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Published in Issue No.143, page 40 to 45
Qualitative and physiological properties of ‘Pompia’, a citron-like fruitS. D’Aquino F. Fronteddu M. Usai A. Palma
Citrus is one of the richest botanical groups of plants with thousands of known cultivars worldwide. The botanical classification of Citrus is very controversial; in fact, the number of species is not well defined and differs according to various botanists. Tanaka (1961) states that the genus Citrus includes 157 species whilst Swingle and Reece (1967) indicate only 16 species exist. More than in other botanical groups, citrus individuals of different species can cross-breed with high frequency, resulting in fertile hybrids which, in turn, create new individuals for cross-pollination. For example, Barret and Rodhes (1976) suggested that lemon is probably a trihybrid between C. medica, C. grandis and Micro citrus.
This justifies the suspicion that traditional botanical classification considers a group of individuals as species that are, in reality, not a species but a botanical variety. In fact, in line with recent studies, the number of true species in the Citrus genus is only 3–4 and all the traditional species should be considered as botanical groups or hybrids derived from these 3–4 species.
‘Pompia’ is a type of Citrus of unknown origin that has been cultivated in Sardinia for centuries. Manca dell’Arca cited it at the end of the 18th century (1780); historical documents dating back to 1760 (Cherchi Paba, 1974–1977) state that it was well known and cultivated in Milis (Oristano, central-west Sardinia). Nowadays Pompia is present all over the island, but is primarily cultivated in central-east Sardinia where there are also some specialized orchards.
Pompia trees bear large, oblate fruit that can weigh up to 500–600 g (Figure 1). The rind colour changes gradually from a deep green to yellow at full maturity (Figure 2). The albedo is very thick. The juice is similar to lemon juice; because the essential oils are different from those of any other citrus fruit, the juice has a distinctive flavour. The juice is used in place of lemon juice to dress special dishes and salads. However, peeled and sliced fruit are especially prized by Sardinians as the primary ingredient in a special Sardinian dessert cake. Recently, this speciality has gained popularity with tourists and is now offered as a typical local product. The peel is also candied and used as an ingredient in fruit-cake and candy. In addition, the rind can be immersed in ethanol thus extracting the essential oils which are then used to produce a typical liquor.
The growing popularityof Pompia has led to this study of the main physiological, biometric and chemical characteristics of Pompia fruit over the five-month ripening period, from the end of October, when fruit are still completely green, to March– April when the peel turns to a deep yellow colour.
Materials and methods
The experiment was carried out in a commercial orchard located in Siniscola (eastern Sardinia). Shoots were grafted onto sour orange trees and planted at 6m x 5m in 1990. During 2000–2001, the most important highly heritable characters, namely, the shape and habit of the tree, the colour of shoot tips, characteristics of the leaves, flowers and fruits and seeds in addition to the phenological phase were recorded according to the Ministero dell’Agricoltura e delle Foreste and the Società Orticola Italiana (1985) descriptors list for lemon.
Six trees located in different parts of the orchard were chosen for periodic harvesting of fruit over the growing period for chemical, morphological and physiological assessments. At each picking date, a sample of 48 fruits of medium size and free of visible defects were collected. In particular, from each tree two fruits from every cardinal point at 1–1.5 m from the ground were chosen. Pomological assessments included: weight, volume, longitudinal diameter (LG), maximum transversal diameter (HTD) and minimum transversal diameter (LTD). Colour of the peel was determined either subjectively—by estimating the percentage of yellow-coloured surface—or objectively by taking measurements on a total of 48 fruits on three equatorial points with a Minolta CR-300 chromameter (Minolta, Osaka, Japan) according to the Commission Internationale de l’Eclarage (CIE) (L*, a*, b*) colour scale. By the means of a 2-mm diameter needle applied to an Effegì penetrometer, the resistance of the peel to puncture was determined, while deformation, as mm reduction of the transversal diameter of the fruit, was measured by applying a 3-kg weight on the transversal diameter for 10 s. Peel thickness (mm) was measured at four sites 90 degrees distant one to another on the equatorial section. The colour of the flesh was assessed in two points of the equatorial section using the same chromatometer and scale as for peel. For each fruit the number of segments and seeds were also counted. The fruits were juiced and the percentage of juice was calculated. Chemical analysis of the juice included: pH, tritatable acidity (TA), total soluble solids (TSS) and vitamin C. TA was determined by titrating 10 mL of juice against 0.1 N NaOH to end point (8.2) and expressed as % of citric acid; TSS, as °Brix, were determined by the means of a digital refractometer (Atago, Tokyo, Japan), whilst vitamin C was evaluated by the indophenol titration method, as described by Ting and Rouseff (1986).
Respiration activity, ethylene production rate and endogenous gas composition were assessed on a total of 12 fruits by choosing at random 3 fruits from each cardinal point out of the 12 collected. Specifically, individual fruits were closed in 3-litre jars for 3 hours. The lid of each jar was fitted with two rubber septa. Through the two septa, two needles connected with a combined CO2/O2 analyzer (Combi Check 9800-1, PBI-Dansensor A/S, Denmarck) were inserted. A pump was used to continually circulate air through the first needle from the headspace. The air, after being analyzed, was re-injected through the second needle into the jar. The analysis was continued until the values of CO2 and O2 were constant; this took about 2 min. One hour after sealing a 2-mL sample from each jar was withdrawn with a syringe for ethylene measurement that was determined by gas chromatography (D’Aquino et al. 1996). The fruit was then removed from the jars and left for six hours at 20°C; after that the fruits were used to determine endogenous atmosphere. Specifically, a 4-mL sample of air was withdrawn from each fruit by inserting a syringe needle into the stem scar through the calyx cavity, with the fruit submerged in water. From this sample 1 mL gas was used for ethylene determination and another 1-mL sample for CO2 and O2 assessment. Ethylene, CO2 and O2 were determined by gas chromatography using two different Varian 3300 gas chromatographs fitted with a FID and a TCD, respectively. Operating conditions and columns were the same as reported by D’Aquino et al. (1996).
Data were subjected to analysis of variance and the LSD was used for means separation using the Statgraphics software (Manugistics, version 5 professional, 2000).
Table 1 shows the principal characteristics of Pompia trees. The Pompia tree is vigorous, has an upright habit that becomes large if not pruned, with sparse branches. The leaves are large, simple, ovate, with entire margin, while petiole wings are absent. The colour of the tips of the shoots is purple. The flowers are large, mainly grouped in inflorescences, less frequently isolated and the colour of the petal is purple-white. In Table 2 the main features of fruit are reported. The fruit shape is oblate, with a smooth/rough/bumpy surface, depending on the period of flowering and the position of the fruit; generally fruit borne inside the canopy are smooth while those more exposed to the light are rough or bumpy. The peel is very rich in essential oils. The essential oils resemble those of lemon but with a distinctive delicate and fragrant aroma. The peel is moderately hard and difficult to peel when fruit are green, but can be removed fairly easily when fruit are ripe. The membranes of the segments are quite delicate and the adherence of the segments to each other is moderate. Fruit axis is hollow and the cross section of the fruit is irregular. The segments present large vesicles, which are quite juicy. Fruits are strongly attached to the tree.
Data reported in Table 3 related to the main biometric parameters are the average of the records collected from October to March, since slight changes occurred during the sampling period. The fruit is very large with the average weight being 320 g; however, there is a wide variability in fruit weight with some fruits weighing more than 500 g and others less than 200 g. Fruit size depends heavily on yield; when trees have a low yield the fruit is larger, sometimes reaching 700 g. The rind is very thick with the average value being 1.5 cm; values lower than 1 cm were never registered. The number of segments is variable and ranges from 11 to 13. The average number of seeds was 10, but a great variability was observed with some fruit having 4–5 seeds and others having more than 15. The seeds are polyembryonic (3–4 embryos per seed); this supports the hypothesis that Pompia is a hybrid since true citron varieties are monoembryonic.
At the beginning of October fruit were still completely green; at the first inspection 17% of the peel was yellow whereas at the 4th inspection the peel was completely yellow (Table 4). From 31 October to 15 November objective analysis of the peel indicated a sharp increase in the L* value; after 15 November only little variations occurred. However, a* values increased progressively over the sampling period with the final value being slightly higher than 0. The b* component was always positive; it increased sharply during the first three inspections and was fairly constant at the subsequent sampling periods (Figure 3A). Objective measurements show that the evolution of the colour of the peel was completed by December with the final values of b* and a* indicating a strong yellow component with a very weak red constituent. However, the flesh underwent little change with the a* component remaining negative and the b* component remaining positive, but considerably less than the b* values of the peel (Figure 3B).
As maturation progressed specific weight decreased progressively (Table 4). A significant decrease was also detected for juice percentage which diminished from 20.8% on 30 October to 15.5% on 12 March (Table 4). Also puncture resistance decreased significantly during the sampling period, while deformation increased significantly only from January on, indicating no important relationship with resistance to puncture (Table 4).
Titratable acidity, pH, total soluble solids and vitamin C, which are reported in Table 5, were typical of lemons. Over the sampling period, pH increased progressively and ranged between 2.35 and 2.69. Titratable acidity accounted for about 90% of the total soluble solids. Both parameters underwent a slight reduction over the sampling period. However, vitamin C content showed the most dramatic change during ripening with a reduction of 33.6% from the first sampling (33.97 mg/100 mL of juice) to the last one. Chemical parameters are very similar to those of lemons grown in similar environmental conditions (D’Aquino et al. 1998; D’Aquino et al. 2002).
Endogenous CO2 increased progressively over the ripening period; at the first sampling CO2 was 1.62 kPa, while at the last sampling it was 2.83 kPa (Figure 4A). Conversely endogenous O2 decreased from 18.9 to 17.4 kPa (Figure 2A). Although a large variability among the single fruits was observed at each inspection, endogenous C2H4 showed an increasing trend but with a final value of 0.13 µL.L -1 (Figure 2A). Respiration activity showed a decreasing trend with a reduction of about 35% at the last assessment with respect to the first one (Figure 4B).
As maturation progressed, C2H4 production increased slightly; however, as for endogenous C2H4, a very large variability was observed amongst the single fruits and differences, from the statistical point of view, were inconsistent (Figure 2A). The overall physiological behaviour of Pompia fruit resembles that of lemon. In different experiments with lemon cultivars, we have noted similar trends and intensities of respiratory activity (data not shown). The slight but gradual reduction in respiration activity intensity observed, which was counteracted by a concomitant raise of endogenous CO2 and decrease of endogenous O2, is typical of citrus fruit (Agabbio et al. 2000; Eaks 1970). In addition to the physiological reduction of metabolic activity due to ageing, this can also be explained by a concomitant reduction of permeability to rind gases probably from the continuous accumulation of wax on the rind that occurs with fruit maturation (El-Otmani and Coggins 1985), which leads to a rise of endogenous CO2 and a consequent reduction of respiration activity.
Although it is not certain if Pompia is really a hybrid between lemon and citron, different features seem to confirm some genetic similarities (Chessa et al. 1994). The rind at full maturity is completely yellow and the number of segments is variable among the fruits of the same tree, as occurs in both lemon and citron (Hodgson 1967). The average weight of the fruit is more similar to that of citron than lemon; however, the oblate shape resembles a grapefruit.
We believe that the increasing importance that local traditional products are gaining in Europe justify more attention to this fruit whose use in the local Sardinian food industry can make an important contribution to the local economy. We recommend that further studies for better characterization of the nutritional values of the various components of the fruit (flavedo, albedo, essential oils) be done along with a definitive study of the genetic origin of Pompia based on molecular methods. We believe this can increase the interest in Pompia from either the commercial or the scientific point of view.
The authors wish to thank Mr Domenico Mura for his valuable technical assistance. This study was funded by the Ministero delle Politiche Agricole e Forestali (MiPAF) within the framework of ‘Liste di orientamento varietale degli agrumi’. The authors contributed equally to this study.
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