- Freezing of Plant Tissues
As depicted in older texts, freeze injury and freeze resistance were simply explained: in freeze-suscepti-
ble tissues, free water froze, forming crystals that disrupted cell membranes, whereas in freeze-resistant
tissues the water was bound in the form of hydrophilic colloids. When this model was subjected to mod-
ern research, however, little if any of it turned out be so simple. Interested readers are referred to two ex-
cellent reviews [23,24]. Freeze-hardy plants have hormonally controlled mechanisms enabling them to
respond to gradual changes in temperature and day length in preparation for winter. Such changes are ob-
vious with deciduous trees, vines, and shrubs, which shed their leaves, often after having displayed dra-
matic changes in leaf color. No such highly visible evidence is afforded by conifers, which, nevertheless,
also need gradual autumnal climatic changes to induce similar hormone-controlled internal adaptation to
prepare for winter [25]. But what of plants that survive a freeze without a prior hardening period? Ex-
pressed very briefly, water in certain woody plants can supercool to a surprising extent, although this pro-
tective mechanism is often negated by the presence of ice-nucleating bacteria [24]. Such bacteria are by
no means ubiquitous, but they are very common and a real factor in freeze injury.
Exposure to freezing but nonlethal temperatures can cause various chemical changes in plant tissue.
Only one is mentioned here. It is very common for oranges that survive a freeze to develop white crystals
clearly visible between the segment membranes. These are hesperidin, the principal flavone in citrus
fruits, and although their presence sometimes causes alarm, they are completely nontoxic. Up to the
1950s, growers placed much credence on estimations of fruit damage as judged by the amount of hes-
peridin crystals. This mindset proved quite fallacious [26].
The once apparently simple field of tissue freezing is further complicated by work with detached
plant parts. Celery pollen has been stored in viable condition at 10°C for as long as 9 months [27]. The
use of “cryoprotectants” has made possible prolonged, very low temperature storage of living tissue for
in vitro tissue culture and propagation. Using such cryoprotectants as polyethylene glycol glucose and
dimethyl sulfoxide, such living material as apices of brussels sprouts [28] and Rubus[29] have been
rapidly cooled, then held at 196°C until needed for tissue culture propagation.
- Dormancy, Bud Initiation, and Fruit Setting
Obviously, it is well that autumnal climatic changes prepare perennials of the temperate zone for the
rigors of winter. It might seem that if no winter was to be expected, such plants could grow happily in
eternal summer. Or so thought the planners of the huge (1 hectare under glass) Devonian Gardens,
located over a large shopping mall in Calgary, Alberta, Canada. Their concept had been to surround the
clientele with familiar summer vegetation in the depths of Calgary’s cold, snowy winter. It was a costly
error. Deprived of their climate-induced cycle, the familiar native plants became spindly and unthrifty
and soon began to die. The thousands of years of evolution that had fitted those plants for the rugged
winter of the Rocky Mountain foothills had produced plants that could not do without it. Instead, the
native plants had to be replaced with (as nearly as possible) “look-alikes” imported from Florida and
California [30].
The dormancy of winter-hardened plants is deceptive. Essential physiological and morphological
changes are progressing and will do so only at the low temperature to which evolution has adapted such
plants. Spring bulbs (tulips, daffodils, narcissi, Easter lilies, etc.), brought indoors and kept in warm tem-
peratures after flowering, will not bloom again. Such bulbs left in the winter ground (or held in correctly
regulated cold storage) undergo histological changes clearly discernible under a dissecting microscope or
even a powerful hand lens. By the time the bulbs are ready to start growing again in the spring, each one
contains all the necessary floral parts, minute but discernible. It is by use of a series of very exact storage
temperatures that today’s scientific flower producers are able to have spring bulbs in bloom timed for such
occasions as Mothers’ Day and Easter. Such imposed temperature regimes are very precise: there are
sharp differences in temperature requirements, not only among genera, but even between individual cul-
tivars [31].
The same thing happens (on a truly microscopic scale) within the fruit buds of deciduous fruit trees
and shrubs. This is why, as horticultural students, we could cut apple boughs in late spring, place them in
water in a warm building, and, apparently miraculously, decorate our Easter dance with apple blossoms.
The same phenomenon explains why a blossom freeze wipes out a deciduous tree fruit crop for a whole
year. Those blossoms came from fruit buds initiated 10 or 11 months before, which had developed while
dormant and apparently inactive during the winter months.
TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS 17