Colleen Doherty is an assistant professor of molecular and structural biochemistry at N.C. State. Here she explains how some plants manage to survive cold temperatures relatively unscathed. Questions and answers have been edited.
Q. What happens if water freezes inside a plant?
A. Freezing temperatures are bad news for a plant. Ice crystals can damage the cell membranes and cause the cells to rupture. The fluids surrounding their cells can freeze solid, ultimately causing plants to become dehydrated. In addition, low temps can sap them of their energy. Plants use photosynthesis to turn light and carbon dioxide into sugar, but in the winter the chemical reactions to make energy have slowed down. The energy that can’t be used damages the inside of the cell. Some plants avoid this issue by shedding their leaves and taking things up again the next spring when the going gets better. However, evergreens are amazing and keep going all winter.
Q. How can plants endure freezing temperatures?
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A. Some tropical plants can’t tolerate any low temperatures. Other plants can handle chilling, but are in trouble when temperatures drop below freezing. However, some plants are freezing tolerant and can endure subzero temperatures.
Even then, these plants can only survive freezing after exposure to cool temperatures. This is what we call cold-acclimation. At a temperature just above freezing, it is like the plant puts on a coat. The plant flips on a special protein called CBF, which acts like a master switch to turn on many other proteins, ultimately making the plant freezing-tolerant.
So are plants that don’t survive freezing missing these CBF “winter coat” proteins? Surprisingly, almost all plants have CBF proteins. For example, tomato, which is very sensitive to freezing, has at least three copies of CBF, but somehow they don’t do a very good job of switching on the other proteins needed to tolerate the cold. Researchers are trying to understand why these proteins work in some plants and not others.
Q. Is it true that some plants cope with the cold by producing antifreeze proteins?
A. Some of the proteins turned on in response to the cold make chemicals called cryo-protectants – compounds that protect the plant from dehydration. Other proteins specifically prevent water from recrystallizing. These antifreeze proteins have some differences from the antifreeze we put in our cars. For example, antifreeze proteins can work at much, much lower concentrations than car antifreeze.
Q. You study how plants use changes in time and temperature to decide how to respond to stress. Can plants really tell time and make decisions?
A. Yes! Just like us, plants have an internal circadian clock they use to tell time. Plants need their clock for almost everything. They use it to get ready for photosynthesis before dawn, so they can capture the first rays of light. The clock helps plants know when a particular herbivore – say, a caterpillar that is only active in the day – is most likely to attack. The clock is also important for freezing tolerance because a working clock is needed to know the seasons are changing. Plants use their clock to keep track of the changing length of days – the longer days in summer and shorter days in winter. This can partly explain why plants are so vulnerable to a late frost in the spring or an early frost in the fall.