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Phase transition - ice nucleation and super cooling in plants

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

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Phase transition - ice nucleation and super cooling in plants

  2. 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. 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. 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. 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. 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. 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. 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.
  9. 9. 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)
  10. 10. 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
  11. 11. 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.
  12. 12.  How to Supercool Water: A SciShow Experiment https://youtu.be/NMSxuORKynI