carotenoid development took weeks, rather than days, hence was commercially unacceptable in a stem-
end rot district.
I still do not know why, prior to picking, cool (below ca. 12°C) temperatures are necessary to destroy
chlorophyll in the peel of citrus fruits, but warm (ca. 30°C) temperatures maximize the rate of ethylene-
mediated chlorophyll disappearance after picking. This paradox does, however, emphasize something
that is too often ignored or forgotten: prior to picking, a fruit is an integral part of the physiology of the
plant as a whole.
- Temperature and Fruit Quality: Preharvest
There is no point in producing fruits commercially unless they are palatable, and in some instances palata-
bility is strongly related to growing temperatures. Again, a citrus fruit, grapefruit (Citrus paradisi), will
serve as a prime example, not so much because of its place in this writer’s past research but because the
internal and external qualities of grapefruit have been extensively studied. All growing districts base their
quality standards on what they do best [61], and since Florida’s climate is so unsuited to the production
of grapefruit with a bright, colorful exterior, standards have been developed largely based on internal
quality. These are expressed in terms of sugar (as degrees Brix), acid (as ratio of citric acid to Brix), and
juice volume (as cubic centimeters per fruit) [62,63]. Internal quality obviously varies widely among
growing districts, leading to some totally unprofitable studies in day/degree relationships. But even the
most casual observations make it apparent that districts famed for the high quality of their grapefruit (such
as the Rio Grande Valley of Texas and the Indian River district of Florida) are areas with warm winter
nights, during which growth of the tree and of the fruit can continue uninterrupted. A controlled climate
experiment with Redblush grapefruit in Florida confirmed this. Maximum internal quality was found in
fruit from little trees, which were grown where night temperatures were not allowed to drop below 21°C
[64].
The shape of grapefruit is very sharply associated with internal quality: the flatter the fruit, the higher
the internal quality. The influences of day and night temperatures, and of day length, were studied under
controlled conditions [65]. A 32 /7°C (day/night) temperature regime produced severely “sheep-nosed”
fruit of very low internal quality. A 32/24°C temperature regime produced flat fruit (axis length diam-
eter) of high internal quality. No correlation between fruit quality and day length was found.
- Wound Healing: Temperature Humidity Time
Some plant products have considerable ability to heal mechanical lesions after harvest. The ability de-
pends on certain ranges of temperature and humidity, however, and the healing takes several days to com-
plete. It has long been known that both sweet potatoes (Ipomoea batatas) and so-called Irish potatoes
(Solanum tuberosum) can heal damage to their own tissue [66]; for this reason, it is advised that potatoes
be harvested, then held for several days at ambient (or higher) temperature and very high humidity before
being placed in cold storage, because such healing occurs only at high temperatures and humidities [67].
Similarly, when seed potatoes are cut into “planting pieces,” they should be “cured” for several days prior
to planting under the warmest conditions available. During this period of comparatively high temperature,
a layer of suberized cells forms over the wounds.
A much more recent finding is that citrus fruits can heal shallow wounds into the flavedo (colored
part of the peel), but only at very high relative humidity (ca. 95% RH) and temperatures as high as
28–29°C (which, fortunately, are the conditions recommended within Florida citrus degreening rooms).
An unusual aspect of this healing of citrus fruits is that it involves lignification, not suberization, and it is
associated with sharp increases in phenolic compounds and of the enzyme phenylalanine ammonialyase
(PAL) [68].
In both these types of healing, the role of comparatively high temperatures is critical. Such wound
healing should not, however, be confused with drying treatments, which are essentially catabolic rather
than anabolic. The “curing” of onions prior to storage is an example of drying. The curing process aims
at killing the outer layers of cells by heat and desiccation, a form of localized necrosis that would be dis-
astrous with living products of most other types.
Attention is again drawn to the different physiological responses of plant organs on and off the
mother plant. After a Florida hurricane, attached citrus fruits will heal severe wounds and continue to
grow to maturity at normal ambient temperatures although badly scared. Fruits with similar injuries that
become detached from the tree promptly rot. Various forms of squash (Curcubitaspp.) carved with a gar-
TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS 21