The structure of the fruit peel in two varieties of Malus domestica Borkh. (Rosaceae) before and after storage

Abstract

The structure of fruit peel of two apple varieties 'Szampion' and 'Jonagold' was investigated using light microscopy as well as scanning and transmission electron microscopy. The samples were taken immediately after harvest and after 6-month controlled atmosphere storage. The Szampion and Jonagold fruit differed in terms of the surface type, number of lenticels, thickness of the cuticular epithelium, height of epidermal cells and thickness of the hypodermis as well as the amount of crystalline wax and the number of microcracks formed on the fruit surface. The 6-month storage resulted in fruit weight loss, increased numbers and depth of microcracks, thickening of the amorphous wax layer and enhanced production of platelet forms of crystalline wax, which filled the microcracks abundantly. Compared with Jonagold, the Szampion fruit exhibited a fewer lenticels, a bigger number of microcracks, smaller amounts of crystalline wax and more substantial weight loss. The apple varieties studied had a reticulate-lamellate cuticle, and at harvest, the epidermal and hypodermal cells contained numerous amyloplasts filled with starch grains, which were not found after the storage period. Additionally, after storage, the cell protoplasts in the apple peel displayed a disorganised structure, and their vacuoles contained fragments of cell membranes, intravacuolar precipitates and deposits, and spherical bodies. The results may facilitate better understanding of changes occurring in fruits of Szampion and Jonagold during storage and help choose the best storage conditions to reduce loss of weight and prevent impairment of fruit quality.

Fig. 1

LM. a–d Fragments of the cross sections through the fruit peel of Szampion (a, b) and Jonagold (c, d) after harvest (a, c) and after storage (b, d). Note microcracks (arrowheads) in the cuticle (Cu) and amyloplasts (arrows) in hypodermal cells. Thickened cuticle filling the cell lumen—visible on the anticlinal and internal periclinal epidermal cell walls. Stars epidermis, H hypodermis

Fig. 2

SEM. a–f Different types of lenticels on the apple surface: a, e, f Szampion; b, d Jonagold. Note the mycelial hyphae (arrowheads). g, h Fragments of the cross sections through the fruit peel of Szampion (g) and Jonagold (h) after harvest. i, j Fragments of the cross sections through the fruit peel of Szampion (i) and Jonagold (j) after storage. Arrows an amorphous wax film on the cuticular layer, Cu cuticle, E epidermis, H hypodermis

Fig. 3

SEM. a–c Fragments of the epidermis surface of Szampion fruit after harvest. Note numerous microcracks (arrows) and horizontal wax platelets between the microcracks (c). d–g Fragments of the epidermis surface of Jonagold fruit after harvest. Note vertical wax platelets (d–f) and horizontal wax platelets (g). Arrowheads granular epicuticular wax

Fig. 4

SEM. a, b Fragments of the epidermis surface of Szampion fruit with microcracks and without vertical platelets of crystalline wax after storage. c, d Fragments of the epidermis surface of Jonagold fruit with microcracks and vertical and horizontal platelets of crystalline wax after storage

Fig. 5

SEM. a, b Microcracks without vertical platelets of the epicuticular wax on the surface of Szampion fruit after storage. c, d Microcracks with vertical platelets of the epicuticular wax on the surface of Jonagold fruit after storage

Fig. 6

TEM. a, b Ultrastructure of the Szampion (a) and Jonagold (b) fruit cuticle after harvest. c Epidermal cell of Szampion fruit after harvest. Note the intravacuolar precipitates and spherical bodies (Sb). d Fragments of the hypodermal cell of Jonagold fruit with chloroplast (Ch) and intravacuolar deposits (D) after harvest. e, f Amyloplasts (A) with starch grains (S) in the hypodermal cells of Szampion (e) and Jonagold (f) fruit after harvest; CP cuticle proper, ECL external cuticular layer, ICL internal cuticular layer, V vacuole, M mitochondrion, P plastoglobules

Fig. 7

TEM. a, b Fragments of the Szampion (a) and Jonagold (b) fruit cuticles with microcracks after storage. c, d Fragments of the epidermal cells of Jonagold (c) and Szampion (d) fruit after storage; V vacuoles, M mitochondrion, A amyloplasts, Sb spherical bodies, Mf myelin figure