observations

Starting from mid-February and throughout the growing season, the main phenological phases were observed. The phenological observations, carried out on four fruiting branches (one for each cardinal point) for each selected tree, were made using the BBCH (Biologische Bundesanstalt, Bundessortenamt und Chemische) scale system [23,24]. In particular, the following phenological phases were defined: beginning of flowering (code 60—first flowers open); full flowering (code 65—50% of flowers open and first petals falling); end of flowering (code 69—all petals fallen); end of physiological fruit drop (code 74—fruits about 40% of final size); fruit color break (code 81—beginning of fruit coloring).

2.4. Fruit Drop Monitoring

At the beginning of November, a square fruit harvesting netting measuring 5 m per side and with central cutting under the canopy of each selected tree was laid. At the same time, all the fruits present on the fruiting branches used for the phenological observations were counted and marked individually using small paper labels with cotton thread. Starting from 200 and up to 410 days after full bloom (DAFB), every 30 days, the fruit drop percentage was evaluated both at the entire tree and selected fruiting branches level. At whole tree level, the fruits fallen on the harvesting net were counted and weighed. On the fruiting branches, the number of fruits dropped was recorded. Furthermore, the detached fruits were classified according to the breakage zone: AZ-A (break between the twig and the peduncle) and AZ-C (break between the peduncle and fruit). The fruit drop rate was calculated as the ratio between the number of fruits dropped during a time frame of 30 days starting from 200 and up to 410 DAFB, and the total number of fruits present on each fruiting branch at the beginning of November, multiplied by 100.

2.5. Yield Determination

The yield per tree was determined both in terms of weight and number of fruits. The total number of fruits per tree was determined, adding up the fallen and sampled fruits during the period between 200 and 410 DAFB with the fruits remaining on the tree and harvested after 410 DAFB. Changes in yield per tree between 200 and 410 DAFB were estimated by multiplying the number of fruits on the tree in different 30-day intervals (calculated by subtracting the number of dropped fruits from the total number of fruits per tree) for the corresponding average fruit weight (i.e., the value it had at that specific stage of fruit growth).

2.6. Fruit Sampling for Physical and Qualitative Analyses

Starting from 200 and up to 410 DAFB, every 30 days, four fruits (one fruit per main cardinal position) were collected from each selected tree for a total of thirty-six fruits per cultivar (12 fruits for each block × 3 blocks). The fruit harvest was performed in all three orientations, internal and external, aiming to avoid the edge effect. The fruits were harvested manually with specific pruning shears. The picked fruits were put into plastic bags, appropriately marked and perforated to avoid the formation of condensation. The fruit samples were then transferred to the laboratory on the same day, where they were immediately analyzed.

2.7. Carpometric and Physical Parameters Measurements

Carpometric measurements were performed by weighing and measuring the fruits individually. Fruit weight was measured using an analytical balance (model BC 2200C; ORMA, Milan, Italy). The volume of each fruit was measured using the method of immersing the fruit in a known volume of water. Each fruit was completely submerged in a 1000 cm³ capacity graduated beaker half-filled with water. The volume was calculated with the following Equation (1):

V (cm³) = V2 − V1

(1)

where

• V is the volume of the individual fruit (cm³);
• V1 is the initial volume of water (cm³);
• V2 is the volume registered after the immersion of the bergamot fruit in the graduated beaker (cm³).

Water temperature during the measurements was kept at 25 °C. Specific gravity of fruits was determined by weighing the fruits in air and then determining their volume in water (specific gravity of fruit = fruit weight/fruit volume). Fruit size was determined by measuring width and height using a digital caliper (mod. 1651DGT, Beta Utensili s.p.a., Sovico, Italy). Flavedo color was recorded on three equidistant points of the equatorial region for each fruit using a Minolta CM-700d spectrophotometer (Minolta Corp., Osaka, Japan). Color was evaluated according to the Commission Internationale de l’Éclairage (CIE) and expressed as L*, a* and b* color values [25,26]. These values were used to calculate the hue angle (H°) and target color (C*) as follows:

H° = arctan (b*/a*)

(2)

C* = (a² + b²)^1/2)

(3)

In addition, the Citrus Color Index (CCI), which is widely used in the citrus industry as a maturation index, was calculated following the method described by Jiménez-Cuesta et al. (1981) [27] using Equation (4):

CCI = (1000 × a)/(L × b)

(4)

Peel thickness was measured at three equidistant points of the fruit after it was horizontally split at the point of the equator using the same digital caliper used to determine the size.

2.8. Juice Quality Parameters Dermination

Juice was extracted from the fruits using a domestic squeezer (Citrus Juicer mod. Apollo 150 W, SIRMAN SpA, Curtarolo, Italy) and filtered before analysis. Three juice samples, from the pooled juice of twelve fruits from three replicates per cultivar, were used for chemical analyses. Juice yield was calculated as follows: (juice weight/fruit weight) × 100. The total soluble solids (TSS), titratable acidity (TA) and Ascorbic acid (AA) were determined. TSS was measured using a digital refractometer (DBR 047 SALT, Giorgio Bormac Srl, Modena, Italy), with the results expressed as °Brix. The acidity content was determined in the same diluted juice solution by titrating to pH 8.2 using 0.1 N NaOH, and the value was expressed as % (w/v) citric acid. Measurements were made at constant room temperature (22.2 °C). Ripening index (RI) was calculated as the ratio between percentage TSS and TA (g citric acid/100 mL fruit juice). Ascorbic acid was evaluated using liquid chromatography following Boninsegna et al. (2024) [28] and 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, injecting 20 μL of sample. Operating conditions were a flow rate of 0.7 mL/min with an isocratic elution of a mobile phase solution of potassium phosphate 20 mM acidified (pH 2.9). The results were reported as g of acid/L of juice, comparing for each acid a calibration curve made with external standards.

2.9. Statistical Analysis

Significant differences in the data were evaluated using analysis of variance (ANOVA), followed by Tukey’s multiple range test for p < 0.05. Statistical analysis was performed with the Systat 13 statistical program (SYSTAT Software Inc., Chicago, IL, USA).

3. Results and Discussion

3.1. Climatic Conditions and Main Phenological Phases

The monthly changes in mean temperatures and rainfall during the years 2021 and 2022 at the experimental site are shown in Figure 2. These data, considered as an average of the period 2021–2022, show that the average annual temperature was 18.6 °C, the average temperature of the coldest month was 11.3 °C, in four months the average temperatures were above 24 °C and the annual temperature range was 16.6 °C. The average annual rainfall was 507.7 mm. The wettest month was November (96.1 mm), while the driest was July (with just 2.2 mm of rainfall). The wettest period of the year coincided between late autumn and early winter, with over 50% of the average annual rainfall. From the comparison of the monthly data of the average temperature and rainfall of the years 2021 and 2022, measured by the meteorological station placed within the experimental site, with those of the historical series (from 1991 to 2020) of the monthly data recorded by the ARPACAL meteorological station located approximately 30 km from experimental site (Figure 2 and Figure 3), it is clear that the climate trend that occurred during the experiment period was very similar to the typical climate of the area. Therefore, from a climatic point of view, the years 2021 and 2022 can be considered perfectly representative of the weather conditions that usually characterize the area in which bergamot is traditionally grown. Phenological observations have highlighted overall quite limited differences between the two cultivars of bergamot studied, except for the blooming period, where the differences were more evident. Overall, the Femminello cultivar has exhibited a slight advance in the main phenological phases compared to the Fantastico. In Femminello, the beginning of blooming occurred in the third week of April, while in Fantastico it occurred a week later. Full blooming occurred at the end of April in Femminello and in the first week of May in Fantastico. In both cultivars, the flowering period lasted about 40 days. The June drop began in the last week of June and lasted for about two weeks. The first signs of color break were recorded in the last week of November. This process occurred a few days earlier in Femminello than in Fantastico.

3.2. Fruit Drop

Fruit drop data showed that, up to 260 DAFB, in both cultivars fruit fall was quite limited (Figure 4). Fruit drop from the start of the measurements (200 DAFB) up to 260 DAFB was 3.5% and 2.2% in Fantastico and Femminello, respectively. Despite a certain increase, the fruit drop remained low in the following two months, with cumulative values of below 9%. In both cultivars, the 10% threshold was exceeded only starting from 350 DAFB. From the analysis of the breaking point up to 350 DAFB, the great majority (over 80%) of the fruits fell due to a mechanical tearing that occurred between the twig and the peduncle (AZ-A). This clearly indicates that this fruit drop is essentially due to mechanical phenomena and is not connected with the evolution of fruit ripening. In fact, although during the experiment the wind speed in the experimental site never exceeded 1.5 m/s and there were no particularly intense rain or hailstorms, it is quite likely that light breezes of air may have caused mechanical stresses that determined the fall of a small quantity of fruit, especially those that were fastened to the peripheral parts of the canopy. The higher fruit drop rates recorded so far in the Fantastico compared to the Femminello can probably be attributed to the larger size of the fruits. Larger fruits are subject to greater mechanical stress than smaller ones. The fruit drop increased significantly in both cultivars after 350 DAFB, going from values just above 10% to values above 20% at 380 DAFB. The analysis of the breakage zone highlights that, starting from 350 DAFB, the fruit fall occurred essentially due to the break between the peduncle and the fruit. This clearly indicates that the increase in fruit drop was due to the advanced stage of fruit ripening. In fact, with the ripening process, a series of physiological and biochemical events that occur in the area between the peduncle and the calyx led to the detachment of the fruit. These are quite complex and articulated mechanisms that first lead to the thickening of the cell walls, following the rupture of the middle lamella, followed by hydration of cell wall components. As abscission progresses, the primary cell wall is degraded. Eventually, a fracture develops across the abscission zone, resulting in fruit separation from the peduncle [29,30,31,32]. Although the causes that determine these processes are not yet completely clear, a very important role is certainly played by ethylene [31,33]. In fact, although citrus fruits have generally been recognized as non-climacteric fruits, since an increase in respiration and ethylene production apparently does not occur after the mature fruit has been removed from the tree [34,35], a quite different story would seem to concern the fruits still attached to the tree. Sawamura (1981) [36], analyzing the seasonal changes in the levels of endogenous ethylene inside the fruits still attached to the tree, observed first a progressive decrease in the values starting from the end of the June fruit drop and up until the proximity of ripening, and then a sharp increase as the ripening progressed. In addition to acting directly by stimulating fruit abscission, it also appears that the presence of ethylene determines an increase in the quantities of mRNA and proteins in these tissues [37]. Although the increase in mRNA and protein is not clearly understood, it is known that some mRNAs and proteins are frequently associated with enzymes cellulase and polygalacturonase [38]. These enzymes are thought to function in the degradation of adhesive cell wall components that link abscission zone cells together, and this increase in enzyme activities is highly correlated with a reduction in the fruit break strength [39,40]. The results obtained highlight that the fruit drop became particularly evident in the subsequent survey (410 DAFB) when the percentage of fallen fruit was higher than 40% in the Fantastico and even close to 60% in the Femminello. In this respect, it should be remembered that during the abscission process the involved organs release significant amounts of ethylene, which first activate its signal transduction pathway, leading to the activation of ethylene responsive transcription factors and ethylene-responsive genes, which in turn regulate abscission [41]. Endogenous ethylene production, induced by methyl jasmonate, is thought to be responsible for fruit abscission in some orange varieties [42]. In addition to ethylene and methyl jasmonate, abscisic acid may also play an important role in the abscission process of citrus fruits. While it is quite certain that abscisic acid plays a role in the formation of ethylene, which induces abscission [43], its direct action on fruit abscission is not yet clear. It is known that the concentration of abscisic acid in citrus fruits increases as ripening progresses [44] and that exogenous applications stimulate cellulase activity in separation zones causes the loosening of citrus fruit [45]. When ethylene levels are lowered due to hypobaric conditions, abscisic acid stimulates cellulase in the separation zones of citrus fruits [45]. In some instances, abscisic acid induces senescence, which brings about more ethylene synthesis by the fruit. The possibility that other enzymes, such as polygalacturonase [46], membrane permeability [47] and a senescence-inducing factor [48] may be influenced through endogenous ABA should not be overlooked. The results obtained also highlight that, starting from 380 DAFB, the cultivar differences, which until that period had been quite limited, become increasingly evident. Starting from 380 DAFB, the fruit drop is greater in the Femminello than in the Fantastico. It should be emphasized, however, that both bergamot cultivars are characterized by the ability to keep the fruit on the tree for an extremely long period of time that exceeds a year. Up to now, no study had reported these cultivar characteristics.

3.3. Carpometric and Physical Parameters of the Fruits

The results of the fruits’ weight showed significant differences both in terms of harvest time and cultivar (

Table 1. Effects of harvest time on the main carpometric characteristics of fruits (mean ± standard error) of Fantastico (FA) and Femminello (FE) bergamot cultivars.

Parameter	Cv ¥	Harvest Time (DAFB)
		200	230	260	290	320	350	380	410	Sign
Fruit weight (g)	FA	224.6 ± 1.44 e	255.8 ± 5.80 d	312.5 ± 1.95 c	336.5 ± 1.94 b	357.1 ± 0.96 a	369.3 ± 0.78 a	358.5 ± 4.67 a	335.4 ± 1.05 b	**
	FE	174.9 ± 3.03 f	211.3 ± 4.11 e	232.4 ± 1.23 d	247.8 ± 1.34 c	260.8 ± 0.67 b	271.5 ± 2.57 a	257.0 ± 0.80 bc	227.5 ± 0.67 d	**
	Sign.	**	**	**	**	**	**	**	**
Fruit height (mm)	FA	74.4 ± 0.34 d	79.3 ± 1.78 c	86.0 ± 0.90 b	87.6 ± 0.42 b	88.7 ± 0.28 b	90.5 ± 1.30 a	92.2 ± 0.36 a	93.6 ± 0.48 a	**
	FE	69.5 ± 0.15 e	75.2 ± 0.40 d	77.8 ± 0.58 cd	79.4 ± 0.32 bc	81.4 ± 1.07 abc	83.0 ± 0.38 ab	84.6 ± 1.55 a	85.1 ± 0.76 a	**
	Sign.	**	n.s.	**	**	**	**	**	**
Fruit width (mm)	FA	80.2 ± 0.57 f	81.1 ± 0.73 f	87.3 ± 0.12 e	89.8 ± 0.09 de	91.8 ± 0.17 cd	92.9 ± 0.40 bc	95.1 ± 0.89 ab	96.7 ± 0.55 a	**
	FE	70.8 ± 0.60 f	75.3 ± 0.55 e	77.6 ± 0.14 d	79.7 ± 0.16 c	81.4 ± 0.36 bc	82.3 ± 0.30 ab	83.7 ± 0.30 a	84.1 ± 0.44 a	**
	Sign.	**	**	**	**	**	**	**	**
Ratio height/width	FA	0.93 ± 0.010	0.98 ± 0.025	0.98 ± 0.011	0.98 ± 0.004	0.97 ± 0.004	0.97 ± 0.017	0.97 ± 0.006	0.97 ± 0.010	n.s.
	FE	0.98 ± 0.009	1.00 ± 0.012	1.00 ± 0.007	1.00 ± 0.005	1.00 ± 0.017	1.01 ± 0.008	1.01 ± 0.015	1.01 ± 0.014	n.s.
	Sign.	**	n.s.	n.s.	*	n.s.	n.s.	n.s.	n.s.
Fruit volume (cm3)	FA	262.3 ± 3.12 f	290.8 ± 5.58 e	327.2 ± 2.35 d	352.4 ± 2.40 c	373.8 ± 1.85 bc	391.3 ± 3.04 b	417.5 ± 9.27 a	437.0 ± 3.43 a	**
	FE	213.1 ± 3.11 f	246.5 ± 2.32 e	253.6 ± 2.37 e	271.8 ± 1.37 d	289.4 ± 1.49 c	301.6 ± 1.05 bc	316.3 ± 8.01 ab	320.8 ± 0.72 a	**
	Sign.	**	**	**	**	**	**	**	**
Fruit specific gravity (g/cm3)	FA	0.86 ± 0.005 b	0.88 ± 0.014 b	0.96 ± 0.006 a	0.96 ± 0.009 a	0.96 ± 0.004 a	0.95 ± 0.008 a	0.86 ± 0.007 b	0.77 ± 0.004 c	**
	FE	0.83 ± 0.023 b	0.86 ± 0.010 ab	0.92 ± 0.004 a	0.91 ± 0.006 a	0.90 ± 0.006 a	0.90 ± 0.006 a	0.82 ± 0.018 b	0.71 ± 0.003 c	**
	Sign.	n.s.	n.s.	**	**	**	**	n.s.	**

^¥ FA: Fantastico; FE: Femminello. Significant level: ** significance at p < 0.01; * significance at p < 0.05; n.s. not significant. The different letters within the same line indicate significant differences according to the Tukey test (p < 0.05).

). Regarding harvest time, in both cultivars, up to 350 DAFB, the fruit weight gradually increased. The increases in fruit weight were high enough up to 260 DAFB, with growth rates around 1.0 g·d⁻¹ in the Femminello and 1.5 g·d⁻¹ in the Fantastico. The growth rates subsequently progressively decreased over time, with values around 0.4 g·d⁻¹ between 320 and 350 DAFB in both cultivars. The increase in fruit weight is mainly due to the increase in juice content in the pulp and the accumulation of total soluble solids (mainly carbohydrates, but also organic acids, proteins, lipids and minerals) inside the juice sacs [22]. Starting from 350 DAFB, the fruit weight began to decrease, first slightly and then more noticeably between 380 and 410 DAFB. In the last 30 days, the fruit weight decreased compared to the previous observation (380 DAFB) by over 6% and 11% in Fantastico and Femminello, respectively. The decrease in fruit weight recorded starting from 350 DAFB is essentially attributable to the drying of the pulp, a very complex phenomenon connected to the advanced state of ripeness of the fruit and to the senescence processes of the pulp [49].

Regarding cultivar differences, on average during the period analyzed (from 200 to 410), the weight of Fantastico cultivar fruits was more than 30% greater than those of Femminello. The fruit weight differences between the two cultivars increased over time. Up to 230 DAFB, the differences were under 50 g. They then increased over time, with differences close to or greater than 100 g starting from 320 DAFB. The results obtained confirm the peculiarity of the Fantastico cultivar to produce larger fruits compared to the Femminello cultivar [50]. The changes in fruit weight, expressed not as absolute values but in percentage terms with respect to the maximum value measured in the period between 200 and 410 DAFB (days after full bloom), highlight that the two studied bergamot cultivars have very similar growth models, represented by two curves that are almost superimposable, described by two second-degree equations (Figure 5A).

These results highlight that, in the later stages of fruit growth, the two bergamot cultivars are characterized by very similar percentage changes in weight and that the final fruit’s weight depends greatly on the values they have reached at the end of phase II of fruit growth. In fact, although the differences in fruit weight, in absolute terms, between the two cultivars amplify over time, the changes in percentage terms remain, instead, very similar. This confirms the crucial role of the biological processes that occur during phases I and II of fruit growth in determining the final weights of the fruits. During phase I (2.5–3.5 months after flower bloom), the number of cells in the ovary tissue increases because of cell division, especially in the pericarp. This stage also sees the beginning of cell differentiation in all fruit tissues. In phase II (3.5–4.5 months after flower bloom), cell elongation begins; this is important for increasing the volume and weight of the fruit resulting from the accumulation of dry matter and water. In phase III (4.5–5 months after flower bloom), which essentially concerns the development of phenomena associated with fruit ripening, the growth rate, in fact, decreases drastically [51,52,53]. The cultivar differences in fruit weight could likely be attributed to different initial floral ovary sizes [54] or to different sink strengths, particularly initial fruit development [55,56,57,58]. A close connection between the sink strength in the initial stage of fruit development and the final size was proposed by Guardiola and García-Luis (2000) [59]. Based on this idea, restrictions on the sink strength at very early stages of fruit development would have irreversible repercussions on later stages, even when the cause is no longer present. The different sink strength between the two bergamot cultivars could be attributable to a different enzymatic profile. In fact, it is believed that sink strength is closely associated with the presence of enzymes with specific actions, such as acid invertase, which is involved in breaking down sucrose molecules [60]. Studies of grapefruit have found that the highest level of acid invertase activity in fruit occurs in albedo tissue (component of pericarp) during the first growth stage, accompanied by high levels of hexoses as reducible sugars. These sugars may be used rapidly in respiration and in biosynthesis. An association between the observed levels of acid invertase and the rate of cell division has also been observed [61]. Although fruit growth is influenced by several internal [20,22,62,63] and external [64,65,66,67,68,69,70,71] factors to the plant, the possibility of representing the bergamot fruit growth, even if only starting from the second part of phase II, through a mathematical expression can be a useful practical tool in the hands of bergamot growers to estimate the fruit weight changes in the last stages of their growth. In fact, by associating the percentage changes that the fruits of the two bergamot cultivars exhibit during the period between 200 and 410 DAFB, with the actual size reached by the fruits at the end of phase II of fruit development, it is possible to estimate with good approximation the changes in fruit weight. This can represent a valid decision-making tool for growers to know well in advance what the final weights potentially achievable by the fruits will be and, consequently, also their commercial sizes. Significant differences between harvest periods and cultivars were also found in fruit height and width. Contrary to what was recorded for fruit weight, the values of these two parameters increased over time with an average increase of approximately 0.1 mm·d⁻¹. As for the fruit weight, the highest values for both parameters were found in the Fantastico cultivar. On average, during the eight harvests, the height and width values of the fruits of the Fantastico cultivar compared to the Femminello were 9 and 13% higher, respectively. The differences found in the height/width ratio were not very significant. Significant differences were found, however, in the volume of the fruit, both in terms of harvest time and cultivar. The fruit volume increased gradually over time. From 200 to 410 DAFB, the fruit volume increased by over 65% in Fantastico and over 50% in Femminello. Except for the interval between 200 and 230 DAFB, when the increase of Femminello was higher than that of Fantastico, from 230 to 410 DAFB, the average daily increases of the Fantastico fruits were substantially higher. This behavior determined that the cultivar differences in fruit volume increased from 23% at the 200 DAFB to 36% at the last harvest time. As with fruit weight, changes in the fruit volume, always expressed as percentage values with respect to the maximum value measured in the period between 200 and 410 DAFB, can be represented, for both cultivars, by two second-degree equations (Figure 5B). Significant differences between harvest times were also recorded regarding specific gravity fruit. In both cultivars studied, the specific gravity fruit increased up to 260 DAFB to stabilize at these threshold values up to 350 DAFB and then decreased in subsequent harvest times. These findings are in corroboration with the findings of Khokhar and Sharma (1984) [72], Bhullar (1983) [73] in sweet orange, Singh et al., (2015) [74] in grapefruit, Rokaya et al., (2016) [75] in mandarin and Shahida et al., (2022) [76] in lemon, who reported that the specific gravity increased with the advancement of maturity and then decreased with maturity decline. Changes in fruit specific gravity are related to internal characteristics such as the dry matter, soluble solids or physical disorders. Increases in fruit specific gravity are likely due to a higher rate of accumulation or synthesis of food materials. In the range between 260 and 350 DAFB (the one in which the fruit specific gravity exhibited the highest values), significant cultivar differences were also found, with values approximately 5% higher in the Fantastico cultivar.

3.4. Color and Thickness of the Peel

Significant differences were found in the time evolution of the peel color (

Table 2. Effects of harvest time on the main peel characteristics (mean ± standard error) of Fantastico and Femminello bergamot cultivars.

Parameter	Cv ¥	Harvest Time (DAFB)
		200	230	260	290	320	350	380	410	Sign
L*	FA	56.7 ± 0.29 d	62.9 ± 0.15 c	65.9 ± 0.37 b	70.1 ± 0.03 a	70.6 ± 0.14 a	69.7 ± 0.11 a	70.5 ± 0.21 a	70.2 ± 0.22 a	**
	FE	59.4 ± 0.27 c	65.5 ± 0.42 b	70.5 ± 0.24 a	70.6 ± 0.51 a	70.5 ± 0.33 a	70.3 ± 0.35 a	70.5 ± 0.21 a	70.3 ± 0.38 a	**
	Sign	**	**	**	n.s.	n.s.	n.s.	n.s.	n.s.
a*	FA	−5.1 ± 0.14 e	0.2 ± 0.39 d	1.8 ± 0.15 c	6.2 ± 0.11 b	7.7 ± 0.32 a	7.9 ± 0.29 a	7.4 ± 0.05 a	6.1 ± 0.15 b	**
	FE	−5.1 ± 0.20 e	1.0 ± 0.40 d	6.0 ± 0.12 c	7.5 ± 0.04 b	9.3 ± 0.25 a	9.2 ± 0.08 a	8.6 ± 0.03 a	7.4 ± 0.06 b	**
	Sign	n.s.	n.s.	**	**	*	**	**	**
b*	FA	29.9 ± 0.05 d	36.9 ± 0.41 b	38.4 ± 0.22 a	38.8 ± 0.15 a	37.7 ± 0.18 ab	36.8 ± 0.41 b	34.5 ± 0.11 c	33.8 ± 0.12 c	**
	FE	30.4 ± 0.44 d	37.6 ± 0.58 b	39.2 ± 0.40 a	39.4 ± 0.23 a	39.6 ± 0.04 a	38.9 ± 0.13 ab	37.1 ± 0.18 b	35.0 ± 0.10 c	**
	Sign	n.s.	n.s.	n.s.	n.s.	**	**	**	**
H°	FA	99.6 ± 0.25 a	89.8 ± 0.57 b	87.4 ± 0.24 c	80.9 ± 0.17 d	78.5 ± 0.42 ef	78.0 ± 0.31 f	77.9 ± 0.09 f	79.7 ± 0.27 de	**
	FE	99.5 ± 0.30 a	88.6 ± 0.66 b	81.3 ± 0.09 c	79.2 ± 0.11 d	76.8 ± 0.34 ef	76.6 ± 0.09 f	76.9 ± 0.10 ef	78.1 ± 0.12 de	**
	Sign	n.s.	n.s.	**	**	*	*	**	**
C*	FA	30.3 ± 0.07 e	37.0 ± 0.42 c	38.5 ± 0.22 ab	39.3 ± 0.15 a	38.5 ± 0.24 ab	37.7 ± 0.46 bc	35.3 ± 0.11 d	34.4 ± 0.10 d	**
	FE	30.8 ± 0.45 d	37.7 ± 0.57 b	39.7 ± 0.41 a	40.1 ± 0.22 a	40.7 ± 0.03 a	40.0 ± 0.13 a	38.1 ± 0.17 b	35.8 ± 0.09 c	**
	Sign	n.s.	n.s.	n.s.	*	**	**	**	**
CCI	FA	−3.0 ± 0.10 f	0.1 ± 0.16 e	0.7 ± 0.06 d	2.3 ± 0.05 c	2.9 ± 0.11 ab	3.1 ± 0.08 a	3.0 ± 0.01 a	2.6 ± 0.07 bc	**
	FE	−2.8 ± 0.09 e	0.3 ± 0.18 d	2.2 ± 0.02 c	2.7 ± 0.02 b	3.3 ± 0.08 a	3.4 ± 0.04 a	3.3 ± 0.04 a	3.0 ± 0.02 ab	**
	Sign	n.s.	n.s.	**	**	*	*	**	**
Peel thickness	FA	4.5 ± 0.10 b	4.9 ± 0.12 ab	5.0 ± 0.03 a	5.2 ± 0.06 a	5.2 ± 0.06 a	5.3 ± 0.07 a	5.3 ± 0.14 a	5.3 ± 0.11 a	**
(mm)	FE	3.3 ± 0.11 b	3.7 ± 0.12 ab	3.9 ± 0.03 a	4.0 ± 0.08 a	4.0 ± 0.02 a	4.0 ± 0.07 a	4.0 ± 0.03 a	4.1 ± 0.08 a	**
	Sign	**	**	**	**	*	*	**	**

^¥ FA: Fantastico; FE: Femminello. Significant level: ** significance at p < 0.01; * significance at p < 0.05; n.s. not significant. The different letters within the same line indicate significant differences according to the Tukey test (p < 0.05).

). The coordinate L*, from initial values of 56.7 and 59.4 measured in Fantastico and Femminello, respectively, at 200 DAFB, registered a progressive increase in values up to 290 DAFB and then stabilized until the last harvest (410 DAFB) around 70–71. Significant differences between the two cultivars were found only in the first three harvest periods. In this period (between 200 and 260 DAFB), the values found in the Fantastico cultivar were significantly lower than those found in the Femminello. From 290 DAFB, the differences between the two cultivars were minimal and, in any case, not significant. Significant differences between harvest times and cultivars were also found for coordinate a*. Regarding the harvest times, at 200 DAFB, in both cultivars, the values were negative (around −5), which is a sign of a certain presence of chlorophyll in the peel. Starting from 230 and up to 320 DAFB, the values of this parameter progressively increased and then stabilized for a period of approximately 90 days at values around 7.4–7.9 in the Fantastico and 8.6–9.3 in the Femminello cultivar and decreased during the last harvest time (410 DAFB). At the cultivar level, except during the first two harvest times, where the differences between the two cultivars were not significant, starting from 260 DAFB, the values of Fantastico were significantly lower than those of Femminello. Significant differences were also found regarding coordinate b*. In both cultivars, the values progressively increased up to 260 DAFB, stabilized in the following 90 days and progressively decreased starting from 350 DAFB. Among the cultivars, significant differences were found only starting from 320 DAFB, with significantly lower values in the Fantastico than in the Femminello cultivar. The hue angle (H°) progressively decreased up to 320 DAFB and then stabilized in the subsequent harvest times and rose slightly during the last harvest time. Significant differences between the two cultivars were found only starting from 260 DAFB, with significantly higher values in the Fantastico cultivar. The Chroma (C*) value progressively increased in both cultivars up to 260 DAFB, then stabilized for a period of approximately 120 days and decreased during the last harvest times. Significant cultivar differences were found starting from 290 DAFB, with significantly higher values in the Femminello compared to the Fantastico cultivar. Regarding the Citrus Color Index (CCI), it progressively changed from negative values found during the first harvest period, to positive values gradually increasing up to 320 DAFB. Once the maximum threshold was reached, the CCI values then stabilized for approximately 60 days, showing a decreasing trend during the last harvest time. Except in the first two harvest periods, the Femminello cultivar had significantly higher values than the Fantastico. Color development, commonly known as peel degreening, is a critical part of ripening and is characterized by a color change in the peel from green to yellow/red/orange [22]. Peel degreening is an important aspect for the marketability of citrus fruit, especially when they are used for fresh consumption [77]. There are two main pathways that have been linked to citrus peel degreening. The first is chlorophyll degradation [78,79,80]. The second pathway is carotenoid biosynthesis [81,82]. Both are processes that occur under genetic control. Genes encoding various enzymes for the main steps of chlorophyll degradation and carotenoid metabolism have been isolated and functionally characterized [83].

The results of this study, which represent the first information on the changes in the peel color of bergamot fruits during ripening, also confirm for this species that the decline in rind chlorophyll is a long process that takes several months, and the onset of carotenoid accumulation almost coincides with the disappearance of chlorophyll [84]. The results obtained also highlight that peel degreening and the associated reduction in the content of chlorophylls and carotenoids coincided with the gradual decline in the minimum ambient temperatures, confirming what had emerged in other studies conducted on other citrus species, namely, that peel degreening in citrus fruit progresses as the ambient temperature decreases [16,83,85,86]. From the analysis of coordinate a*, it is clear that it reaches positive values (a sign that the chlorophyll in the peel has now degraded) starting from December, when the average air temperature drops significantly, going from 16.8 °C in November to 12.5 °C in December. It is likely that this lowering of temperature leads to the color change. In experiments carried out in Florida with the Hamlin, Parson Brown and Pineapple oranges on the relationship between climatic conditions and peel color, it was concluded that the color does not change until 12.8 °C is reached [87]. The rapidity with which the hue reaches a maximum depends upon the severity of the temperature drop and the continued occurrences of minimum temperatures below 12.8 °C [88,89]. The slight cultivar differences regarding the changes in coordinate a* are likely to be attributed to a different effect of temperature on the two bergamot cultivars. Quite different effects of the temperature drop on the values of coordinate a* of the peel were found by Manera et al. (2012) [16] on the lemon varieties Eureka, Lisnon and Fino. The study conducted by Manera et al. (2012) [16] also highlighted that once coordinate a* has reached a positive value, a further drop in temperature will have little influence. Similarly, an unchanged mean temperature can give rise to two different values (as a function of the value reached in the previous weeks). The values of coordinate b* of close to 30 at the time of the first harvest clearly indicate that at 200 DAFB the synthesis of carotenoids was in an advanced stage and the yellow tones were already evident but were still masked by the green tones of chlorophyll. The drop in temperature also had effects on the other colorimetric parameters taken into consideration. The data relating to Chroma (C*) show that the values increased with the fall in temperature mainly due to the increase in coordinate b*, which influences Chroma more strongly than coordinate a*. However, once the characteristic yellow color was reached, the Chroma values stabilize or at least vary very little, independently of the temperature. With the decrease in temperature, the degree of hue angle (H°) also tends to decrease. When it reaches values around 90°, the peel has lost all its chlorophylls and the fruit tends to assume its characteristic yellow color. The values of coordinate a* (greater than zero), the hue angle (close to 90°), the Chroma (stabilized at values around 40) and the CCI (greater than 0) indicate that in mid-December, in both bergamot cultivars, the peel was now completely yellow. As regards the slight greening of the peel observed during the last harvest time, evidenced by the slight decrease in the values of coordinate a*, the hue angle and the CCI, it should probably be attributed to the significant increase in the average air temperature. In this regard, it should be noted that the chloro- to chromoplast conversion is a reversible process, even from fully differentiated chromoplasts. The increase in temperature and day length determines a significant increase in nitrogen uptake by the roots and its translocation to aerial parts, including the fruit peel; however, the growing leaves are a major sink of sugars and the concentration in the peel decreases. These changes presumably lead to a greening of the peel of the fruits still hanging on the tree [90]. Significant differences were also found in the thickness of the peel (Table 2). The Fantastico cultivar had significantly higher values than the Femminello.

3.5. Qualitative Characteristics of the Fruits

The juice yield progressively increased up to 260 DAFB (

Table 3. Effects of harvest time on juice content and the main qualitative characteristics of juice (mean ± standard error) of Fantastico and Femminello bergamot cultivars.

Parameter	Cv ¥	Harvest Time (DAFB)
		200	230	260	290	320	350	380	410	Sign
Juice content (%)	FA	45.4 ± 1.27 c	47.4 ± 1.73 bc	52.9 ± 0.61 ab	53.7 ± 1.00 a	51.7 ± 0.58 ab	44.2 ± 0.58 c	37.7 ± 1.41 d	35.4 ± 1.61 d	**
	FE	46.4 ± 0.77 b	45.2 ± 0.44 b	54.6 ± 0.77 a	54.7 ± 0.84 a	51.0 ± 0.80 a	43.0 ± 0.97 b	34.3 ± 2.24 c	27.5 ± 0.25 d	**
	Sign	n.s.	n.s.	n.s.	n.s.	n.s.	n.s.	n.s.	**
Total soluble solids (°Brix)	FA	9.3 ± 0.03 a	9.3 ± 0.03 ab	8.9 ± 0.12 bc	9.0 ± 0.03 abc	8.8 ± 0.07 cd	8.4 ± 0.15 de	7.8 ± 0.12 f	8.0 ± 0.06 ef	**
	FE	8.9 ± 0.03 ab	9.1 ± 0.21 a	8.4 ± 0.09 bc	8.4 ± 0.15 bc	8.0 ± 0.03 cd	7.6 ± 0.15 de	7.3 ± 0.07 e	7.2 ± 0.12 e	**
	Sign	**	n.s.	*	*	**	*	*	**
Titratable acidity (g·L−1)	FA	51.3 ± 0.92 a	45.4 ± 1.00 b	39.1 ± 0.61 c	36.7 ± 0.56 cd	35.5 ± 0.30 cde	33.5 ± 0.35 def	31.8 ± 1.24 ef	30.5 ± 0.88 f	**
	FE	55.5 ± 0.81 a	54.8 ± 1.17 a	42.4 ± 0.94 bc	42.9 ± 1.64 b	40.4 ± 0.71 bcd	38.2 ± 0.51 cd	37.3 ± 0.56 d	37.9 ± 0.76 cd	**
	Sign	*	**	*	*	**	**	**	*
TSS/TA ratio	FA	1.8 ± 0.03 d	2.0 ± 0.04 cd	2.3 ± 0.06 bc	2.5 ± 0.03 ab	2.5 ± 0.03 ab	2.5 ± 0.06 ab	2.5 ± 0.12 ab	2.6 ± 0.06 a	**
	FE	1.6 ± 0.02 c	1.7 ± 0.07 bc	2.0 ± 0.06 a	2.0 ± 0.09 a	2.0 ± 0.04 a	2.0 ± 0.05 a	2.0 ± 0.01 a	1.9 ± 0.02 ab	**
	Sign	**	**	*	**	**	**	*	**
Ascorbic acid (g·L−1)	FA	0.78 ± 0.009 a	0.74 ± 0.013 a	0.64 ± 0.022 b	0.60 ± 0.024 b	0.59 ± 0.007 bc	0.52 ± 0.006 cd	0.46 ± 0.015 de	0.44 ± 0.007 e	**
	FE	0.67 ± 0.007 a	0.67 ± 0.004 a	0.56 ± 0.010 b	0.53 ± 0.019 b	0.52 ± 0.003 bc	0.48 ± 0.006 cd	0.44 ± 0.010 de	0.42 ± 0.009 e	**
	Sign	**	**	*	n.s.	**	**	n.s.	n.s.

^¥ FA: Fantastico; FE: Femminello. Significant level: ** significance at p < 0.01; * significance at p < 0.05; n.s. not significant. The different letters within the same line indicate significant differences according to the Tukey test (p < 0.05).

). Once the maximum threshold was reached, the values remained almost similar for another two months. Starting from 350 DAFB, the values progressively decreased in the following months. Although both cultivars showed the same decreasing trend, the decrease found in the Femminello cultivar was particularly evident, with a value of less than 30% during the last harvest time. However, it should be noted that, except for the last two harvest periods (380 and 410 DAFB), the juice yield was consistently above 40% for six months, even exceeding the 50% value in the period between 260 and 320 DAFB. In view of the increasing interest in bergamot juice by consumers, the two bergamot cultivars studied have proven to be very valid for juice production. The decrease in juice yield starting from 350 DAFB is attributable to the gradual drying of the juice sacs. This is a particular physiological disorder that characterizes many citrus species, which, however, remains not fully understood [43]. Studies conducted on this topic have shown that during the drying process a series of metabolic changes occur in the juice sacs. The disordered juice sacs become hardened, enlarged, dry, colorless and opaque [91,92,93]. Goto (1989) [94] also found that this phenomenon begins with a gelation process, followed by real drying of the juice sacs. A study conducted on pummelo showed that during this process the cell walls of elongated cells and juice cells were both thickened during segment drying, together with the occurrence of opaque white regions in transparent tissue [95]. Similarly, it was found that the cell wall of the juice sacs thickened and the intracellular content decreased with the development of segment drying in Lanelate navel orange [96]. Although the causes of this physiological disorder may be multiple, it is known that it becomes ever more evident as fruit ripening advances [97] and increases the longer the fruit is held on the tree [98]. These findings led some researchers to hypothesize that the drying of the juice sacs was a process of fruit senescence attributed to an imbalance in ROS metabolism [92,99]. However, this hypothesis is not agreed by all scientists; most of them believe that the causes underlying this phenomenon are multiple, with processes modulated by structural genes, transcriptional factors and microRNAs [100,101]. The total soluble solids (TSS) content of the juice showed a decreasing trend during the harvest times, with a decrease between the first and last harvest period of 14 and 19% in the Fantastico and Femminello cultivars, respectively. On average, over the eight harvest periods, the TSS content found in the Femminello juice was approximately 7% lower than that of the Fantastico. Similarly to the total soluble solids content, the titratable acidity (TA) values also decreased as the fruit ripening advanced, going from 51 to 31 g L⁻¹ in the Fantastico and from 56 to 38 g L⁻¹ in the Femminello cultivar over the eight months of harvest. On average, the TA of the Femminello juice was 16% higher than that of the Fantastico.

The decrease in the TSS during fruit ripening is strictly connected to the decrease in the TA. In fact, contrary to moderately acidic citrus fruits, in which carbohydrates represent about 80% of the TSS [102], in high-acid citrus fruits, such as bergamot, the predominant share of the TSS is represented by organic acids (especially citric acid). Therefore, while in moderately acidic citrus fruits the trend of TSS during fruit ripening is usually increasing, since the continuous increase in carbohydrates starting from the end of phase II and throughout phase III of fruit development far exceeds the decrease in acids, in high-acid citrus fruits, the increase in carbohydrates (which proportionally have very little impact on the computation of the TSS) does not compensate at all for the decrease in organic acids (which instead represent the relevant part of the TSS). The decrease in TA, which usually occurs starting from the second half of phase II and throughout phase III of fruit development [103,104,105,106,107], is instead due to catabolism and a dilution effect by fruit growth [22,51,90,108,109]. The E/A ratio in both cultivars increased up to 260 DAFB and then stabilized during the following harvest times. Between the two cultivars, Femminello recorded significantly lower values than Fantastico. The ascorbic acid (AsA) content progressively decreased during the eight harvest times, with a decrease from the first to the last harvest time of about 43% in Fantastico and 38% in Femminello. During the eight harvest times, the AsA in Fantastico was on average 10% higher than in Femminello. Even with different changing patterns, these results confirm what was found in previous studies on bergamot [50] and in other citrus species [110], namely, that AsA decreases with fruit ripening. Regarding cultivar differences, they could be likely due to a different metabolism of accumulation and degradation of AsA. A study on the pulp of two cultivars of oranges (Egan No. 2 and Newhall) suggested that differences in AsA concentration were associated with differences in the expression of genes of the l-galactose pathway as well as in the activity of enzymes involved in AsA degradation [111]. As with the weight and volume of the fruit, the internal characteristics development of the fruit in the period between 200 and 410 DAFB expressed as percentages with respect to the maximum values reached in this period of time can also be represented by specific mathematical expressions: second-degree equations for the juice yield, the TA and AsA, and a first-degree equation for the TSS (Figure 6). Similarly to fruit weight, this can be a useful decision-making tool for bergamot growers as it can enable them to estimate the internal characteristics development of the fruit by knowing the values of these parameters at the end of the second phase of fruit growth (200 days after full bloom).

3.6. Definition of Optimal Harvest Time

The joint analysis of the evolution of fruit drop and fruit weight allowed us to estimate the changes in fruit production per tree during the period between 200 and 410 DAFB (Figure 7). In both cultivars, the production per tree increased up to 320 DAFB. The increase in production was very evident in the first 30 and 60 days in Femminello and Fantastico, respectively, and then it became increasingly slight. During this time, production losses due to fruit drop (still quite limited) were largely overcome by increases following the increase in fruit weight. At 320 DAFB, the trees had a quantity of fruit of just over 60 kg in Fantastico and around 50 kg in Femminello. In the following 30 days, the production per tree remained almost constant. In fact, the loss of production due to the fruit drop was compensated for by the increase, albeit slight, in the average weight of the fruit. After 350 DAFB, the amount of fruits present on the tree began to decrease. The reasons for this decrease in production can be attributed both to the increasingly marked fruit drop and to an average fruit weight reduction following the drying of the pulp. At 410 DAFB, the harvested production was over 50 and 60% less in Fantastico and Femminello, respectively, compared to that present on the trees at 320 DAFB. In view of the results obtained, to maximize production in quantitative terms, the harvest should not occur before 260 DAFB in Femminello and 290 DAFB in Fantastico. By postponing the harvest by a few months, the yields could increase further, albeit slightly. In both cultivars, however, it is not advisable to postpone the harvest beyond 350 DAFB, as the yields are significantly reduced.

The time window between 260 and 320 DAFB (Figure 8) also appears to be the best time to enhance the qualitative characteristics of bergamot fruit. During this period, the fruits have the highest juice yield and in terms of the acidity, on the one hand it is no longer excessively high to make the fruit’s flavor excessively sour, and on the other it has not yet suffered the collapse that makes the taste flat. This time interval also represents a good compromise regarding the nutraceutical aspects of the fruit (AsA content). In fact, although between 290 and 320 DAFB the AsA is clearly lower than in previous harvest periods, its content is still quite good, remaining considerably higher than that of many other citrus species.

4. Conclusions

The results of this study, conducted with a holistic approach that allowed the analysis of both purely bioagronomic aspects and parameters strictly connected with the quality of the fruits, provide new information regarding the changes in the external and internal characteristics of bergamot fruits during ripening in the Fantastico and Femminello cultivars. Until now, no study had ever evaluated the changes in fruit characteristics over such a long period of time (from 200 to 410 DAFB). The results obtained show that during the ripening process, significant changes occur in bergamot fruits that significantly affect the fruit’s quality. Significant differences were found at the cultivar level in terms of fruit size, fruit specific gravity, peel thickness, TSS, TA and AsA content. The fruits of the Fantastico cultivar showed larger dimensions and higher TSS and AsA contents than those of the Femminello cultivar, which instead were characterized by a higher TA of the juice. The study also allowed us to define the optimal harvest period, that is, the time window that maximizes the yield of the trees and enhances the qualitative characteristics of the fruits. In this regard, it emerged that the time window between 260 and 320 DAFB seems to be the best time to maximize yields and enhance the qualitative characteristics of the fruit. These results also highlight that, compared to the harvesting of fruits for the extraction of essential oils, which usually occurs in these cultivation environments between late autumn and early winter, for fruits for the fresh market the harvesting period must necessarily be later. In fact, with the early harvest of the fruits not only is the productive yield penalized, as the fruits have not yet completed their growth, but so are the internal qualitative characteristics (lower juice yield and excessively sour taste). In this regard, the two bergamot cultivars studied have shown that they are characterized by the ability to keep the fruit on the tree for a very long period without excessive drops and quality declines, at least up to 350 DAFB. The new information acquired with this study appears very useful for rationalizing the bergamot harvest and enhancing the quality characteristics of the fruits of this particular citrus fruit. However, the results obtained certainly do not exhaust the subject but highlight the need for further studies regarding the possible quality changes in bergamot fruits during the post-harvest period. Also worthy of further study are the aspects relating to the physiological mechanisms that regulate the ripening and abscission of bergamot fruits and, in particular, the role played by ethylene, methyl jasmonate and abscisic acid on these processes.