Phenolic Composition of Malus baccata Fruit

Abstract

Natural populations of Malus baccata (L.) Borkh. occur in eastern Siberia and Primorsky Krai in addition to the Mongolian People’s Republic and northern China. The plant grows in river valleys, on islands, and on steppes and shrubsteppes. It is an exceptionally frost-resistant representative of the genus. The trees can withstand temperatures as low as –50°C. Polyphenolic compounds, primarily flavonoids contained in apples, are known to be involved in the regulation of human physiological functions. The qualitative and quantitative compositions of phenolic compounds (PC) in various species and varieties of apples and even a single variety growing under different ecological conditions vary regularly [1–4]. The spectrum of polyphenols is typically species-specific for apples and consists of 16 compounds, i.e., quercetin and its glycosides, phloridzin, phloretin, procyanidins, catechin, epicatechin, and several cyanidin glycosides and chlorogenic, cinnamic, and caffeic acids [4–7]. The literature contains much information on the PC contents in cultivated varieties grown in Europe, America, and China [2–7]. The biochemical composition of fruit from M. baccata, which is unique for its ability to fruit under extreme conditions, is practically unstudied. Therefore, the goal of the present work was to analyze the qualitative and quantitative compositions of PC in fruit from two forms of M. baccata. The dry weight (~30% of the raw mass) of M. baccata fruit was slightly greater than that of cultivated varieties (~24%). This seemed logical if the comparatively small (0.75 cm in diameter) size of M. baccata fruit was considered [8, 9]. The ratio of sugar content to acid number was reported for dessert varieties (25–30), for Antonovka (~10) [10], and for the studied fruit (~1.5). The vitamin C content in M. baccata fruit was comparable (6.5–7.5 mg%) to that in cultivated varieties grown in European Russia (from 5 to 50 mg%). We obtained samples of two ecological forms of M. baccata, tall and dwarf, grown under identical conditions in order to estimate intraspecies variations in the fruit chemical composition [9, 11]. The principal studied parameters were the dry fruit mass, sugar content, acidity, and vitamin C content and did not differ significantly for both forms (Table 1). The PC contents (calculated as quercetin) in both ecological forms differed considerably and were 1.01 0.5 mg/g of the fruit raw mass for tall and 1.56 0.12 mg/g for dwarf. The PC content for cultivated dessert varieties varied from 0.3 to 5.0 mg/g of raw mass [2, 3]. This was comparable with their contents in both forms of the studied fruit. Table 2 shows that the fruit PC included 10 principal compounds, i.e., procyanidin B1, anthocyans, phloretin, phloridzin, (+)-catechin, (–)-epicatechin, and quercetin glycosides and chlorogenic, cinnamic, and caffeic acids. The major constituents were procyanidin B1 (from 324.5 28.3 to 636.4 52.8 g/g of raw mass), quercetin glycosides (from 256.5 23.1 to 465.7 34.5 mg/g of raw mass), and phloridzin (331.3 25.3 g/g of raw mass). Cultivated varieties also had high contents of these flavonoids [2, 3, 5, 7]. A unique feature of the chemical composition of M. baccata fruit was the ratio of flavan-3-ols. The contents of (+)-catechin and (–)-epicatechin in fruit of both cultivated and wild representatives were high and comparable with the contents of their dimeric derivatives, i.e., procyanidins [2, 3, 5, 7, 12]. They totaled about 80–90% of the total polyphenol contents. This flavonoid class was represented in M. baccata fruit mainly by procyanidin B1 and reached only 30–40%. It is still unclear why procyanidin B2 was absent. First, it could be caused by the formation of larger unhydrolyzed oligomers, e.g., procyanidin B5 and tannins. Second, the biosynthesis could follow an anthocyan formation pathway from the same precursors as for the flavan-3-ols, i.e., leucoanthocyanidins [6]. This strategy could be more favorable for plant survival because anthocyan glycosides have greater cryoprotective potential. Many researchers relate their content to the frost-resistance of the trees [13–15].