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Thanks to numerous adaptations and variable responses, plants can survive over a wide range of temperatures. Most crucially, a plant can differentiate between hot and cold. Sensing temperature is essential to optimizing response.
Cold
Just as knowing when to flower is important, so too the need to anticipate the chill of winter. The pathways that protect plants from freezing that were inactivated in spring are prepared in autumn for the arrival of crisp weather. Seasonal sense and activity optimize allocation of resources.
Plants that move in response to temperature are thermonastic. The rhododendron evolved in the mountains where low temperatures are common. On a temperate day, a plant’s leaves are outstretched to soak up the Sun. But when the temperature drops below freezing, leaves curl inwards and roll up. Each leaf then droops to reduce the risk of frost damage.
Cooling slows the rate of photosynthesis and has marked effects on respiration. Protein synthesis is also inhibited.
Tropical plants are especially sensitive to chilling temperatures of 10–15 °C. Cold-sensitive plants have higher levels of saturated fatty acids in their cell membranes. Their membranes solidify more quickly when it turns cold than more tolerant plants.
Generally, highly unsaturated fatty acids, which are important in maintaining membrane fluidity, predominate in plants acclimated to cold climates.
Some plants avoid the worst of the cold by hiding from it. Belowground rhizomes, roots, and tubers are less susceptible to freezing than parts aboveground, which are sacrificed as necessary.
Soil temperature falls more slowly than air temperature. Subterranean plant parts have higher inbuilt frost resistance, as these organs suffer less heat loss.
Alpine and arctic plants get some protection from frost by a layer of snow. Nonetheless, these plants minimize aboveground presence.
The stress from freezing takes more of a toll on the intercellular spaces within a plant than it does on the cells themselves. Plants cells accumulate low-weight organic solutes, such as sugars and amino acids, to lower their freezing point. Such solutes only lower the freezing point a few degrees, but they also protect enzymes from dehydration.
Ice readily forms from particulate nuclei around which ice crystals materialize. Pure water freezes at –40 °C, not 0 °C, which is the transition point for normal water.
Some plants resist freezing by supercooling. Select tissues in cold-hardy plants seem as if they contain pure water. Solute content is largely absent. Such regions can be chilled to minus 38 °C before ice forms.
Such protection is particularly important for dormant buds and the xylem of woody plants during winter. While many cells suffer intercellular ice formation and dehydration, some tissues survive by supercooling.
Some trees grow in habitats that drop below –40 °C in the winter. The above tricks to reduce freezing point don’t work. Instead, intracellular freezing is prevented by withdrawing the cell’s water to the apoplast: the diffusional space outside the plasma membrane. Only thin layers of water molecules are left to protect macromolecules. In effect, plant tissues hibernate freeze-dried.
The temperature rarely drops below freezing suddenly in Nature. Usually, autumn ambient temperature gradually decreases for weeks before the first freeze.
During that time, plants anticipate future freezing. Only 1 or 2 days of near-freezing temperatures are enough to bring about acclimation.
Innumerable minute chemical changes, notably in proteins and polypeptides, create antifreeze properties. Metabolic changes for cold acclimation are epigenetically controlled.
Plants learn how to manage the stress of being cold. Acclimated tolerance improves as a plant matures.
