Growth and Development of Horticultural Crops (1+1)

Physiological development of fruits

• As a general principle, fruit development in terms of weight and volume tends to be sigmoidal. A period of very rapid cell division, but very little increase in fruit size (stage I), is followed by a period of rapid increase in size as small, newly differentiated, dense cells develop vacuoles and assume their roles as specific tissues (stage II). In the final stage, as the fruit reaches physiological maturity, increase in size slows and may even stop, although biochemical changes may continue (stage III). There are about as many variations on this pattern as there are different types of fruit, but the sigmoidal mode is usually discernible. The orange, apple, and apricot are considered as typical examples of the development of citrus, pome, and drupe fruits.
A. Hesperidium, e.g., Orange (Citrus sinensis)

• The duration of growth and maturation varies sharply with variety. For early varieties such as Hamlin and navels, harvesting commonly starts 6 to 7 months after bloom. For the late Valencia variety, harvesting starts about 12 months after bloom. Harvesting can continue for a “tree storage” period lasting several months, during which late oranges have two crops on the tree at the same time. Herein lies a critical difference between citrus and deciduous fruits. The latter must be picked soon after maturation is complete or they will fall from the tree. Citrus fruits have no such sharply defined abscission period, something that is frustrating to would-be developers of mechanical harvesting equipment, but an enormous advantage in marketing the crop over a period of weeks or months in which the crop is “stored on the tree.”
• Stages of development
• Stage I lasts a month or less, during which cell division is extremely rapid but fruit enlargement is trivial. At this stage the cuticle has not yet developed, making the little fruitlets extremely vulnerable to superficial damage. Although most cell division takes place in this period, some cell division can continue in the peel until maturation, particularly with navel oranges, making such fruit very vulnerable to water damage. Stage II is the period of cell (and hence fruit) enlargement. The fruit expands rapidly, as does CO2 output per fruit, although CO2 evolution per unit weight (the usual way of expressing respiration) declines sharply. During this period, the juice sacs are enlarging and developing their distinctive solutes. Increases in whole fruit and pulp radii and whole fruit, pulp, rind, and albedo volume during fruit development follow single sigmoidal patterns. Such solutes are initially high in organic acids and low in sugars. As the orange matures, sugars increase steadily while acids decline. Legal maturity standards for citrus fruits are usual in major producing areas. In this, every district sets its standards according to what they do best. European citrus districts, South Australia, California, and other districts with Mediterranean-type climates (cool winter nights, bright days, and low rainfall) can rely almost entirely on external standards to sell their oranges. Florida, with its blossom-period winds and humid, subtropical climate, cannot compete on appearance and so relies principally on standards based on the high sugar content of its oranges. These maturity standards are based not only on sugar content but also on the ratio of total soluble solids (TSS, mainly sugars) to acids (titratable as citric acid), with a sliding scale throughout the season. At the beginning of the season, Florida oranges must have 8.0% TSS with a TSS/acid ratio of 10.5:1 (Figure 3). By the end of the season, this ratio may exceed 20:1, but with the proviso that (for fresh fruit sale) acid cannot be below 0.4% lest the oranges taste too insipid.
• Regardless of growing district, consistent gradients occur within a citrus fruit, particularly in terms of sugar content. The vascular system extends down the central axis of the fruit, reaching the blossom (stylar, distal) end first, them ramifies back up the carpels to the stem (calyx, proximal) end of the fruit. Apparently as a consequence of this distribution of photosynthates, sugars are higher in the blossom end. A very thorough study reported that the proximal halves of mature California Valencia oranges averaged 7.2 g of sugar per liter of juice as compared with 9.5 g/L for the distal (blossom, stylar) halves, a difference clearly discernible by taste. When sharing a grapefruit, canny citrus people give the stem-end half to their companion, retaining the blossom end half for themselves. In the jungles of southeast Asia where citrus first evolved, all are still green when mature. The extent to which the expected orange or yellow colors develop depends on the growing area having cold enough nights to stress the fruits. In subtropical areas such as Florida and Brazil, early varieties may mature while still green, necessitating postharvest removal of the green chlorophyll with ethylene.
B. Pome, e.g., Apple

• The typical growth curve of any main crop apple variety is only slightly sigmoidal. Very early varieties, such as Early Harvest, Yellow Transparent, and Melba, mature to acceptable eating quality before any deceleration of growth. Apples that mature this early are very frail and suitable only for local consumption. The longer it takes an apple variety to reach maturation, the more sigmoidal its growth curve. In general, the later an apple variety matures, the longer its potential marketing life. Initially, all cells of the apple are alive. Cell division in the epidermis ceases at the end of stage I. Marked elongation and flattening of the epidermal cells occur throughout stage II, during which period the epidermal cells extrude waxy, cutinous material.
• In fully mature late-season apples, the epidermal cells are separated, dead or dying, embedded in the continuous cuticle (a heterogeneous polymer of fatty acids overlaid with a layer of wax). The cuticle can continue to develop after harvest. During the stage II growth period, the epidermis is penetrated by stomata that tend to cork over at full maturity. Under the epidermis in some varieties is the periderm, a thin layer of cork cambium. If the epidermis is injured early in stage II growth, as by mechanical abrasion or frost, the periderm develops a protective layer of corky cells: biologically an excellent protection for the fruit but a “grade-lowering defect” for the packer and the consumer. Parenchyma tissue from the fused bases of the calyx, corolla, stamens, and receptacle constitutes the major part of the edible tissue of the mature fruit. Cell division having ceased at the end of stage I (usu-ally 3 to 5 weeks after anthesis), the considerable enlargement of the fruit comes from cell enlargement and their partial separation to form a considerable volume of air-filled intercellular spaces. Except for the petals (which abscise and fall after fruit set), all the original parts of the flower persist in the fully developed apple.
C. Drupe, e.g., Apricot (Prunus armeniaea)

• The growth curve of the apricot, indeed of all fleshy, succulent drupes, is exaggeratedly sigmoidal. Stage II growth is interrupted by “pit hardening,” in which the endocarp thickens and lignifies to form the hard, stony “pit” enclosing the seed. During this period, the fruit ceases to increase in size. Biochemical changes continue, but without cell enlargement. The apricot pit is smooth and, at maturity, quite free from the edible mesocarp tissue, being attached only at its proximal end by the persistent vascular system. In other drupes, the pit is seldom so separate, although in “freestone” peaches the deeply incised pit is nearly free from the edible mesocarp. In “clingstone” peaches, the endocarp and mesocarp interfaces adhere. Such characteristics are of commercial significance. “Freestone” varieties (cultivars) are preferred for the fresh fruit market. Because of their considerably firmer flesh (mesocarp), clingstone varieties are preferred by the canneries. (A machine neatly removes the clingstone pits). In the mango (Mangifera indica), the ultimate example of a “clingstone drupe,” the pit is covered profusely with tough fibrous “hairs” that usually extend into the edible flesh. The date (Phoenix dactylifera), the ultimate “oasis crop,” is a specialized drupe that develops so much sugar that its cells plasmolyze and ultimately die. Initially, most of the sugar is sucrose, but during maturation, all the sucrose converts to glucose and fructose. When fully mature, all that is still living is the embryo within the stony seed. After harvest, the date is therefore handled as a confection rather than as a fruit. Very occasionally, a drupe may have multiple seeds within the boney endocarp. One such is the highly poisonous, but attractive seeming, fruit of the manchineel (Hippomane mancinella), the so-called poisonous guava. Two familiar dessert nuts are the seeds of drupes. The almond (Prunus communis) and the pistachio (Pistacia vera) are drupes in which the mesocarp fails to develop any further after pit hardening, thus resulting in a growth curve that is definitely not sigmoidal.