This is a Euphorbia plant showing plant frost tolerance at its best.When I took this photo the temperature was about -3 degrees C and everything around was very frozen, including my fingers.
Some plants are able to tolerate a lot of freezing, and when the weather warms up, they carry on exactly as before, and looking as if nothing had happened. This Euphorbia is a good example.
Some plants are not frost tolerant at all. I discovered this first-hand once when my plant growth room became confused by a heatwave outside and turned into a freezer inside. For a few hours the Kalanchoe plants inside looked like frozen versions of themselves, but when the room warmed up again they turned to mush.
As I understand it, this is because the water in their cells had frozen into shards of ice and punctured the cell walls. In their normal state, plants are like massive structures made entirely from water-filled balloons. The little balloons are the tiny plant cells. If a plant has no frost tolerance mechanism, then in a good freeze, the ice shards will pop all of the little balloons that make up a plant's structure. When the water thaws again there is no structure to hold the plant up, and the whole thing just melts down to mush.
Some plants, like the Euphorbia above, have mechanisms to prevent the cells from being popped. Scientists call these processes "cold acclimation".
Summarised in a recent-ish paper (Hannah et al., 2005), it seems that plants are invoking a whole lot of different schemes to keep their cells safe during the coming frost. Plants are turning the expression of hundreds of different genes on or off, or up or down. Consequently, a whole range of activities in the plant are altered. Growth changes, and the cell walls are adjusted to make them more able to withstand the different challenge of cold weather. Salts and water are moved around to different locations in the plant. As I understand it, this is for safer storage of the water, as it might puncture cell walls if it was allowed to freeze in the wrong place, or in the wrong way. Lastly the production of antioxidants goes up, though I'm not clear on why that is or what it does.
The real message for me is that cold acclimation is not just one single technique, or even five different techniques used by five different families of plants. It is a hugely complex system with many different layers of activity going on.
The other interesting point is that these processes seem to occur when the plant starts to feel a bit chilly, in preparation for the actual freeze to come. This means that if we take a frost tolerant plant from a warm place, and plunge it into freezing temperatures without warning, then it might not be nearly so happy about the situation. There's a warning in that for those of us who buy plants from warm shops and take them straight home to the garden.
So there we are. That's a little foray into the world of cold acclimation. As the winter weather ramps up this year, we will know that apparent inactivity of our garden plants is really just a graceful front. The plants have it all going on inside, as they get ready to glide effortlessly through another winter.