Plants use a combination of freeze avoidance and freeze tolerance mechanisms, which often involve a process called cold acclimation, to protect themselves from freeze damage.
Freeze Avoidance Systems
These systems prevent ice from forming within the plant tissues:
- Dormancy: Many plants enter a state of dormancy during winter, reducing their metabolic activity and water content in above-ground parts (e.g., deciduous trees shed their leaves to reduce water loss).
- Water Relocation (Dehydration): Water is actively moved out of the plant cells into the extracellular spaces (apoplast), where ice can form without rupturing the cell membranes.
- Supercooling: Plants can keep water in a liquid state even below its normal freezing point (below 0°C) by removing ice-nucleating agents from cellular fluid and accumulating compatible solutes (osmolytes). The formation of ice within the cell is usually fatal.
- Physical Barriers:
- Waxy Coatings & Hairs: Some leaves have a thick, waxy cuticle or fine hairs (trichomes) that repel water, preventing ice from forming directly on the leaf surface.
- Bark: Thick bark on woody plants provides insulation to the living vascular tissues beneath.
- Needles: Conifer needles have a small surface area and waxy coatings, which reduces water loss and helps shed snow.
- Hairy surfaces trap air spaces, insulate, and trap air/moisture; (trichomes) prevent contact with ice and change icing dynamics essentially acting as a buffer
Freeze Tolerance Systems
These systems allow plants to survive even when ice forms in their extracellular spaces:
- Accumulation of Cryoprotectants: During cold acclimation, plants synthesize and accumulate various compatible solutes, such as soluble sugars (sucrose, glucose, raffinose), specific amino acids (proline), and polyamines. These act as natural “antifreeze” by lowering the freezing point of the cell sap (cytosol) and stabilizing cell membranes and proteins.
- Antifreeze Proteins (AFPs): Plants produce specialized AFPs that do not necessarily lower the freezing point significantly, but rather bind to existing small ice crystals in the extracellular spaces and inhibit their growth and recrystallization into larger, damaging crystals.
- Membrane Modification: Plants alter the lipid composition of their cell membranes, increasing the proportion of unsaturated fatty acids. This maintains membrane fluidity and integrity at low temperatures, preventing them from becoming rigid and rupturing.
The Cold Acclimation Process
Many temperate plants have the ability to increase their freezing tolerance through a process called cold acclimation or hardening. This is triggered by environmental cues like shortening day length and gradually cooling temperatures in the autumn. This process involves complex molecular signaling pathways (such as the ICE-CBF-COR gene cascade) that activate the physiological and biochemical changes necessary for winter survival.
Sugars, minerals, and volatile substances all
lower the freezing point of water through a phenomenon called freezing point depression. This effect is a colligative property, meaning it depends primarily on the concentration of solute particles in the solution, rather than their specific chemical identity.
Mechanism
When a substance dissolves in water, the solute particles disrupt the formation of the highly ordered hydrogen-bonded crystal structure required for pure ice to form. The water molecules require a lower temperature (less kinetic energy) to slow down enough to arrange themselves into a solid crystalline lattice, thus lowering the freezing point of the solution compared to pure water (0°C or 32°F).
Comparison of Effects
The extent of freezing point depression depends on the total number of dissolved particles (moles of solute particles per kilogram of solvent, or molality).
-
- Minerals (Ionic Solutes): Minerals typically dissolve as ionic compounds, meaning they dissociate into multiple ions in water. For example, table salt (sodium chloride, NaCl) breaks into two ions (
Na+Na raised to the positive power
Na+
and
Cl−Cl raised to the negative powerCl−
). This dissociation results in more particles per mole of dissolved substance, making minerals very effective at lowering the freezing point. Calcium chloride (
CaCl2CaCl sub 2CaCl2
), which dissociates into three ions, is even more effective. This is why salt is commonly used on roads to melt ice.
- Sugars (Covalent Solutes): Sugars (like sucrose or glucose) are covalent compounds and do not dissociate into multiple particles when dissolved in water; each sugar molecule remains a single dissolved unit. Therefore, a sugar solution of the same molality as a salt solution will have fewer dissolved particles and thus a less pronounced effect on the freezing point compared to minerals.
- Volatiles (Volatile Solutes): Volatile substances (like alcohol/ethylene glycol used in antifreeze) also lower the freezing point of water. Their effect follows the same colligative property principle: more dissolved particles mean a lower freezing point. The primary difference is that because they can vaporize easily, the system’s behavior can be more complex to model than with non-volatile solutes, which remain in the solution. However, in a stable solution, they effectively lower the freezing point, and can be very effective, with strong solutions of ethylene glycol and water freezing at temperatures as low as -45°C (-49°F).
- Minerals (Ionic Solutes): Minerals typically dissolve as ionic compounds, meaning they dissociate into multiple ions in water. For example, table salt (sodium chloride, NaCl) breaks into two ions (
In summary, all three types of substances lower the freezing point, but minerals are generally more efficient due to their dissociation into multiple ions, while sugars and volatile substances have an effect proportional to their molecular concentration.
Combined with Microclimates
When water freezes, it gives off heat to its surroundings.
This process is called an exothermic process. When water changes phase from a liquid to a solid (ice), its molecules arrange themselves into a more ordered, stable crystal lattice structure. The excess energy that the molecules had in their more freely moving liquid state is released into the environment as heat.
Key Concepts:
- Latent Heat of Fusion: The amount of energy involved in this phase change is known as the latent heat of fusion (or enthalpy of fusion). For water, this value is approximately 334 joules per gram (or 80 calories per gram).
- Constant Temperature: This heat release happens without a change in temperature once the water reaches its freezing point (0°C or 32°F). The temperature of the water/ice mixture remains constant at 0°C until all the water has completely frozen.
- Practical Implications:
- This is why putting a container of water into a freezer actually makes the freezer work harder—it has to remove both the sensible heat (to cool the water to 0°C) and the large amount of latent heat released during the freezing process itself.
- In agriculture, farmers sometimes spray their crops with water during a light freeze. As the water freezes, it releases heat, which helps keep the plant tissues inside the protective layer of ice at a temperature of 0°C (slightly warmer than the surrounding air), preventing damage.
- likewise the use of wet sheats to prevent frost damage
- likewise wet mulch of straw or leaves to help overwinter tender perennials