Winter is here

Frigid temperatures in Arkansas this weekend have inspired an icy topic.  If you’ve ever wondered why your lettuce wilts when it accidentally freezes in the refrigerator, or your basil dies after the first frost then this post is for you. 

Contrary to what I believed and maybe what some of you do to, plant cells themselves rarely freeze. The water in between cells freezes much more readily than the cells themselves; this is the start of the plant’s problems. 

Dehydration is the most common culprit for cell death at cold temperatures. It seems counter intuitive that dehydration would occur as a result of freezing water, but it makes sense when you begin to think like a plant cell. All living cells exist in a state of equilibrium with their surroundings. Ions, gases, small molecules and water are constantly moving around the plant, going in and out of cells as needed. The concentrations of these species inside and outside of the cell are carefully regulated by the plant. Since cells have stringent rules on what kinds of molecule can cross the cell membrane, often solute concentrations are kept at the optimum level by osmosis. 

Osmosis is the diffusion of water across a semi-permeable membrane, that is, a membrane that only some molecules can cross. The movement of water during osmosis depends on the concentration of solutes on either side of the membrane. If the concentration of solutes inside of the cell is high compared to outside of the cell, water will move into the cell and vice versa. So how does freezing induce dehydration?

As I mentioned earlier, water on the outside of the cell freezes first. As this extracellular water freezes, the effective concentration of solutes outside of the cell increases. The plant attempts to alleviate this change in concentration by sending water out of its cells. But at frosty temperatures more and more extracellular ice is likely to form and more and more water leaves the cells to compensate. 

Eventually the strain on the cell is too high; the cell wall and membrane crumble and the cell will be unable to regain its shape, even after thawing. Hence, the sad looking tree outside my window and wilted lettuce in a fridge that is too cold.

You may be wondering about the plants that do tolerate freezing, like winter wheat. Indeed, some plants are equipped to handle temperatures down to -40 and -50 degrees Celcius! There is some evidence that special proteins called antifreeze proteins (AFPs) are partially responsible. While antifreeze proteins can’t prevent water from freezing, they do change the size and shape of ice crystals and they affect which part of the plant freezes first, giving the plant a fighting chance in a chilly environment. I wish I could get some antifreeze proteins in my toes, which feel like they have ice crystallizing inside them…

Sources and further reading:

Atici, O; Nalbantoglu, B. 2003. Antifreeze proteins in higher plants. Phytochemistry 64: 1187-1196.

Pearce, R S. 2001. Plant Freezing and Damage. Annals of Botany 87: 417-424.

Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 15.8, Osmosis, Water Channels, and the Regulation of Cell Volume. Available from: http://www.ncbi.nlm.nih.gov/books/NBK21739/
Background image from: http://home.howstuffworks.com/lawn-garden/professional-landscaping/basics/10-winter-plants.htm