On December 22nd, 2015 we learned about

The ways plants endure winter and the danger of internal icing

While water is critical to life on Earth, in cold temperatures it can also mean death. Low temperatures can freeze water inside an organism, creating hard, sharpened ice-crystals that can slice up or explode soft tissues from the inside. Animals usually avoid this kind of damage by migrating away from the cold, growing layers of waterproof insulation, or running inside to drink hot coffee under a snuggie. Since plants aren’t mobile, they’ve had to evolve other mechanics to survive winters in good enough shape to be ready for spring.

Expire before icing

The first trick plants employ is to minimize their exposure to the cold. On one extreme, this means avoiding the cold by dying before temperatures get too low. Summer annual plants, like peas, and most domesticated grains, grow, mature and reproduce in the warmer months, leaving their seeds to germinate over the winter.

Leave fewer leaves

For plants with longer life-spans, a more moderate scaling back means dropping leaves in the fall, leaving only heartier, more enclosed stems and trunks to deal with winter’s cold. Flat, green leaves can be a liability in the winter not only because they can collect heavy, branch-breaking snow, but they also offer a lot of surface area for water to freeze, even when there’s no precipitation. The same pores that are key to the leaves’ respiration in warmer months also expose the plant’s water stores to cold, and so dropping them for darker winter days, when the plant isn’t photosynthesizing as much, avoids that risk.

Evergreens’ exposure

Since they keep their needle-shaped leaves year-round, evergreen trees need further protection still. Their thin, narrow leaves not only avoid snow build-up, but also minimize their exposure to cold. Internal moisture is further protected through the waxy outer layer of the needle, which acts as insulation.

Isolating the ice

Internally, many plants safegaurd against water freezing with forms of antifreeze. The most basic version of this is sugar, conveniently already available as part of the plant’s food production. Mixing sugar into water lowers its freezing point, and so keeping those elements circulating together in the plant is a low-cost way to avoid freezing on the inside even when pure water may be freezing outside. The next degree of protection comes from specialized proteins, which are created only when temperatures start to drop outside. These proteins surround tiny ice crystals as they start to form, isolating them from contacting other water molecules. This keeps them from growing big enough to really destroy cell walls and other delicate biology in the plant. Spruce trees use this mechanism to create a multitude of tiny but isolated ice crystals when temperatures get too low, minimizing the damage they’d cause compared to what would happen with fewer but larger structures growing within the leaf.

Importantly, all these methods are reversible. Isolated ice crystals can thaw. Leaves can be regrown, or new seeds can germinate. This way, plants can be ready to grow again in warmer months, since they don’t otherwise have the luxury of picking their favorite weather as the seasons change.

Source: How plants prepare for winter by Stephen Heard, Scientist Sees Squirrel

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