and transported for weeks on the heads of slaves [53]. Such aging of tropical seeds is not necessarily com-
pletely temperature independent. Seeds of Plantago ovata(an annual herb grown in India), although com-
pletely recalcitrant at harvest, germinated freely after a single day at 15°C plus treatment with gibberel-
lic acid (GA3) [54].
A record for temperature-related recalcitrance is held by American ginseng (Panax quinquefolius).
It is no wonder that this wild herb has been hard to domesticate: it is reported [55] to need cool-warm-
cool stratification over a period as long as 540 days to 18 months!
Temperature may or may not prove to be important in the storage and germination of a particular type
of seed, but it can never be ignored as a possibly critical factor.
- Temperature-Induced Ethylene Effects
Ethylene (C 2 H 4 ) is the universal growth regulator. Until the advent of the gas chromatograph, it was be-
lieved that biosynthesis of ethylene was confined to certain plant tissues (e.g., apples) and was not pre-
sent in others (e.g., oranges). As analytical equipment improved, it became apparent that under various
forms of stress any plant tissue can produce ethylene, and the extent of this effect is temperature depen-
dent [56]. Among the more striking temperature-induced effects of endogenous ethylene are the “fall col-
ors” in deciduous woodlands, which result from the reaction of ethylene with plant pigments.
Bright colors are not only attractive but, as long as consumers insist on relying on their eyes rather
than their taste buds, they can be very valuable. Thus, temperature-modulated ethylene effects become es-
sential tools in the marketing of certain fruits and vegetables. Citrus fruits afford an excellent example.
Citrus fruits grown at sea level in the humid tropics, where the species originated, are all green: no bril-
liant oranges or yellows gleaming amid the jungle foliage.
But for centuries, citrus fruits have been grown in cooler, usually more arid, areas, principally around
the Mediterranean Sea. There, the considerable stress of cool nights on a tropical fruit forces production
of minute amounts of ethylene, with consequent loss of chlorophyll and development of carotenoids.
Thus, we have the obvious “fact” that oranges should be orange and lemons should be yellow. This con-
sumer prejudice presents citrus growers in milder climates such as Florida and Brazil with a very real,
temperature-induced problem. In such districts, early varieties may mature and pass their optimum matu-
rity without ever developing “typical varietal color.”
It has long been axiomatic among Florida citrus growers that their fruit would not change color
without “a week of cool nights” (which in many years comes after the early varieties are over). A 1942
study confirmed this [57]. No significant color break was observed as long as night temperatures were
above 55°F (12.8°C), and a week of nights below 50°F (10°C) resulted in good orange color on early
varieties of oranges. Grapefruit, however, responded to the stress of low night temperatures much less
predictably.
In California, an ingenious experiment studied the effect of temperature on the coloring of Valencia
(late) oranges under controlled conditions. Fruit-bearing branches were grafted onto young potted root-
stocks, and air and soil temperatures were controlled separately [58]. Both variables were found to affect
fruit color, the best orange color being achieved with 7°C soil temperature and 20°C air temperature. In-
ternal analyses found no correlation between fruit color and fruit maturity.
Various attempts by this author to reproduce such temperature-induced color changes with detached
fruit have been unsuccessful. Once the fruit has been detached from the tree, exogenous ethylene must be
supplied and the effect is, again, sharply temperature dependent, but with a relationship quite different
from that observed for attached (nonpicked) fruit. In an early Florida study [59], we found a very sharply
defined optimum for chlorophyll destruction in oranges at 85°F (29.4°C) and a very ill-defined optimum
for grapefruit at approximately the same temperature. Such ethylene “degreening” had no apparent effect
on carotenoids; the degreened oranges were pale yellow. California packinghouses that commonly de-
greened at 75°F (23.9°C) reported development of a deep orange color, but the process took 8–10 days,
a prohibitive period in Florida because of endemic stem-end rot (caused by Diplodia natalensis), which
is strongly stimulated by ethylene.
Nearly 20 years after the Florida work just described, the carotenoid development /chlorophyll de-
struction effect was studied in detail with very much more sophisticated equipment [60]. This time ethy-
lene-induced carotenoid accumulation was shown to be (1) temperature sensitive and (2) inhibited at 30°C
and above. The work was continued and showed that very high levels of specifically identified
carotenoids could be achieved with concentrations of ethylene as low as 0.1 ppm. However, induced
20 GRIERSON