Frost, the delicate appearance of ice crystals on surfaces, is a major threat to plant life. This phenomenon is not merely a surface coating; it triggers a cascade of physical and biological events within the plant’s tissues. Understanding how frost kills plants requires looking beyond the freezing point of water to examine the precise mechanisms by which ice formation causes cellular damage and eventual death.
The Critical Temperature for Freezing Injury
Plant tissues do not necessarily freeze the moment the air temperature drops to \(32^{\circ}\text{F}\) (\(0^{\circ}\text{C}\)). Water inside plant cells contains dissolved sugars and salts, which act as solutes and depress the freezing point, a process called supercooling. This allows the plant’s internal water to remain liquid well below \(0^{\circ}\text{C}\) without forming ice crystals. The true threshold for injury is the nucleation temperature, the point at which ice crystallization finally begins.
This temperature is often determined by the presence of ice-nucleating agents, such as certain bacteria or dust particles, which can initiate freezing between \(-2^{\circ}\text{C}\) and \(-5^{\circ}\text{C}\). The rate at which the temperature drops also determines the severity of the damage. A slow temperature decline allows the plant to manage ice formation safely, but a rapid drop can cause water to freeze instantly inside the cells, which is lethal.
Cellular Dehydration and Osmotic Stress
The primary mechanism by which frost kills most plants is not the physical bursting of cells, but severe dehydration. When temperatures fall below the nucleation point, ice crystals first form in the extracellular spaces, the gaps between cells. The water in these spaces, being relatively pure compared to the water inside the cells, freezes first.
As the extracellular ice crystals grow, they dramatically lower the water potential outside the cells, creating an osmotic gradient. Water then rushes out of the cells and across the plasma membrane to dilute the newly concentrated extracellular solution and deposit onto the growing ice crystals.
This rapid water loss causes the cell to shrink, a process known as plasmolysis, where the cell membrane pulls away from the rigid cell wall. The shrinking and resulting mechanical stress can irreversibly damage the delicate plasma membrane, which regulates all cellular transport. Cell death occurs because the compromised membrane can no longer function.
Visible Symptoms of Frost Damage
The microscopic cellular damage caused by dehydration quickly translates into macroscopic, visible symptoms on the plant. After a freeze, the formerly turgid tissues often appear water-soaked and translucent, which is a direct consequence of ruptured membranes leaking cellular contents. As the tissues thaw, they lose all structural rigidity, becoming limp and wilted because the cells can no longer maintain turgor pressure.
The characteristic darkening, turning brown or black, is a sign of necrosis, or tissue death. This discoloration occurs because the freeze-damaged membranes release compartmentalized enzymes and phenolic compounds from the cell. These chemicals mix and react with oxygen, causing rapid oxidation that manifests as a black or bruised appearance on leaves and stems. The speed and extent of this darkening indicate the severity of the internal cellular collapse.
Biological Strategies for Cold Survival
Some plants, particularly perennial species in cold climates, possess sophisticated biological mechanisms to tolerate freezing temperatures. The most significant of these is cold acclimation or hardening, a process triggered by exposure to gradually decreasing temperatures above freezing. This physiological adjustment prepares the plant for sub-zero conditions.
One key strategy involves increasing the concentration of solutes like sugars and the amino acid proline within the cells. This increase lowers the freezing point of the cytoplasm even further, reinforcing the supercooling mechanism and helping to retain water against the osmotic pull of extracellular ice. This effectively reduces the risk of lethal dehydration.
Plants also produce specialized antifreeze proteins (AFPs), which do not necessarily lower the freezing point significantly, but rather inhibit the growth of any ice crystals that do form. These proteins bind to the surface of small ice crystals, preventing them from growing into large, destructive masses. By limiting the size and shape of the ice, these proteins mitigate the mechanical damage that can occur in the extracellular space.