Gibberellins were among the first GRs to become available in commercial quantities, thus greatly fa-
cilitating research showing that for a very wide range of plants, gibberellin could inhibit flower bud for-
mation and sometimes induce parthenocarpy if applied after flower bud initiation [11–17]. For details of
the histology of flower induction in apples, see Buban and Faust [18].
Research on the role of GRs in bud initiation has been facilitated by the finding that tracheal sap
is a convenient source of naturally occurring GRs [19]. Abscisic acid (ABA) is now known to be very
much involved, not only in flower bud formation but also in fruit development [20,21]. Growth regu-
lators control messenger RNA (mRNA), which generates necessary enzymes de novo for fruit devel-
opment following anthesis [15,22,23]. Much research has involved manipulation of bud differentiation
and fruit development using exogenous application of both natural and synthetic GRs [24–26]. But
flower bud initiation, and hence the entire cycle of flowering and fruiting, can be controlled solely by
intelligent manipulation of temperature and light. A 39-week cycle (repeated at 4-week intervals) has
been developed using dark and lighted cold rooms and greenhouse or nursery facilities to provide a
continuous supply of three varieties of container-grown apples for year-round harvesting, a remarkable
feat [27].
Normally, pollination is necessary for fruit set; however, there are notable exceptions. With the
buying public increasingly demanding seedless fruits of various kinds, parthenocarpy has become
highly prized for many types of fruits. This is certainly so for citrus fruits, for which pollination had
long been deemed unnecessary, even undesirable, as it increases the number of seeds in supposedly
“seedless” varieties. That was before the introduction of a number of human-made crosses such as the
tangelos (tangerine grapefruit). Tangelo varieties that are apparently fruitful when grown in small
trial plots were almost completely barren when planted in large multihectare blocks. Thus it was found
that for some hybrids, such as Orlando tangelo, pollination by some other variety was as necessary as
it is for apples and pears [2]. A remarkable example of parthenocarpy is the navel orange, which has a
small secondary fruitlet at the stylar end and which is always seedless. Fruit set of navel oranges, which
is often uneconomically light in Florida, is sharply affected by ambient temperatures prior to and dur-
ing fruit set [28]. Because seedless table grapes may fetch more than twice the price of seedy grapes,
parthenocarpy is highly valued [29]. In the popular Thompson Seedless variety, fruit set is dependent
on GRs involved in pollen tube development, even though the pollen tube does not reach, and hence
does not fertilize, the ovule [30].
III. MORPHOLOGICAL CATEGORIES OF FRUITS
Fruits have evolved so many diverse forms that Soule lists 46 different morphological fruit
types [1]. Although anatomical and taxonomic considerations cannot be ignored completely, only a
few general categories can be considered within this context of fruit physiology. Nondessert fruits are
discussed only insofar as is necessary to establish their place in the wide general category of fruits.
For a detailed histological treatment of the various tissues that can be involved in fruit development,
see Esau [31]. All fruits are the products of matured ovaries. Some, in addition, incorporate other
floral parts. This is particularly true for fruits derived from inferior ovaries (epigyny), that is, fruits
such as apple and pear, in which the other floral parts (stamens, petals, and sepals) are above the
ovary.
A. Achene
An achene is a hard, dry, fully matured simple ovary. Achenes are usually thought of as “seeds” (although
some may contain two seeds). A grain of wheat is an achene, each flower within a head of wheat (inflo-
rescence) having matured individually to form an achene. A grain of corn (maize, Zea mays) is an ach-
ene, corn on the cob being an unusual example of an intact, nondehiscent inflorescence. Achenes are, in
general, nutritious and have been utilized as foods since antiquity, not only in the form of our well-known
cereal grains but also as such lesser known species as the sumpweed (Iva annua), gathered by native
North Americans, and amaranth (Amaranthus caudatusandA. quitensis), a staple of the pre-Columbian
Aztecs, the cultivation of which has persisted in remote Andean valleys and which is currently an inter-
est of “health food” devotees [32].
FRUIT DEVELOPMENT, MATURATION, AND RIPENING 145