The lowest temperature a plant can survive has no single answer, as the limit varies dramatically across species, tissues, and environmental conditions. Plant hardiness is defined by a species’ ability to endure seasonal cold, and this ability determines its geographical distribution. The challenge low temperatures pose is the freezing of water, the solvent for all biological processes. Plant survival depends on whether the species can prevent ice from forming inside its cells or tolerate the damage caused by ice formation.
The Cellular Mechanisms of Cold Injury
Cold temperatures inflict damage on plants through two distinct physiological conditions: chilling injury and freezing injury. Chilling injury affects tropical and subtropical species that are damaged by low, non-freezing temperatures, typically between \(0^\circ \text{C}\) and \(15^\circ \text{C}\). These plants have cell membranes adapted to warmth, and cold exposure makes the lipid bilayer stiff and brittle, disrupting its function in nutrient transport and respiration.
Freezing injury, which occurs below \(0^\circ \text{C}\), involves the physical formation of ice crystals and presents two primary lethal mechanisms. The first is extracellular freezing, where ice forms in the intercellular spaces outside the living cells. Water is drawn out of the cell through osmosis to maintain equilibrium, leading to severe cellular dehydration.
This dehydration concentrates solutes and salts within the cell, which can be toxic and disrupt essential metabolic processes. The cell membrane can also be mechanically damaged as the cell shrinks and collapses against the rigid cell wall. The second, and typically immediate, lethal mechanism is intracellular freezing, where ice crystals form directly inside the cytoplasm.
Intracellular ice formation is fatal to nearly all plant cells because the crystals mechanically rupture the cell membrane and organelles. This type of freezing is avoided by hardy plants but can occur in unhardened tissues or when the temperature drops too rapidly.
Biological Strategies for Developing Cold Tolerance
Plants inhabiting temperate and arctic regions have evolved sophisticated physiological mechanisms to survive freezing temperatures. The most significant is cold acclimation, or hardening, where plants increase freezing tolerance after exposure to mild cold and shorter days. This preparatory process involves genetic changes, such as activating the CBF (C-repeat Binding Factor) regulon, which signals the production of protective molecules.
A primary defense is the accumulation of compatible solutes, or cryoprotectants, within the cells. Sugars (like sucrose and raffinose), alcohols (like glycerol), and amino acids (like proline) act as natural antifreeze, lowering the cytoplasm’s freezing point. These osmolytes also stabilize cell membranes and proteins, protecting them from damage caused by dehydration during extracellular freezing.
Another crucial strategy is supercooling, a process of freezing avoidance where water remains in a liquid state even at sub-zero temperatures. This is achieved by removing ice-nucleating agents from the cell interior and increasing the concentration of solutes. While supercooling protects the liquid contents inside the cell, it has a physical limit: water will spontaneously freeze through homogeneous nucleation at approximately \(-40^\circ \text{C}\).
Woody perennial plants and seeds enter deep dormancy, involving metabolic shutdown and significant tissue dehydration. This reduction in cellular water content prevents ice crystal formation and allows tissues to withstand extremely low temperatures. Specialized protective proteins, such as dehydrins, accumulate during this period, stabilizing membranes and macromolecules against freezing and dehydration stress.
Categorizing Survival Thresholds
Plant survival thresholds are classified based on their tolerance to minimum winter temperatures. Tender, or tropical, plants are the least cold-tolerant, with species like bananas suffering injury at temperatures as high as \(10^\circ \text{C}\) or \(15^\circ \text{C}\). Semi-hardy perennials and temperate crops tolerate light frost, surviving brief drops below zero, especially if they have undergone cold acclimation.
The most common system for predicting survival thresholds is the USDA Plant Hardiness Zone Map, which divides geographical areas based on the average annual extreme minimum winter temperature. Each zone represents a \(10^\circ \text{F}\) band, providing a standardized measure for gardeners to select plants that can survive the coldest night of the year. For example, a plant rated as hardy to Zone 6 is expected to survive where the minimum temperature averages between \(-10^\circ \text{F}\) and \(-5^\circ \text{F}\) (\(-23.3^\circ \text{C}\) to \(-20.6^\circ \text{C}\)).
Extremely hardy species, such as boreal conifers like spruce and pine, represent the upper limit of natural cold tolerance in living plant tissues. These species, which dominate the Siberian and Canadian taiga, are adapted to environments where temperatures frequently drop below \(-40^\circ \text{C}\). Their tissues can survive this through a combination of supercooling and deep tissue dehydration, allowing them to withstand temperatures down to \(-60^\circ \text{C}\).
The absolute lowest temperature a plant structure can survive is found in specialized tissues that have been completely desiccated, such as seeds or dormant buds. These structures can be cryopreserved by immersion in liquid nitrogen at \(-196^\circ \text{C}\). At this temperature, all biological processes cease, and water is vitrified into a glass-like state, preventing the formation of damaging ice crystals, allowing for indefinite survival.