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Phase transition - ice nucleation and super cooling in plants
PHASE TRANSITION - ICE NUCLEATION
AND SUPER COOLING
• The term phase transition (or phase change) is most commonly used to
describe transitions between solid, liquid and gaseous states of matter,
and, in rare cases, plasma.
MELTING POINT OR FREEZING POINT?
The melting point of a substance is defined as the temperature at which a
solid, when given enough heat, is turned into liquid depending on the
purity of the substance and the pressure that is applied to it.
The freezing point of a substance is defined as the temperature at which
matter or a substance is changed from its liquid state into solid
The melting point is considered as a characteristic property of a substance
while the freezing point is not
Nucleation is typically defined to be the process that determines how long an
observer has to wait before the new phase or self-organized structure appears
Ice nucleation refers to phase transition of liquid to solid phase by formation of
Two types of ice nucleation occurs in nature
Homogeneous ice nucleation – it uses preformed ice and it happens in pure water at
Heterogenous ice nucleation – it uses other compounds to initiate the nucleation.
This is common in plants as they contain solutes in water. This is an efficient way for
nucleation and crystal formation catalyzed by the presence of dust, salts, organic
molecules or ice-nucleation active (INA) bacteria
HOW CRYSTAL FORMS
The heterogeneous nucleators act as a template that make it easier for
water molecules to begin to take on a crystalline arrangement.
Once a core of water molecules has assumed this crystalline
arrangement (ice nucleus), the ice nucleus acts as a catalyst to induce
the freezing of the surrounding water molecules
The speed of ice nucleation spread will be up to 27 cm per minute in
the surrounding areas
TYPES OF ICE NUCLEATING AGENTS
There are two types of ice nucleating agents
Intrinsic ice nucleators which are found in plants itself
Extrinsic ice nucleators which are foreign materials to the plants
The water droplets on the surface is an extrinsic nucleator and it
aggravates the process of nucleation. Hence the plant surface have to
be dry to avoid this (row covers).
This type of extrinsic nucleation spreads through broken cuticle.
The plants with thick cuticle avoids this spread
Even application of hydrophobic film prevents this type of spread as in
case of tomato which could withstand upto - 6 °C
Also known as undercooling,
It is the process of lowering the temperature of a liquid or a gas
below its freezing point without it becoming a solid.
The ability of some plants to maintain symplastic water in an
unfrozen condition and without movement of water into the
Pure water has the ability to supercool to temperatures as low as
−40°C (homogeneous nucleation temperature) and perhaps even
to temperatures as low as −100°C
Antifreeze proteins (AFPs), also known as hysteresis proteins (THPs), inhibit
ice crystal growth in a non-colligative mechanism, lowering the freezing
point of water below the melting point, thereby producing a thermal
The separation of the melting and freezing temperature is usually referred
to as thermal hysteresis
They are identified in fishes, insects and plants
Thermal hysteresis activity of plant AFPs is low (0.2–0.5°C) compared with
fish (0.7–1.5°C) and insects (3–6°C).
But they reduced freezing injury by slowing the growth and recrystallization
Anti-nucleators (compounds that inhibit ice nucleation activity but do
not exhibit hysteresis) have been identified from a variety of sources
including microorganisms, insects, plants and synthetic polymers
Eg: PCA 60 (dehydrin)
Deep supercooling of bud and xylem parenchyma tissuesof woody
plants is one of the most enigmatic aspects of biological ice nucleation
and cold hardiness
Water in these tissues exists in the liquid phase to temperatures as low
as −50°C by being isolated from internal, heterogeneous ice nucleators
including extracellular ice
This is an ancestral trait evolved before the development of antifreeze
proteins and dehydrins
In order for tissues to supercool, the cells within the tissue must:
(i) be free of heterogeneous nucleating agents that are active at
(ii) have a barrier the excludes the growth of ice crystals into the
supercooled cells from the surrounding apoplast;
(iii) have a barrier that prevents the rapid loss of cellular water to sites
of extracellular ice despite the presence of a large vapour pressure
(iv) have cell walls with sufficient tensile strength to counteract the
negative hydrostatic pressures that result from a large vapour
How to Supercool Water: A SciShow Experiment