Many plant species increase in frost hardiness when exposed to
environmental conditions such as lowering temperatures (Levitt, 1980).
The growth of extracellular ice causes progressive cellular
dehydration to such an extent that cells contract and cell walls may
bend inwards. The flexibility of cell walls will affect the extent of
disruption and cell survival (Palta, 1990).
During cold acclimation, cell wall thickening and changes in the cell
wall properties, probably caused by lignification and suberization
have been observed in epidermal cells of Secale cereale (Griffith and
Brown, 1982). The rigidity of cell walls is improved through
impregnation with lignin, while suberization of cell walls may reduce
desiccation of tissues. Such changes might result in development of
negative turgor pressure (tension) in the cell during freezing (Burke
et al., 1983). This will limit the loss of cellular water and the
contraction of the cells. The assumption that thick lignified cell
walls do not bend inwards during freezing is confirmed by micrographs
of cold acclimated and no-acclimated ray parenchyma cells of Cornus
sericea (Ristic and Ashworth, 1994).
Cells and tissues which have developed a certain degree of tolerance
to freezing-induced dehydration stress will permit extracellular ice
crystal formation before the appearance of damage. Tolerance to
freezing-induced dehydration stress is the main survival strategy for
most plants in the temperature regions. The response to freezing
stress may differ between acclimated and non-acclimated plants, as the
latter presumably exhibit limited or no-tolerance to freezing-induced
dehydration stress. In non-acclimated plant material, the
freezing-induced stress will soon pass the threshold at which the
damage associated with cellular dehydration will be irreversible. In
contrast, cold acclimated cells may have changed their cellular
function and structure in such a way that they can sustain the
freezing-induced dehydration stresses (Fircks, 1994).
Water stress has been shown to enhance freezing resistance, which has
been attributed to both increased avoidance and tolerance to
freeze-induced dehydration stresses. Mild water stress enhances the
development of cold acclimation. Excessive drought stress, however,
decreases the capability of plant material to cold acclimate,
emphasizing the importance of adequate water supply throughout the
growing season (Fircks, 1994).
As extracellular freezing takes place, plants have to endure a number
of stresses: concentration in pH and in electrochemical potentials,
reduction of volume, osmotic and mechanical stress, loss of functional
and structural water, together with the mechanical stresses caused by
growing extracellular ice masses. Such a wide array of stresses will
result in multitude of effects which in various combinations may lead
to freezing injuries. Furthermore, differences in response among
tissues, plant parts, genotypes and growth stages are other factors
which make it difficult to apply results from isolated cells, tissues
or plant parts to single plants (Fircks, 1994).
