(miniature drupes), each from a separate floret within an inflorescence. Improving the inherent structural
weakness of the raspberry has become a challenge for research workers [39,40].
- Strawberry (Fragaria virginiana F. chiloensis)
The strawberry is an accessory fruit, one in which the conspicuous fleshy part is composed of tissues ex-
ternal to the pistil. (The Annonas, soursop, sweetsop, and cherimoya are tropical examples of accessory
fruits.) The succulent flesh of the strawberry is receptacle tissue. The “seeds” embedded in its exterior
surface are achenes and thus true fruits.
- Pineapple (Ananas comosus)
The pineapple is a multiple fruit, one formed from many pistils of an inflorescence. The pineapple fruit
develops from separate lavender-colored flowers distributed around the length of the central axis of the
inflorescence. The entire flowers become incorporated into the fruit, much of the flesh being formed from
the fleshy bracts subtending each flower. Individual varieties are self-infertile; hence pineapples grown
in monocultures of a single variety are always seedless. However, in areas such as the Caribbean, where
small plots of various varieties are common, it is usual to have pineapples with occasional small black
seeds.
IV. PHYSIOLOGICAL DEVELOPMENT
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 spe-
cific 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 vari-
ations on this pattern as there are different types of fruit, but the sigmoidal mode is usually discernible.
The orange, apple, and apricot are discussed next 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 dif-
ference between citrus and deciduous fruits. The latter mustbe 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 are shown in Figure 3.
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. In growing areas such as Florida, where stage I coincides with the strongest winds of
the year, just brushing against an adjacent leaf causes major “windscars” on the mature fruit. This prob-
lem is exacerbated in areas such as Brazil and Florida, where rains in the postbloom period facilitate su-
perficial infection of such windscars by waterborne spores of the melanose fungus (Diaporthe citri). Al-
though 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 [38].
Stage II is the period of cell (and hence fruit) enlargement. The fruit expands rapidly, as does CO 2
output per fruit, although CO 2 evolution per unit weight (the usual way of expressing respiration) declines
sharply (Figure 3). 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 devel-
opment follow single sigmoidal patterns (four-parameter logistic function, R^2 0.99) [41]. 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
150 GRIERSON