Freezing, in the context of plant life, occurs when temperatures drop below the freezing point of water, 0°C (32°F), causing ice to form within or around plant tissues. Because the water within plant cells contains dissolved solutes, it typically freezes at a slightly lower temperature, often a few degrees below 0°C. This exposure to sub-zero temperatures is a primary factor limiting where plant species can survive and grow across the globe. Understanding the physical process of ice formation and the cellular damage it causes is foundational to understanding how plants have evolved complex survival mechanisms.
The Mechanics of Cold Injury at the Cellular Level
The most common cause of freezing injury is the formation of ice in the space outside the cells, known as the apoplast. When ice crystals begin to form in the apoplast, they reduce the water potential, creating an osmotic gradient. This gradient draws water out of the living plant cells in a process similar to drought stress, causing the cell to shrink and suffer from dehydration.
This freeze-induced dehydration can damage the plasma membrane, the boundary separating the cell’s interior from the outer environment. If the cooling is too slow, or if the plant is tolerant, the cells dehydrate slowly, and the plant has a chance to survive the water loss. The other, more immediately lethal form of injury is the formation of ice inside the cells.
Intracellular ice formation typically occurs when the temperature drops too rapidly, preventing the cell from losing water fast enough to the apoplast. When water freezes inside the cell, the expanding ice crystals physically rupture the delicate cell membranes and organelles. This mechanical damage leads to the immediate death of the cell.
Recognizing the Signs of Freeze Damage
The damage caused by freezing at the cellular level results in visible symptoms. Immediately after a freeze, tender foliage may appear water-soaked or translucent because the damaged cell membranes have leaked their contents into the intercellular spaces. This initial appearance quickly gives way to permanent signs of tissue death.
After the tissue thaws, the damaged areas typically turn dark brown or black, a process known as necrosis. Leaves may become limp and wilted, and the edges or tips of the leaves often look brittle and discolored. For woody plants, severe cold can cause the bark on stems or trunks to split, known as a frost crack, due to the rapid expansion and contraction of the tissue.
The full extent of the damage may not be apparent immediately and may only become visible days or weeks later as the plant attempts to resume growth. In addition to direct tissue damage, a process called frost heave can occur when ice expands in the soil. This physical expansion lifts plants out of the ground, ripping and exposing the roots, which leads to dehydration and death.
Plant Strategies for Surviving Sub-Zero Temperatures
Plants have developed two main biological approaches to survive freezing: acquiring tolerance or avoiding the formation of ice altogether.
Cold Acclimation (Freeze Tolerance)
The ability to acquire freezing tolerance is achieved through a process called cold acclimation or cold hardening. This adaptation is initiated by exposure to gradually cooling, non-freezing temperatures, often between 0°C and 10°C, and can be triggered by changes in day length. During cold acclimation, plants alter their cellular composition, which enhances their ability to withstand the coming freeze. They accumulate high concentrations of soluble sugars, such as sucrose, and other compounds like proline, which act as cryoprotectants. These solutes increase the concentration of the cell sap, effectively lowering the freezing point of the water inside the cell and helping to stabilize cell membranes against dehydration.
Plants that utilize freeze tolerance allow ice to form safely in the extracellular spaces. These species control the rate of ice formation and manage the resulting dehydration stress. They achieve this by having cell membranes that are flexible and robust, allowing them to shrink significantly as water moves out without suffering damage upon rehydration. The success of this strategy relies on the plant’s ability to control where the ice forms, ensuring it stays outside the cell to prevent lethal intracellular freezing.
Freeze Avoidance (Supercooling)
Freeze avoidance, or supercooling, is a mechanism where plants prevent water from freezing even when the temperature is below 0°C. The water remains liquid well below its normal freezing point, sometimes dropping as low as -40°C in specialized tissues like overwintering buds. This is achieved by limiting the presence of ice nucleators, which are particles that promote ice crystal formation, and by using specialized antifreeze proteins that inhibit ice growth.