Most varieties of pears (Pyrus communis) do not ripen to acceptable eating quality on the tree. Once
picked, pears have to be eitherripened for immediate use (preferably at 20 to 25°C) orheld in cold stor-
age at only a degree or two above their freezing point. Pears, particularly the popular Bartlett variety, will
neither ripen nor store at intermediate temperatures, particularly in the range 8 to 12°C. Instead, they be-
come rubbery in texture and virtually inedible.
This is necessarily an abbreviated and simplified account of the complex physiology of climacteric-
type fruits. The extraordinary development of nonchemical analytical equipment has stimulated much
postharvest research. Some surprising results are being encountered, such as a newly developed thornless
blackberry being strongly climacteric [55]. Those interested in further reading are referred to a 1985 sym-
posium [56], particularly the paper by McGlasson [57].
Handling of nonclimacteric fruits is very much simpler. There are no significant physiological
changes involved in separation from the tree and no postharvest ripening cycle. Those signs along the
Florida highways saying “TREE-RIPENED CITRUS,” although misleading, are legally defensible, all
citrus fruits being “tree ripened.” With no climacteric rise to suppress, nonclimacteric fruits such as cit-
rus of various types, grapes, and certain vegetables that are botanically fruits do not benefit nearly as much
from prompt refrigeration as do climacteric-type fruits. Indeed, for fruits susceptible to chilling injury, de-
layed storage may be beneficial by enabling the fruit to adapt to lower storage temperatures [46]. Sooner
or later, of course, any fruit can be expected to abscise if left on the tree long enough. Modern research
shows this to be a surprisingly complicated biochemical and histological process [58]. Such abscission is
always due to trace amounts of ethylene at the abscission zone. Typically, this is induced by ABA (ab-
scisic acid), the growth regulator produced in response to such environmental stresses as low temperature
or drought. Deciduous fruit trees have deciduous fruits that fall when fully mature. Such natural abscis-
sion can be delayed with “stop drop” sprays, but at a loss of some postharvest shelf life.
Citrus fruits, typical fruits of evergreen trees, have no such programmed abscission, making har-
vesting much more onerous than for deciduous fruits. [Typically, a Valencia orange must be removed
with a pull force of 18 to 22 pounds (8 to 10 kg) as compared to ca. 4 to 5 pounds (1.8 to 2.5 kg) for a
McIntosh apple.] Research [59] has shown that the abscission-causing ethylene in citrus fruits can also be
triggered by endogenous IAA (indole acetic acid).
B. When to Harvest
This discussion is out of chronological order in terms of the fruit because it is necessary to understand
something of postharvest fruit physiology before dealing with optimum picking dates for various types of
fruits.
- Citrus Fruits
It is fairly simple to set legal maturity criteria for nonclimacteric fruits such as citrus and grapes. These
undergo no considerable physiological change at harvest, nor do they abruptly abscise and fall. Maturity
standards, either legal or voluntary, can be set in terms of sugar content, sugar/acid ratio, and juice yield.
Moreover, citrus fruits can be “tree stored.” Early tangerine varieties can be picked over a period of sev-
eral weeks, at the end of which period they start to dry out rapidly. Orange varieties, particularly the late,
main crop Valencia variety, can be picked over a period of 2 to 3 months, sometimes more. Grapefruit
from a single bloom can be harvested over a period of 6 months or more. (As this is being written, the
same Florida grapefruit that might have been picked in October 1992 are still being harvested during the
first week of June 1993). This is a great convenience in marketing, provided that the shipper does not try
to extend marketing by storing grapefruit that has already used up its storage potential during prolonged
tree storage [60].
- Apples (Malus sylvestris)
Deciding on a harvesting date is very much more difficult for climacteric-type, temperate-zone decidu-
ous fruits for which only a narrow window of opportunity is available. “It is exceedingly important that
apples be harvested at the right time. The exact degree of maturity at which a given variety should be
picked depends in large part on what disposition is to be made of the fruit.... If apples are picked too
soon and then stored for any length of time they are subject to storage troubles such as bitter pit and scald.
... Almost every measure or index of maturity has to be defined for not only a given variety but for a
FRUIT DEVELOPMENT, MATURATION, AND RIPENING 155