Frost types
Frost can be divided into two categories depending in its origin.
The first is called radiation and originates at the site as a reuslt
of outgoing long-wave radiation during clear, calm nights. The second,
advection frost, consist of a thick layer of cold air moving over the
area concerned. Very severe frosts occur when the two are
simultaneous. Such conditions occur when the cold advective air is
further cooled through heat losses by radiation during cloudless, calm
nights.
Radiation Frost
Forests or other dense stands of crops are open ecosystems which
exchange energy with other ecosystems and wiuth the atmoshpere, where
radiation is the most essential factor for the environmental heat
exhange. The radiation balance at a site is the sum of net incoming
solar radiation (shortwave radiation) and net outgoing terrestrial
radiation (longwave radiation). During daytime, the net shortwave
radiation exceeds the net long-wave radiation, resulting in a
possitive energy balance at the earth's surface. During the night,
especially in calm and cloudness conditions, the earth emits radiation
direclty to the atmosphere and space, resulting in a negative energy
balance and low temperature, especially at the earth's surface. In
open areas with sparse vegetation such radiation frosts are especially
likely to occur. In contrast, the presence of closed forests or areas
with shelterwoods will have an effect similar to that of clouds or
fog, in that trees will partly reflect the radiation, absorb the
ground-derived thermal radiation heat and emit back both absorbed and
stored heat.
Advection Frost
The movement of air masses in a horizontal direction and the
resulting energy exchange with the surroundings is thermal advection.
Advection of large cold air masses from the north, particularly during
spring and autumn, brings about drastic falls in temperature,
sometimes causing advection frost. Climatic factors such as
cloudiness, for or mist will not prevent the occurence of advection
frosts. The southwards advecting air masses are considered as 'clean'
and dry, but during their movement southwards they become enriched
with moisture and 'polluted' with particles.
Freezing injuries in plants
Freezing injuries in non-acclimated and cold
acclimated plant material are associated with ice crystal formation in
plant tissue. Freezing of tissue water in plants is by extra- and
intracellular ice crystal formation. Extra-cellular ice crystal
formation is associated with conditions in which plant material
underwent equilibrium freezing, when the liquid to ice phase
transition in the extracellular spaces is a continuous function of
temperature (Olien, 1967). This occurs when plant material is cooled
at a moderate rate after mild transient supercooling and is
accompanied by liberation of non destructive free energy from
crystallization of water.
However, induction of extracellular ice crystal formation requires
that equilibrium freezing is preceded by a nonequilibrium condition
which liberates non-disruptive energy of crystallization. Large
displacement from equilibrium caused by rapid heat removal (high
cooling rates), extensive supercooling, a large amount of "easily"
freezable water as well as low membrane permeability to water are
singly or in combination the causes of liberation of such a large
amount of free energy of freezing through which the state of
nonequilibrium freezing is attained. Such conditions may lead to
cellular disruption, intracellular ice formation or both.
Extracellular ice formation
Sakai and Larcher (1987) suggest that extracellular
crystal ice formation occurs after mild supercooling through
heterogeneous nucleation. From the nucleation site(s), freezing is
assumed to spread to all plant parts mainly through the vascular
system (vessels) and extracellular spaces.
When
extracellular freezing has taken place, a difference in water vapor
pressure develops between the extracellular ice masses and the
intracellular liquid water. Differences in water potential between
supercooled cellular water and extracellular ice on the cell wall
surface cause the migration of cellular water to the extracellular
ice masses, where it crystallizes. This condition causes dehydration
of cells until a thermodynamic equilibrium of equilibrium freezing has
been established between the cell sap and extracellular ice. When
plant material is exposed to lower temperatures, a larger fraction of
cellular water is frozen out extracellularly to obtain a new
thermodynamic equilibrium and the cells experience increased
dehydration stresses. Depending on the rigidity of the cell walls,
cells may be exposed to shrinkage or even inward bending of cell walls
during progressive dehydration. The osmotic potential of the cell sap
will determine the extent to which cells can be dehydrated.
Intracellular
ice formation
In general, intracellular ice formation in plant
cells is avoided in plant material if the cooling rate is less than 6
oC / h. A high freezing rate in combination with low permeability of
cells to water migration leads to nonequilibrium freezing which can
result in intracellular ice formation. Conditions leading to
intracellular ice formation also result from moderate supercooling of
samples below -10 oC. For rye protoplasts, the risk for nonequilibrium
freezing is very low when ice crystal formation begins in the
temperature range between -2 oC to -5 oC, even when cooling rates
varied between 120 oC to 7200 oC / h.
The risk of
being killed by intracellular ice formation may be lowered by
improvement of the water permeability of the cell membrane, which is
attained by increasing the lipid unsaturation of membranes during cold
acclimation. For plant material in the temperate region, lowering of
the phase transition temperature is probably independent of the
genotype.
In nature, intracellular ice crystal
formation is uncommon, because early ice nucleation and relatively
slow cooling rates most commonly occur. One of the exceptions to this
is the phenomenon of supercooling of ray parenchyma cells. There is
indirect evidence that supercooling of xylem ray parenchyma cell
causes intracellular ice formation. However in these studies very high
cooling rates were used, which are unlikely to occur in nature.
Results by Kaku et al. (1980) imply that the cooling rate can affect
the supercooling ability of Rhododendron flower buds and hence
also may determine the temperature range for intracellular ice
formation in ray parenchyma cells. The effect of high cooling rates is
demonstrated by appearance of low temperature exotherms (LTE) in stem
sections of apple cooled at a rate of 60 oC /h. The same study showed
no LTE in similar sections cooled at a rate of 4 oC /h. These results
suggest that low cooling rates initiate cellular dehydration while the
high cooling rates induce intracellular ice formation.
Irrespective of whether the material is i a cold-acclimated or
non-acclimated state, intracellular ice formation is always lethal to
cells. The formation of intracellular ice in isolated plant
protoplasts is probably caused by mechanical damage to the plasma
membrane. These conditions are attained during supercooling of cells.
In contrast, intracellular ice formation can not occur as long as
cells undergo cell dehydration in the presence of extracellular
freezing and without altering test conditions. However, limitations in
the applied techniques mean that, up to now, there is no direct
evidence for intracellular ice formation in laboratory tests or in
nature.
Freezing damage and recovery
A number of studies have emphasized that the plasma
membrane is the primary site of freezing damage (Levitt, 1972, 1980;
Palta and Li, 1978; Palta & Weiss, 1993). Evidence has accumulated
that the integral membrane-located proteins involved in active
transport of K+, coupled to plasma membrane H+-ATPase pumps, are among
the components which are progressively impaired during ongoing
freezing (Palta & Li, 1978). The increased efflux of potassium (K+)
and sugars from cells exposed to freezing has been associated with the
initial stages of freezing damage. Results presented by Palnt
suggested that the semipermeability of the plasma membrane remained
unaltered at the initial stage of freezing. Total loss of
semipermeability may take place when freezing conditions of the cell
have passed the threshold for recovery. Hence, the change of the
observed influx/efflux rate of K+ is caused by an impairment of the
plasma membrane ATPase activity (Palta & Li, 1980). The mechanism(s)
by which the ATPase activity is impaired have not yet been clarified.
In addition, efflux of Ca2+ (Ca2+ concentration was about 100 times
less that of K+) has been observed in lethally injured cells. Results
indicate that Ca2+ plays an essential role for recovery of freeze
stressed cells. Alteration of the Ca2+ concentration in membranes and
cytoplasm, caused through freeze-induced dehydration, may be an
essential mechanism for sensing cellular disorder and translocating
this signal via CA2+ - calmodulin and protein kinase to activate H+
-ATPase for influx of ions (Palta & Weiss, 1993).
During recovery, leaked ions must be transported back into the cell
against a concentration gradient. This requires a functional
membrane-located H+ -ATPase pump. Intracellular accumulation of
cations due to active transport enables the cells to regain their
former turgor and volume through absorption of intercellular water.
The capacity to restore cell functions and structure after exposure to
freezing stress may be of importance for the maintenance of the
productive capacity of plant material (Fircks, 1994).
