Handbook of Plant and Crop Physiology

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 devel-

oped apple.

C. Drupe, e.g., Apricot (Prunus armeniaea)

The growth curve of the apricot, indeed of all fleshy, succulent drupes, is exaggeratedly sigmoidal (Fig-

ure 5). 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. Bio-

chemical changes continue, but without cell enlargement. Morphological development in the peach

(which is essentially similar to apricot) has been reported in considerable detail [48].

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 “cling-

stone” 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 dactylif-

era), 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. Af-

ter 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 re-

sulting in a growth curve that is definitely not sigmoidal (Figure 5).

FRUIT DEVELOPMENT, MATURATION, AND RIPENING 153

Figure 5 Growth in diameter of fruits of Ne Plus almond, Royal apricot, and Kerman pistachio. (From
Ref. 36.)