C. Definitions
- Fruit is the product of a matured ovary.
- Maturationis the completion of the development of a fruit to the point at which it is physiolog-
ically mature enough to be separated from the parent plant. Typically, this is the point at which
its seeds are viable. There is no necessary relationship with market maturity, for which imma-
ture fruits may be required (e.g., cucumber, okra) or for which arbitrary legal standards may be
set for external color and/or sugar or acid content (e.g., citrus, grapes). - Ripeningand maturation can be synonymous for nonclimacteric fruits (e.g., grapes, strawberries,
and citrus) that are edible at the time of picking and have no postharvest ripening cycle. However,
they are quite different for climacteric-type fruits: those that are considered unripe until they have
entered on a distinctive postharvest respiratory rise in which ethylene is evolved, CO 2 output in-
creases (sometimes as much a tenfold), tissues soften, starch/sugar or acid /sugar changes occur,
and typical external color changes may be involved. Tomatoes, apples, pears, avocados, and ba-
nanas are typical climacteric-type fruits with distinctive postharvest ripening cycles. - Berryis used quite differently by plant scientists and by the general public. Botanically, a berry
is the product of a single pistil, fleshy throughout, usually indehiscent, and homogeneous in tex-
ture [1]. Thus a grape is technically a berry, but a strawberry is not. - Anthesisis the stage of flowering at which pollination can take place, usually considered to be
the initiation of fruit development. - Parthenocarpyin its narrowest sense is defined as the ability of a plant to develop fruit without
sexual fertilization. More broadly, it is the ability to produce fruit without seeds [2].
Readers interested in further details of terminology are referred to two publications: Watada et al. for
general terminology relating to developing horticultural crops [3] and Gortner et al. for the biochemical
basis for terminology used in maturation and ripening of fruits [4].
II. PREREQUISITES FOR FRUIT FORMATION
As long ago as several hundred years BC, it was recognized that all fruit came from flowers. The ancient
Greeks named one exception, the fig, “the only fruit not preceded by a flower.” This was because they did
not realize that the fig is an aggregate fruit with many minuscule flowers insidethe enlarged, fleshy re-
ceptacle.
Flowers must be preceded by buds specifically differentiated for flower formation. In deciduous
fruits, this starts some 10 or 11 months prior to bloom (i.e., initiation of fruit bud formation for the next
year’s crop starts almost as soon as the new crop is set). A study of the rate of flower bud development in
deciduous fruits indicates that each species follows a sigmoidal growth pattern within a temperature range
specific to that species [5]. In citrus fruits, fruit bud differentiation is initiated only a few weeks prior to
bloom [6]. For both deciduous and citrus fruits, blossom formation occurs on wood at least 1 year old.
(“Fruiting spurs” on apple trees may bear fruit almost every other year for a dozen years or more.) Grapes
are in sharp contrast to this pattern. Skilled grape pruners remove almost all woody growth (canes) from
the previous year, leaving only a few buds (how many depends on the variety, district, and vigor of the
plant). From these few buds grow long canes on which leaf and fruit bud differentiation has to take place
rapidly enough to provide for the current crop. Grape flower development has been described in detail [7].
Bud formation in tropical fruits is controlled mainly by water availability and temperature and thus can
be less predictable than for deciduous fruits. An extreme example is papaya (Carica papaya). Although
basically dioecious, under various temperature, moisture, and nutrient stresses, carpels can metamorpho-
size into stamens, and vice versa [8].
Thus flower bud initiation is a necessary precursor to fruit formation. Particularly after the landmark
1918 paper by Kraus and Kraybill on fruiting in the tomato [9,10], it was believed that flower bud initia-
tion was dependent on the balance between carbohydrates and nitrogenous compounds in developing tis-
sues (the C/N hypothesis). Within the last 50 years, it has been realized that in any plant, flower bud ini-
tiation and hence fruit formation are controlled by growth regulators (GRs). Development of GRs and the
balance between them is, in turn, controlled by environmental forces, notably temperature and light.
144 GRIERSON