- Other Plant Organs
The urgency of immediate postharvest temperature and humidity control is related to the maturity of the
plant part involved [101]. Grain crops, mature root crops, and cabbage are typical of storage organs that
enter a resting stage preparatory to winter. Their respiration rate is very low, and thus prompt postharvest
refrigeration is of little consequence. Young actively growing tissues, such as asparagus, green peas, and
sweet corn, have very high respiration rates that need to be reduced by refrigeration as soon as possible.
The same is true of cut flowers, an intrinsically ephemeral product.
There is a tendency to forget the economic consequence of unrestricted respiration rate in crops for
processing. Nevertheless, particularly when crops are paid for on the basis of sugar content, excessive res-
piration rates due to prolonged exposure to high temperature (as with truckloads of oranges waiting in the
sun outside a Florida cannery) deplete sugar content, hence the cash value of the product. Even sugarcane
stacked in the sun by the roadside after harvest is losing sugar for which the grower would otherwise be
paid [109].
B. Prestorage “Curing”: Temperature HumidityTime
Traditionally, those who handled horticultural crops for shipment or storage were advised to refrigerate as
soon as possible after harvest. It is now known that there are marked exceptions to this general rule. One
such exception is the group of products that need to be “cured” prior to storage to heal mechanical wounds
(see Sec. II.B.7). The outstanding example is sweet potato, for which Rhizopusdecay in cold storage was
often calamitous until is was demonstrated that prior “curing” at ambient (or higher) temperature and very
high humidity for several days healed wounds that otherwise would have been invasion sites for Rhizopus
[110]. The same benefit can occur, although usually to a less marked extent, with other root and tuber crops.
C. The Chilling Injury Syndrome
Perhaps the most intriguing response of plants to temperature is the chilling injury syndrome exhibited by
many plants of tropical origin (which include such familiar crops as cotton, soybeans, tomatoes, citrus,
and cucumbers, commonly grown in the temperate zone). Morphological and biochemical responses of
corn (Zea maysL.) to field chilling conditions have been reported in considerable detail [111]. CI-sus-
ceptible plants (and their detached plant organs) are severely injured by temperatures well above freez-
ing. Critical temperatures vary, but typically injury occurs at temperatures below 10°C. Preharvest chill-
ing injury can occasionally be troublesome, particularly with cotton seedlings [112] and mature, but
unripe, tomatoes [113]. But CI is particularly important after harvest, not only because of the products
lost due to incorrect storage or transit temperatures but also (perhaps more significantly) because of se-
vere limitations on marketing. If Florida grapefruit could be stored and shipped at the same temperatures
as Florida oranges, markets for grapefruit growers would be enormously expanded.
The symptoms of CI can be either superficial or metabolic. Superficial effects are typically various
forms of peel injury, which may be uniform (e.g., the darkening of the peel of a banana held in a house-
hold refrigerator) or highly irregular (e.g., discrete, necrotic sunken areas of grapefruit or cucumbers, sur-
rounded by healthy tissue).
The metabolic origin of CI is so profound that a remarkably precipient study demonstrated a paral-
lel between behavior of mitochondria in CI-susceptible versus nonsusceptible plants and of mitochondria
from poikilothermic (cold-blooded) versus homeothermic (warm-blooded) animals [114].
The tomato is an example of a climacteric-type fruit that is metabolically sensitive to CI. A mature
green tomato that has been chilled will never ripen, even when treated with exogenous ethylene.
The literature on CI is dispersed among many types of plants and journals; moreover, research re-
ports often deal solely with individual reactions or systems isolated from ecological considerations. Much
of this literature up to 1986 has been reviewed [115].
Nevertheless, this account reviews the 25-year-long series of reports on grapefruit (and occasionally
bananas, limes, and avocados, when grapefruit were out of season) at the University of Florida’s Citrus
Research and Education Center in Lake Alfred. There are several reasons for this duplication.
- Grapefruit is uniquely suited for CI research in that fruit can be harvested from a single bloom
on an individual tree for as long as 8 or 9 months (typically from September to May). Moreover,
TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS 27