This presentation discusses the phase transition of water and its relevance to freezing tolerance in plants. It discusses the ice nucleation and supercooling

1. PHASE TRANSITION - ICE NUCLEATION
AND SUPER COOLING
-J.Delince

2. PHASE TRANSITION
• 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.

3. 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

4. ICE NUCLEATION
 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
crystal nucleus
Two types of ice nucleation occurs in nature
 Homogeneous ice nucleation – it uses preformed ice and it happens in pure water at
-40°C
 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

5. 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

6. 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

7. SUPERCOOLING
 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
apoplast
 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

8. ANTIFREEZE PROTEINS
 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
hysteresis
 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
of ice.

11. ANTINUCLEATORS
 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)

12. DEEP SUPERCOOLING
 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

13. In order for tissues to supercool, the cells within the tissue must:
(i) be free of heterogeneous nucleating agents that are active at
warm temperatures;
(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
gradient; and
(iv) have cell walls with sufficient tensile strength to counteract the
negative hydrostatic pressures that result from a large vapour
pressure gradient.

15.  How to Supercool Water: A SciShow Experiment
https://youtu.be/NMSxuORKynI