* https://en.excelentesprecios.com/cherry-tree-varieties, (3 June 2022); ** https://m.blog.naver.com/PostView.naver?isHttpsRedirect=true&blogId=2jw67&logNo=90164242596 1 June 2022. Tm = trademark. During early fruit development, there is constant communication between the developing fruit and the developing seed. These signals can be hormones, such as auxin and gibberellins, produced in the endosperm or in the seed coat and are involved in the developmental synchronization of all these different structures ^{[41][42][43][44]}[41,42,43,44]. The disruption of this communication is one of the triggers of fruit drop—the loss of a significant proportion of their fruit before ripening. This loss, which is often referred to as ‘June Drop’ or ‘Cherry Run Off’, varies from year to year and, in some seasons, can result in a total loss of the crop.

3.2. Physiological Changes during Cherry Fruit Development

The developmental growth rate of the drupe can be described by a double sigmoid curve, where three well-marked stages can be identified (Figure 23). In the first phase (stage I), the ovary either aborts or starts all the subsequent morphological changes that will lead to the formation of the fruit ^{[45][46][47][48][49][50][51]}[45,46,47,48,49,50,51]. The traditional definition of phase I, beginning at anthesis, also includes the acceleration of the growth of the ovary approximately two weeks before anthesis [52]. The outer epidermis of the developing cherry fruit consists of a single row of cells that are covered externally by a cuticle, which is interrupted only where the stomata are present (see Section 4). During the pre-flowering period, the cells of the epidermis are already well-differentiated, with an elongated shape and no hairs, and increase their size slowly, mainly in the radial direction. The mitotic activity is low since the increase in the number of cells is very limited, while it is high during the first half of stage I (Figure 34). The second half of stage I is characterized by an increase in cell size, both radially and tangentially. Moreover, during the second half of stage I, the cuticle reaches its full thickness. No cell division is observed during stage II, which is characterized by slight tangential elongation of cells and additional cell wall thickening. Lastly, during the final phase (stage III), cells start to enlarge by as much as 100-fold, contributing to the final fruit size. After these three phases, a green immature fruit is formed, which has the dimensions of the mature fruit and starts the maturation process ^{[45][46][47][48][49][50][51]}(see Section 3.3) [45,46,47,48,49,50,51]. A more than two-fold tangential enlargement of the cells occurs, with a decrease in the radial diameter and cuticle thickness, in order to generate a large increase in the fruit surface area [53]. During the pre-bloom period, the fleshy mesocarp is composed of isodiametric parenchymatous cells, which elongate according to a gradient tangentially to slightly radial from the epidermis to the stony endocarp, respectively. The increase in tissue thickness, more than double, is due mostly to cell division. The first half of stage I is characterized by cell mitotic divisions, accomplishing an increase of about 20–30% in the number of cells from the epidermis to the stone, and no further divisions are observed after completion of this stage [53]. The cells increase in diameter more than four-fold, mostly during the second half of stage I. Moreover, differentiation of a hypodermal layer, five or six cells thick, occurs just under the epidermis, by considerable thickening of the cells. These cells become slightly greater in size than those of the epidermis and become tangentially elongated. On the other side, adjacent to the stone, a layer of small isodiametric cells, three or four cells wide, differentiates, during the second half of stage I. During stage II, some cell enlargement, less than 6%, occurs and intercellular spaces remain prominent. The cells just under the epidermis continue slowly to elongate tangentially, while limited cell division can be found in the thin layer of small cells adjacent to the stone, otherwise, there is none. Stage III is also called “the final swell” and during this period, four parts of the fleshy mesocarp can be recognized (from the epidermis to the stony endocarp):

• The hypodermal layer of collenchyma;
• The peripheral layer of thin-walled parenchyma, extending to a line just inside the ring of vascular bundles;
• The layer of radially elongated cells, extending from this line nearly to the stone;
• The thin layer of small cells adjacent to the stone.

The transition from one layer to the other is not precise, since there is a gradual transition from a tissue in which one cell type predominates to one in which another one is dominant. A gradient of cell elongation can be identified, from tangential in the hypodermal cells to radial in the ones close to the stone. The rapid cell enlargement brings a decrease in the thickness of the cell walls and the disappearance of the intercellular spaces that are now difficult to identify [53]. The ventral and the dorsal capillary bundles do not enter the stony endocarp, while the two bundles that supply the two ovules (in peach called “funicular bundles”) are at either side of the ventral suture and adjacent to the ovarian cavity. Pit bundles traversing the stony endocarp, of the type found in peach, are not present in cherry. During the pre-bloom period, the differentiation of a ring of vascular bundles, which extends through the fleshy mesocarp parallel to the central axis, occurs. However, the walls of xylem elements do not become sufficiently thickened to become conspicuous until about the full-bloom period. The number of bundles is fixed, so that as the fruit develops, the vasculature system extends progressively throughout the flesh, mostly by a transverse divergence of small bundles from the main ones. At maturity, a skeleton network of conductive tissue is present [53]. The innermost ovary wall comprises a single layer of cells, called the inner epidermis, and three or four layers of pericarp cells. These cells, which are going to become part of the stony endocarp, are smaller than the cells that are going to become the fleshy mesocarp, and this size difference persists to maturity. During the pre-bloom period, the inner epidermal cells elongate tangentially and divide transversally just before full bloom, to form with adjacent pericarp cells with a similar orientation—a “hoop” shape that surrounds the ovary cavity. The presence of this hoop is also reported in other species ^[54][55][54,55]. The group of cells that differentiate into the stony endocarp do not give rise to the fleshy mesocarp and vice versa. Stage I is characterized by a large increase in both the number and size of the inner layer cells (tangential elongation) and the outer layer cells (central radial elongation). No cell divisions are observed in the inner layer during the second half of stage I, while there is a greater increase in the mitotic divisions and cell size in the outer layer. Cell walls become progressively thicker but do not harden significantly until the end of this stage, which is when the pit cannot be sectioned without special treatment [53]. The thickening and hardening of the cell walls are the main events that occur during stage II, and no cell divisions are observed after the mid-point of this stage. Hardening does not occur uniformly in the stony endocarp, but it begins near the apical end at the hilum, and continues downwards toward the chalaza before finally including the whole endocarp. Although lignification occurs, cells are living and nuclei are abundant, even though the cellular lumen is frequently scarcely larger than the nucellus. At the end of stage II, the stony endocarp is made of 26–27 layers of very thick-walled lignified cells. This number of layers is the same as that present in the fleshy mesocarp, although the thickness is much less due to the very different destinies of the two tissues. During stage III, there is a slight increase in hardness and brittleness that is associated with a loss of moisture. It is possible to find occasional living cells with active nuclei [53].

3.3. Final Ripening Stages of Mature Cherry

Fruit ripening is a highly variable process with numerous differences when different plant species are compared; however ripening is often characterized by the accumulation of pigments, such as carotenoid sand anthocyanins (Figure 45) [56]. Despite these differences, fruit ripening has been classified into two distinct groups, climacteric and non-climacteric, which differ in their patterns of ethylene production and respiration at the onset of the ripening process ^[57][58][57,58]. Climacteric fruit, such as tomato, display a rise in respiration and a burst of ethylene modulating the differentiation of chloroplast to chromoplast and the accumulation of carotenoids ^[59][60][59,60]. In contrast, cherry fruit are non-climacteric and ripening is promoted by abscisic acid (ABA) [61]. No burst of ethylene and respiration has been observed in sweet cherry during fruit development and ripening. Moreover, the exogenous ethylene treatment has no significant effects on the fruit’s respiration rate or on its softening ^[62][63][62,63]. Ethylene concentrations remain low ^[64][65][64,65], although ethylene may affect the accumulation of anthocyanins [66]. However, it should be noted that the ethylene concentration in some sweet cherry cultivars has been shown to increase sharply before harvest [67], suggesting that it may play some role in fruit ripening during the maturation phase of development [68]. In sweet cherry, ABA levels increase before maturation and decrease near harvest ^[64][69][64,69]. Furthermore, in grapes, another non-climacteric fruit, the onset of ripening is ABA related; ABA modulates the changes in colour and the accumulation of sugars and has a role in fruit softening in the later stages of ripening ^[70][71][72][70,71,72]. Cherry ripening is associated with changes in colour, sugar, vitamin and organic acid content (Figure 45). In sweet cherries, ABA has been associated with the regulation of anthocyanin synthesis and the organoleptic properties (ratio of total soluble sugars to total acidity) ^[64][66][73][64,66,73]. The colour of the fruit is attributed to the accumulation of these water-soluble health-promoting anthocyanins (a class of flavonoids), which are responsible for the blue, purple and red colours ^[74][75][74,75]. In addition to ABA, cytokinins, jasmonic acid (JA), gibberellins (GAs), and auxins, all play roles in the ripening of non-climacteric fruit. Auxin (1-naphthaleneacetic acid (NAA)) treatment has been shown to enhance anthocyanin production in sweet cherry by upregulating key anthocyanin regulatory, biosynthetic and transport genes [76]. This study demonstrated that treatment with NAA alters ethylene production, induces ripening, and enhances anthocyanin production, probably through ABA metabolism [76]. Size, colour, firmness, sweetness and flavour intensity are considered the most critical attributes that drive consumer acceptance and, ultimately, the value of the crop to the producers ^{[77][78][79][80]}[77,78,79,80]. However, consumers from different countries and regions place different values on their requirements for a good cherry. For example, consumers in Norway prefer dark, large cherries ^[81][82][81,82]. Similar results were found in the UK [83] and in American markets [84]. These consumer requirements may differ from those of the growers and supermarkets. Growers want cherries that are resistant to cracking and can be left on the trees longer to ripen, are easy to harvest (long peduncles) and firm enough to resist damage during picking, processing, and transporting. Supermarkets want cherries with long shelf lives, even weeks once in store.