QotW: Why don't plants freeze to death?

The answer to this was coming up cold until James Tytko found Proffessor Howard Griffiths from the University of Cambridge...

Howard- Plants from cooler climates have a number of mechanisms which allow them to get used to lower temperatures- as winter approaches, their metabolism adjusts- and plants become ‘hardened’ to the cooler conditions during autumn.

James - And how do they do it?

Howard - To avoid chilling damage, plants change the composition of cell membranes and incorporate shorter unsaturated fatty acids - exactly the ones that are good for us! These changes prevent the membranes becoming more solid, or gel like, under chilling temperatures, and help to maintain normal metabolism.

James - So, eating your greens is even more beneficial in winter due to the plants’ anti-chill techniques. A silver lining to those endless grey clouds, I suppose. But what about freezing?

Howard - In order to avoid damage from ice crystals during freezing, many of our plants shed their leaves in winter- becoming ‘deciduous’.

For the plants which keep their leaves, they can tolerate freezing temperatures by “supercooling”, which can protect plants down to about -12oC. This prevents what’s known as nucleation: the first step in ice formation.

James - Supercooling, nucleation. You’d be forgiven for thinking these were processes going on in some sprawling thermal power plant - but this is all going on inside the most ordinary plants we encounter every day, as Howard explains:

Howard - It’s essentially the same thing as when we add antifreeze chemicals to our car radiators, and salt to de-ice roads: these solutes help to lower the temperature at which water freezes. Plants also accumulate extra solutes in their tissues, and can also make specific antifreeze proteins or ‘cryoprotectants’ which prevent ice formation and help to lower the freezing point of cell tissues.

James - That’s why you shouldn’t walk over a lawn while the grass is still white with frost. The grass underfoot may be supercooled, with your clumsy steps disrupting this delicate process, causing ice crystals to form and putting an end to the poor grass’ resistance.

Howard - Other plants like conifers, which survive extreme temperatures in Siberia and northern Canada, have thickened cell walls and repackage proteins and pigments to prevent damage, and can also tolerate deep supercooling down to -40oC.

James- As seen on TV. David Attenborough covered the boreal forest at the start of episode 3 of The Green Planet if you’d like to see these amazing trees for yourself.

Howard - These processes can have added benefits for us humans as well. Maple syrup, for example, is made from sugar solutions which maple trees use to repair and refill their water transport cells (or ‘xylem’) after surviving a long, cold winter.

James - The sweet taste of victory.

Well, Krzysztof, I hope that answers your question. Plants have a few ways to beat the chill, from changing the makeup of their membranes by incorporating shorter, unsaturated fatty acids, to lowering the temperature at which water freezes within them via additional solutes. It’s all rather remarkable, isn’t it?