Geeking Out: Why does a well-hydrated plant survive colder temperatures than a thirsty one?

As I prepare for my upcoming Chemistry class, I thought it would be good practice to do a little bit of research into what happens to a plant at the cellular level when it freezes. Having no idea how complex the subject would be, I promised to share this information with any of you other geeks who might be interested (I know there are at least a couple of you out there). 

There are a lot of online articles covering the practical things you need to know about protecting your plants from frost damage, including a great publication by Linda Reddick, published by the Arizona Cooperative Extension.  While some people wouldn't consider it to be practical knowledge, this post is about how the plant's cells are damaged.  Why does a well-hydrated plant resist freezing better than one that is thirsty?

I've read this fact in numerous publications, and I've witnessed it to be true, but I had little understanding of exactly why it's true.  The answer is much more complex than I had thought. Beyond the fact that a well-hydrated plant simply functions better than one that is thirsty, there's a lot to know about how hydration relates to frost damage.

There are several ways that plant cells can be damaged by colder temperatures, but they do have a few tricks up their...uhh...cell membranes.  One of these tricks takes advantage of osmosis to decrease the temperature at which its insides can freeze.

This happens when ice crystals begin to form in the spaces between plant cells.  The solutes that were previously suspended within the water molecules are left behind when these crystals form.  Water molecules outside of the cells continue to form crystals, until the concentration of solutes (sugars, etc) in the water left behind lowers the solution's freezing temperature enough to keep a small amount of (highly concentrated) solution lingering between plant cells.  *Phew, that was a long sentence*

This extracellular solution begins to draw water from inside of the cell because its osmotic pressure is higher than the solution within the cell.  In turn, the solute concentration increases within the cell and lowers the freezing temperature.  

With this in mind, it’s easy to see how a well-hydrated plant cell would be better prepared for freezing temperatures than one that has little water to begin with.  A well-hydrated cell can give up more water without suffering damage associated with severe dehydration.

Of course, there are other ways that freezing temperatures can damage plant cells…but we’ll have to save that for another edition of Geeking Out.  For now, we've answered the question of exactly why a well-hydrated plant has better chances than a thirsty one when the temperatures get low.  

I'll leave you with one parting thought that should make a little bit of sense if you've read through this post.  Plants that are resistant to freeze damage are often also fairly tolerant of drought.