2. Temperature: The Pace-Setter for Life

Every plant has a set of cardinal temperatures—a minimum, optimum, and maximum temperature—for each of its biological functions. These temperatures are not fixed; they can differ between plant species, for different processes within a single plant (like photosynthesis versus respiration), and even at different stages of a plant’s life.

Waking Up from Winter: Temperature’s Role in Germination and Budding

Ever wonder why seeds you planted in the fall don’t sprout during a warm January thaw? This is the reason. Many plants native to temperate or colder regions have evolved a crucial survival mechanism: their seeds and buds will not break dormancy until they have experienced a required period of low temperatures. This ensures they don’t begin to grow during a brief warm spell in winter, only to be killed by a subsequent frost.

• Peach buds (Prunus persica) require 950 hours at 7°C or below to break dormancy.
• Blueberries (Vaccinium spp.) need about 800 hours at or below 7°C.
• Common ragweed seeds (Ambrosia artemisiifolia) must be exposed to moist conditions at 4–7°C for 6 to 10 weeks before they will germinate.

Surviving the Cold: How Plants Adapt

The process by which plants improve their ability to withstand low temperatures is called cold hardening. As temperatures drop in the fall, hardy plants undergo several cellular chemical changes that help them survive freezing conditions. According to Klages (1947), these changes include:

1. An increase in bound water (water less likely to freeze)
2. A decrease in total water content
3. An increase in the osmotic pressure of cell sap
4. The conversion of starch to sugars. This effectively lowers the freezing point of the water inside the plant’s cells, acting as a natural antifreeze.
5. An increase in cellular compounds called pentosans
6. The conversion of certain proteins to amino acids

Adaptation to Local Climates: Temperature Ecotypes

Plants are often genetically adapted to the temperature conditions of their geographic origin. A plant species that grows in both cool and warm regions will often contain different “ecotypes,” or populations adapted to their local climate. A study by Miller (1960) on two types of grass clearly demonstrates this principle.

While temperature governs the speed of a plant’s internal machinery, the sun provides the raw energy and critical signals that power it all: light.