The temperature at which a plant dies has no single answer. Plant fatality results from thermal stress, which causes irreversible damage to cellular machinery at both extremely high and low temperature ranges. The specific lethal temperature varies dramatically depending on the plant’s species, its natural habitat, and whether the temperature change occurs suddenly or gradually. Understanding plant death requires examining the distinct cellular breakdowns that occur when temperatures push beyond biological limits.
The Thresholds of Cold Damage
Plant death from low temperatures occurs through two distinct mechanisms: chilling injury and freezing injury.
Chilling Injury
Chilling injury affects sensitive plants, typically tropical or subtropical species, when exposed to low, non-freezing temperatures (0°C to 15°C). The primary cause of damage is the rigidification of cell membranes, as lipid components transition from a fluid state to a semi-solid gel. This loss of fluidity disrupts embedded proteins and enzymes, leading to metabolic imbalances, slowed growth, and eventual tissue collapse.
Freezing Injury
Freezing injury happens below 0°C and involves the formation of ice crystals. Ice typically forms in the extracellular spaces first, drawing water out of the cells in a process called freeze-induced dehydration. This water loss concentrates the cell’s internal solutes, which lowers the freezing point of the remaining intracellular water and prevents lethal ice crystals from forming inside the cell.
If the temperature drops too rapidly or too far, the cell wall cannot contain the pressure of the dehydrated cell, leading to cellular collapse. Conversely, ice nucleation can occur inside the cell if the temperature drops below the cytoplasm’s supercooling point, resulting in swift intracellular freezing. Ice crystals forming within the protoplast cause physical rupture of organelles and cell membranes, leading to rapid, irreversible death. For most non-acclimated temperate plants, the lethal threshold often falls between -5°C and -10°C.
The Mechanisms of Lethal Heat
High temperatures kill plants by causing biological failure within the cells, generally beginning above 40°C.
Protein Denaturation
One immediate lethal mechanism is protein denaturation, where heat causes essential enzymes and structural proteins to unfold and lose their shape. This irreversible destruction halts metabolic processes, including photosynthesis and respiration, leading to cellular failure. Temperatures between 45°C and 55°C are typically lethal to most temperate plants if sustained for more than a few minutes.
Membrane Destabilization
Lethal heat also directly impacts cellular membranes, causing destabilization. Elevated temperatures increase the kinetic energy of membrane lipids, leading to excessive fluidity. This makes the cell structure permeable and leaky. This loss of integrity causes ions and essential cellular contents to escape, disrupting the chemical gradients necessary for life.
Water Stress and Desiccation
A third mechanism involves water stress, or desiccation, amplified by high temperatures. Heat dramatically increases the rate of transpiration (water evaporating from the leaves), overwhelming the plant’s ability to replenish lost moisture from the roots. The plant cannot transport water quickly enough to cool itself and maintain turgor pressure. This rapid, uncontrolled water loss leads to wilting and death.
Acclimation and Species Specificity
The wide range of lethal temperatures is explained by a plant’s genetic makeup and its ability to undergo acclimation, or “hardening.” Acclimation is a gradual physiological adjustment to non-lethal, pre-stress temperatures, allowing the plant to survive extremes. For cold acclimation, plants alter cell membrane composition to maintain fluidity and accumulate cryoprotectants, such as specialized sugars, which lower the cytoplasm’s freezing point.
The baseline survival range is determined by the species’ habitat of origin. Tropical rainforest species possess no cold-acclimation mechanism and can be killed by temperatures just above freezing. Conversely, xerophytes, like desert cacti, have evolved specialized heat-tolerant proteins and mechanisms to slow water loss, allowing them to withstand temperatures exceeding 60°C for short periods.
The ultimate lethal temperature is influenced by external environmental factors. The duration of exposure is important, as a short burst of extreme temperature is often survivable, while prolonged exposure can be fatal. Factors such as high humidity reducing evaporative cooling or wind increasing desiccation can significantly lower a plant’s effective thermal limit.