Photosynthesis and Temperature

Nothing... Photosynthesis does not occur at freezing temperatures. The rate of photosynthesis in relation to temperature forms a bell curve. At low temperatures the enzymes responsibility for photosynthesis have very little energy so the rate of photosynthesis is very slow. If it is cold enough for water to freeze it can be very harmful to a plant. As the temperature increases, the enzymes get more energy so the rate of photosynthesis increases. If it gets too hot the enzymes begin to lose their shape (denature). This means they are unable to function properly and the rate of photosynthesis decreases again. Also, at higher temperatures the stomata close to prevent water loss. This also stops gas exchange which slows photosynthesis even further. If it is hot enough for water to boil this can be very harmful.

Water is the problem in cold. Water expands when it freezes so the plant has to protect itself from this. If a plant cell freezes, it does what a frozen water pipe does: it ruptures.

Plants use two methods to become dormant so they prevent the damage. They stop all reactions that require water. This means they stop growth, they store food for spring start up, then they use plant antifreeze to avoid damage.

They do this when the day gets short before the first frost date usually.

If the region gets well below freezing the plant may do more to protect itself. They also move water out of cells leaving the substances that tolerate cold without freezing inside. Water sits frozen between cells but not breaking the cell open.

In extreme dormancy they also count the number of hours they spend below freezing. Plants in this phase can not wake up til enough cold time has passed so if a few warm hours happen they do not respond. These plants can survive zones 1- 3.

Basic life support or basal respiration continues year round in trees that under go ectodormancy in regions of zone 4 or higher. Without chloroplasts they may not photosynthesize but respiration continues in mitochondria throughout the sapwood volume of the plant. The maintenance respiration rate continues depending on the temperature in the woody tissue. Stem respiration increases with spring bud break over several weeks but bole respiration is only slightly increased over winter levels. There is no above ground growth during dormancy but the plant continues basic metabolic functions and minimal below ground growth/repair as long as the water remains unfrozen.

Herbaceous perennials expect and allow die back of all above surface structures protecting only the root and crown to regrow from.

Sometimes temperatures are used in connection with day length to manipulate the flowering of plants. Chrysanthemums will flower for a longer period of time if daylight temperatures are 50°F. The Christmas cactus forms flowers as a result of short days and low temperatures.

Temperatures alone also influence flowering. Daffodils are forced to flower by putting bulbs in cold storage in October at 35 to 40°F. The cold temperature allows the bulb to mature. The bulbs are transferred to the greenhouse in midwinter where growth begins. The flowers are then ready for cutting in 3 to 4 weeks.

Thermoperiod refers to daily temperature change. Plants produce maximum growth when exposed to a day temperature that is about 10 to 15°F higher than the night temperature. This allows the plant to photosynthesize (build up) and respire (break down) during an optimum daytime temperature, and to curtail the rate of respiration during a cooler night. High temperatures cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to occur, photosynthesis must be greater than respiration.

Low temperatures can result in poor growth. Photosynthesis is slowed down at low temperatures. Since photosynthesis is slowed, growth is slowed, and this results in lower yields. Not all plants grow best in the same temperature range. For example, snapdragons grow best when night time temperatures are 55°F, while the poinsettia grows best at 62°F. Florist cyclamen does well under very cool conditions, while many bedding plants grow best at a higher temperature.

Buds of many plants require exposure to a certain number of days below a critical temperature (chilling hours) before they will resume growth in the spring. Peaches are a prime example; most cultivars require 700 to 1,000 hours below 45°F and above 32°F before they break their rest period and begin growth. This time period varies for different plants. The flower buds of forsythia require a relatively short rest period and will grow at the first sign of warm weather. During dormancy, buds can withstand very low temperatures, but after the rest period is satisfied, buds become more susceptible to weather conditions, and can be damaged easily by cold temperatures or frost.

Review of Temperature Effects on Plant Growth:

Photosynthesis: Increases with temperature to a point.
Respiration: Rapidly increases with temperature.
Transpiration: Increases with temperature.
Flowering: May be partially triggered by temperature.
Sugar storage: Low temperatures reduce energy use and increase sugar storage.
Dormancy:Warmth, after a period of low temperature, will break dormancy and the plant will resume active growth.