Introduction

Turkey is located in the eastern Mediterranean and is seen as a bridge between Europe and Asia. The landscape of the country possesses natural food resources including wild edible fruits due to its vast geographical diversity in terms of topography and climate. In particular, the country consists of three different bio-geographic regions, each with its own endemic wild edible fruit species and natural ecosystems (Ercisli et al. 2008a, 2008b; Sengul et al. 2011).

Among the wild edible fruits, the strawberry trees, due to their commercial and nutrient values, are of great significance for rural community. Strawberry tree called Arbutus species (Arbutus unedo and Arbutus andrachne) grows naturally in the forests consisting of red pine, oak, wild olive, myrtle trees and shrubs in the Mediterranean, Aegean, Marmara and Black Sea regions of Turkey (Serce et al. 2010).

Compared to Arbutus andrachne, Arbutus unedo, with its evergreen shrub structure bearing orange colored fruit in autumn, is more attractive and more promising due to its better fruit characteristics. Typically seen as a fruit species native to Mediterranean countries such as Algeria, Morocco, Tunisia, Turkey, Syria, Greece, Croatia, France, Portugal and Spain, Arbutus unedo is not only used for domestic consumption but also for medicinal purposes (Ruiz-Rodriguez et al. 2011).

Morphological and biochemical markers have been widely used in breeding studies and in the investigations into diversity of species and the relationship between genotypes, cultivars, cultivated species and their wild parents. Conventional plant breeding depends on morphological (phenotypic) selection in the first step (Ercisli et al. 2012). More recently, biochemical content, in particular, bioactive content (phytochemicals) of fruits has been widely searched in different laboratories in terms of their human health benefits. The breeders are now searching to find genotypes that have higher bioactive content in order to use them in cross breeding activities for the purpose of obtaining new cultivars that possess high nutrient value for health (Vidrih et al. 2013).

Therefore, understanding the chemical composition of foods by focusing particularly on the phytochemical fingerprinting has become central to research in order to address the increasing demand for nutraceuticals in foods. Directly involved in process of growth, maturation, and senescence, organic acids are of great significance in fruits as intermediates in the metabolic processes of the fruit. (Poyrazoglu et al. 2002). Sugars are very important parameters for market quality of fruits not only because consumers would like to buy more sweety fruits but also because sucrose and fructose have beneficial effects on gastrointestinal health (Canan et al. 2016).

In this study, some morphological and biochemical characteristics were identified for the 15 genotypes belonging to wild strawberry tree (Arbutus unedo).

Material and Methods

Plant Material

A total of 15 strawberry tree genotypes were used and fruits were harvested in June 2016 and 2017 from wild grown Arbutus unedo plants grown naturally in the forests of Mugla province in Aegean region of Turkey when they reached full maturity stage. The fruit samples were placed into sample containers and transported to laboratory for analysis. The fruit samples were collected randomly from all parts of the branches of the tree and they were stored at −20 C° until the analysis.

Pomological Characteristics

A total of 30 fruits were sampled for traits such as fruit weight (g) and color. The fruit color was measured using a portable Minolta Chroma Meter (Minolta, Model CR-400), which provided CIE L*, a* and b* values (McGuire 1992).

Extraction and Identification of Organic Acids

For organic acid analysis, the method by Bevilacqua and Califano (1989) was used.

Identification of Total Antioxidant Activity

In order to determine antioxidant capacity, TEAC assay was used. ABTS was dissolved in acetate buffer and prepared with potassium persulfate, as described by Rice-Evans et al. (1995).

Extraction and Identification of Ascorbic Acid (Vitamin C)

Ascorbic acid content was determined following the modified HPLC (isocratic program) (Agilent 1100 series HPLC G 1322 A, Germany) analytical procedure outlined by Cemeroglu (2007).

Total Phenol Identification

The total phenolic content of strawberry tree fruits was determined spectrophotometrically at 765 nm following the Folin-Ciocalteu method as described by Singleton et al. (1999). A standard calibration curve was plotted using gallic acid (Merck, Germany) in the concentration range of 1–500 mg/L. The results were expressed as milligrams of gallic acid equivalents per 100 g of fruit.

Total Dietary Fibre Identification

Dietary fibres were determined according to the method of AOAC (1999).

Total Fat Identification

Total fat were determined according to the method of AOAC (1999).

Statistical Analysis

All data were analyzed using SPSS software and procedures. Analysis of variance tables were constructed using the Least Significant Difference (LSD) method at p < 0.05.

Results and Discussion

There were no statistical differences between years, so the data were pooled. Table 1 shows fruit weights, the number of fruit per cluster and color values of genotypes. There were statistically significant differences (p < 0.05) among genotypes for all pomological parameters (Table 1). Fruit weight came out as between 4.67 and 7.88 g and the number of fruits per cluster as between 3.77 and 8.60 (Table 1). L, a and b color indices ranged from 39.10 to 48.72, 31.02 to 45.22 and 19.81 to 30.68, respectively (Table 1). Islam and Pehlivan (2016) studied on 40 Arbutus unedo genotypes from Marmara region in Turkey and they reported average fruit weight, the number of fruit per cluster and L, a and b values of 40 genotypes as 5.00 g, 5.21, 47.26, 37.07 and 26.89, respectively. Karadeniz et al. (1996) reported fruit weight as between 3.38 and 9.74 g in Arbutus unedo genotypes grown in Black sea region. The differences could be due to genotypes and environmental factors.

Table 1 Some pomological characteristics of Arbutus unedo genotypes (Average of 2016–2017 years)
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Five organic acids were determined for all Arbutus unedo fruits and malic acid was dominant acid (0.94–2.76%) followed by citric acid (0.31–0.76%). The rate of Oxalic acid was identified as between 0.13 and 0.30%, Succinic acid as between 0.15 and 0.38% and Fumaric acid as between 0.11 and 0.34% (Table 2). There were statistically significant differences (p < 0.05) among organic acids except oxalic and succinic acid (Table 2). Malic and citric acid has been reported as the main organic acid in Arbutus (Vidrih et al. 2013) and this compound occurs naturally in all fruits and in many vegetables; it contributes to the pleasantly sour taste of fruits, and it is used as a food additive.

Table 2 Organic acid content of Arbutus unedo genotypes (Average of 2016–2017 years) (g/100 g FW)
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The genotypes significantly differed from each other in terms of fructose and glucose content but there was no difference in sucrose level (p < 0.05) (Table 3). Fructose turned out to be main sugar for all Arbutus unedo genotypes indicating the range of 7.21 g/100 g FW and 10.68 g/100 g FW and glucose level was determined as 4.07 g/100 g FW and 6.81 g/100 g FW and sucrose level as 0.41 g/100 g and 1.37 g/100 g (Table 3). Ruiz-Rodríguez et al. (2011) reported fructose level as 12.34 g/100 g, glucose level as 6.50 g/100 g and sucrose level as 0.34 g/100 g in Arbutus unedo fruits. Contents of fructose, glucose and sucrose in our study are in accordance with the results of Ruiz-Rodríguez et al. (2011).

Table 3 Sugar content of Arbutus unedo genotypes (Average of 2016–2017 years) (g/100 g FW)
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There were statistically significant differences (p < 0.05) among the examined strawberry tree genotypes in terms of total phenolic content, total dietary fibre, vitamin C and total antioxidant capacities (Table 4). Total phenolic content, total dietary fibre, vitamin C and total antioxidant capacity were between 483 and 627 mg GAE/100 g FW, 9.74 and 13.28 g/100 g FW, 46 and 62 mg/100 g FW and 18.07 and 33.41 μmol TE/g FW, respectively. There were no statistical differences among genotypes for total fat content (Table 4). Vidrih et al. (2013) reported average 590 mg total phenols in 100 g strawberry tree fruits; however, Ruiz-Rodríguez et al. (2011) determined total phenol levels ranging from 951 to 1973 mg/100 g FW, which is more than those of our study. Vitamin C is quite variable in Arbutus unedo fruits. The average vitamin C levels in our results were higher than those of Spanish strawberry tree fruits given as 6.03 mg per 100 g (Pallauf et al. 2008), but lower than those of Turkish strawberry tree fruits, which ranged from 98.0 to 280.0 mg 100 g (Celikel et al. 2008). Ruiz-Rodríguez et al. (2011) and Pallauf et al. (2008) reported that Arbutus unedo fruits were rich in antioxidant capacity. Antioxidant capacity of strawberry trees ranged from 18.07 to 33.41 μmol TE/g FW indicating a great diversity which increases its vitality of antioxidant rich genotypes for future strawberry tree breeding.

Table 4 Total phenolic, total dietary fibre, total fat, antioxidant capacity and vitamin C content of Arbutus unedo genotypes (Average of 2016–2017 years)
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Conclusion

The results point out that using wild grown strawberry trees for human nutrition as a good substitute for other edible fruit species may increase quality of human diet thereby improving natural vitamin intake without any side effect. Some genotypes investigated in this study displayed high antioxidant activity thanks to their total polyphenol content. The values found in this study may be helpful for nutritionists as well as berry growers and breeders who can promote the cultivation of species and new cultivars with higher phenolic content and antioxidant activity.