Characteristics of Fruit (Prunus domestica L.) Skin: Structure and Antioxidant Content

Figures & data

TABLE 1 Fruit characteristics of Prunus domestica cultivars^[1,2]

Fruit features	Bluefre	Sweet Common Prune	President
Time of fruit ripening	Mid-September	Mid-September—End of October	Beginning of October—End of October
Size of fruit	Large	Middle	Large and very large
Shape of fruit	Spherical-elliptical	Elliptical and ovoid	Oval
Color of skin	Dark blue, with abundant, bright bloom	Brown-blue to dark blue, with dense, whitish bloom	Burgundy-violet to dark blue with violet-blue bloom
The pit separates from the flesh	Quite difficult	Very easy	Easy

FIGURE 1 SEM. Surface of the Prunus fruits; A, D, G, J—“Bluefre;” B, E, H, K—“Sweet Common Prune;” C, F, I, L—“President.” Visible microwrinkles (A, C) and undulations (B) centered around the stomata. D–F—visible wax platelets (E; arrow heads) and stomata (D–F; arrows) surrounded by wrinkles. G–L—visible crystalline wax in the form of vertical plates (arrows; G, H, J, K) and microgranules (I–L), and numerous microcracks in the wax layer (H, K), asterisks—wax microgranules in high density.

FIGURE 2 SEM. Surface of the Prunus fruits with microcracks and stomata; A–C—“Bluefre;” D–F—“Sweet Common Prune;” G–I—“President.” F, L—LM. Note stomata with wax microgranules visible on the cell surface and in the interior of stomata (A, D, G), and stomata with microcracks (arrows; B, C, E, H, I). F—A microcrack (arrows) in the skin of “Sweet Common Prune.”

TABLE 2 Fruit skin characteristics and antioxidant contents of Prunus fruits

Parameters	Bluefre	Sweet Common Prune	President
Number of stomata per mm^2 of the fruit surface area	9 ± 0.9^a	8 ± 2.1^a	6 ± 2.1^b
Length of the stomatal porus (µm)	47.2 ± 4.5^ac	50.9 ± 2.7^a	45.0 ± 6,1^bc
Thickness of the cuticle (µm)	4.6 ± 0.9^a	4.7 ± 1.0^a	4.9 ± 0.9^b
Height of the epidermis cells (µm)	24.2 ± 6.4^a	22.1 ± 5.2^b	21.5 ± 9.9^b
Number of hypodermis layers	3-5 ± 1^a	3-5 ± 1^a	3-5 ± 1^a
Thickness of the hypodermis cell walls (µm)	7.8 ± 1.9^a	8.4 ± 1.7^a	7.6 ± 2.0^a
Thickness of the hypodermis layer (µm)	110.1 ± 25.3^a	88.3 ± 24.3^b	75.9 ± 22.5^b
Total thickness of skin (µm)	129.3 ± 25.4^a	115.6 ± 36.8^b	108.5 ± 27.7^b
Dry mass (% in 100 g)	16.3 ± 1.2^a	18.4 ± 2.1^a	19.4 ± 1.0^a
Total phenol content (mg·100 g^−1 dry weight)	990 ± 0.3^a	130 ± 0.0^b	1010 ± 0.3^a
Flavonoid content (mg·100 g^−1 dry weight)	40 ± 0.0^a	17 ± 0.0^b	310 ± 0.1^c

Values are mean ± standard deviation (SD). The different letters within a row mean statistically differences (p < 0.05).

FIGURE 3 The cuticle on the “President” fruit surface. A—visible a brightly shining layer of the cuticle observed under LM. B–D—The cuticle observed under TEM. B, C—Note two layers of the cuticle: brighter—cuticle proper and darker—cuticular layer and the border of the occurrence of the epicuticular wax layer (arrow heads). D—Visible cuticle penetrating between the cells of the epidermis (asterisk). c: cuticle, cp: cuticle proper, cl: cuticular layer, cw: cell wall, e: epidermis, n: nucleus, v: vacuole.

FIGURE 4 LM. Fragments of sections through the fruit surface layer; A—‘”Bluefre,” B—“Sweet Common Prune,” C—“President.” Visible chloroplasts in many hypodermal and parenchymal cells (arrows). e: epidermis, h: hypodermis, p: parenchyma.

FIGURE 5 TEM. Skin and parenchymal cells of “President” fruits; A—epidermis and hypodermis cells with nuclei, plastids with starch grains and numerous vacuoles with fibrous and granular deposits. B, C—fragments of epidermis cells with plastids containing starch grains, mitochondria, and vacuoles with membranes forming compartments of various shapes (asterisks). D, E—chloroplasts (arrows) with parenchymal cells. c: cuticle, cw: cell wall, n: nucleus, p: plastid with starch grains, m: mitochondrion, v: vacuole.

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