Even normally hardy plants, such as oak trees, need time to adapt to winter temperatures. A Florida
neighbor of this author grew oak seedlings and shipped them to Michigan in late winter. They were im-
mediately killed by freezing.
In addition to cold hardiness, cold hardening can induce disease resistance [32] in addition to the cus-
tomary physiological effects.
It is very different with citrus fruits. For one thing, fruit buds on deciduous fruit trees are clearly rec-
ognizable to anyone cognizant in such matters. Fruit and leaf buds are indistinguishable on citrus trees
[33], however, and the initiation of fruit bud development takes place only a few weeks before bloom.
The citrus industry and the literature usually speak of “dormant” citrus trees, but such dormancy is in no
way comparable to that of deciduous fruit trees. “Quiescent” is a far better term. Blooming of quiescent
citrus trees is usually initiated by the termination of a long cool spell or drought [34]. The best and most
uniform blooms come when mild stresses from cool weather and drought are relieved simultaneously. A
mild winter, followed by a warm, moist spring, tends to give a straggly bloom, spread over many weeks
or even months, with consequent poor yield, low fruit quality, and difficult harvesting.
When hormonal control of chilling injury was still a very new theory, a colleague and I sprayed a
number of grapefruit trees with various combinations of growth regulators in November. We definitely
affected susceptibility to chilling injury of the fruit harvested in the following fall, although not in any
clearly discernible pattern [35]. What was tantalizing about the test was that with one treatment we got a
highly significant increase in yield, which we felt we could not publish. Temperatures were so mild that
winter that bloom straggled on and on for many weeks—except on one of our growth regulator treatments,
for which the bloom was a “snow bloom,” on schedule in mid-March. The treatment would become use-
ful only if long-range weather forecasts were so precise that each November they could forecast whether
temperatures between November and March would be uniformly, and atypically, mild.
Obviously, the occasional chilly spells so resented by winter tourists initiate the hormonal activity
necessary for a desirably brief, early full bloom.
Even when fruit trees have bloomed satisfactorily, temperature can be a determinant of whether a
good crop will be harvested. Most deciduous fruits need pollination, which is normally done by honey-
bees. It can be very difficult to get the attention of apple or pear growers whose trees are in full bloom if
the temperature suddenly drops below that favored by the bees. If the temperature is not right, the bees
just quit flying, and that can mean a very poor crop indeed. Even if the bees fly and pollen is spread, the
pollen must germinate and the pollen tube grow down to the ovule, a process that can be severely re-
stricted by unseasonably low temperatures [36]. And even when pollination has been successful, growth
of individual grape berries (botanically, grapes are berries) can be restricted by both too high and too low
temperatures [37]. Too high temperatures are more likely to affect fruit set of citrus than of deciduous
fruits. In California, extremely high temperatures after fruit set can cause excessive fruit shedding of navel
oranges [38]. In Florida, trouble is more apt to come from a combination of high temperature and high
humidity, resulting in fungal invasion of the fruitlets [39].
Such problems are not limited to dessert fruits. The buying public having developed an unreasoning
prejudice against seeds in fruits and vegetables of many types, parthenocarpy has become highly desir-
able. For some cucumber varieties, parthenocarpy can be induced with sprayes of chlorfuorenol—unless
the night temperatures are too high. Night temperatures between 16 and 21°C have been reported as fa-
vorable, with parthenocarpy very much reduced when the thermometer reaches 21°C [40].
- Seed Dormancy and Germination
A very helpful specialist in seed science whom I consulted on the preparation of this chapter sent me, in
addition to various published papers, a page-long list (which he considers incomplete) of textbooks, sym-
posia, and so on dealing with the handling and storage of seeds. With temperature so often a critical fac-
tor in storage and germination of seeds, this account can be only the briefest of introductions for the non-
specialist.
An important temperature-related difference should be noted between seed-bearing plants of the tem-
perate zone and those originating in the tropics or subtropics. In areas that experience killing winter
freezes, seeds must notgerminate until the following spring. Exceptions to this principle are seeds of
plants that bloom early enough in the spring to be able to establish mature plants before the onset of win-
ter. The dandelion (Taraxacum officinale) is a familiar, and usually unwelcome, example. Seeds of plants
that evolved in tropical areas need no such protective device and so usually (but not always) can be ger-
18 GRIERSON