Thermonasty And Chromoplasticity

How plants cope with extremes

There’s a theory that the natural world seeks equilibrium, and that after one extreme there is a sort of elastic band snap in the other direction. This was on my mind as I noted the fresh snow on the ground and the -6°C, that’s 21°F, on my van’s dashboard. Some of you will be thinking this is nothing – people garden in places with far colder winter temperatures than this – but bear in mind that the climate of Britain’s South West region is generally mild and benign; this sort of cold is highly unusual for an area that barely saw frost at all last winter.

Last winter was incredibly mild

Plants cope with cold in several ways. Some of course don’t cope at all; the cold destroys their tissues and turns them to mush. Many cacti and succulents are able to cope with remarkable amounts of cold providing they’re dry, and this can also apply to certain bulbs. Herbaceous perennials abandon their above-ground parts and retreat to resting structures below or at the soil’s surface, and most hardy woody plants drop their delicate leaves in favour of protecting their small buds, the construction of their bark and wood protecting the all-important vascular system from too much damage. Some plants have their own anti-freeze that prevents sap from freezing, while others are just incredibly tough and can cope with anything sent their way.

Evergreen plants find themselves at a disadvantage. Leaves must be thick and waxy to cope with cold, or in the case of cold-climate evergreen trees like abies and piceas have evolved into needles to protect themselves from extremes of cold. It’s interesting to note that the same principle is also employed by some trees and shrubs from hotter climates as protection from heat and drought.

Evergreens have to be tough in cold climates

We tend to have certain ideas about how plants should behave, and when plants don’t conform there is concern that they’re unwell. This usually the case when we see leaves curl up in cold weather; the assumption is that the plant is in difficulty, as is the case when we see the same behaviour during drought. However the rolling of leaves in winter is a different process, one called thermonasty.

I’ve fallen for the idea that certain evergreens roll their leaves because their water is frozen and thus unavailable. Essentially a very cold drought. However there are some big flaws to this idea; sure a plant that’s frozen solid in its pot will have less access to water, but if it’s really cold anyway then why would I expect the plant to be drinking water in the first place? And why do plants in the ground exhibit this behaviour when the frost has only penetrated a fraction of their root systems? Cold weather isn’t good for respiration, so plants go into a semi-dormant state until conditions improve.

These leaves curled inwards when it turned cold

Thermonasty is a defence mechanism for the plant. When a plant’s photosynthetic processes have slowed down it has a huge problem: the sun is still shining. With leaves unable to use that energy the sun’s rays risk damaging the cells in the leaf. In solar-electrical systems there is often a switch that cuts off the supply from the photovoltaic cells to prevent the battery from overloading; thermonasty is similar in that reducing the leaf surface exposed to sunlight reduces the chance of damage.

An alternative approach to this problem for plants is chromoplasticity. This is found in certain plants like cryptomeria and microbiota that experience very cold temperatures in the wild. The plants protect their precious chlorophyll and replace it with a reddish brown pigment; this acts like an internal sunscreen to protect the cells of the plant from sunlight light when they can’t use it.

Unlike thermonasty, which can be essentially turned on and off according to current conditions, chromoplasticity is a longer-term thing that lasts weeks or months. Plants that use this method to protect themselves, such as thuja, cryptomeria, junipers, microbiota, some rhododendrons and other evergreen shrubs, will turn a sort of reddish-brown or bronze colour during cold weather and turn green again in spring when temperatures have increased.

Microbiota decussata in green in summer and mild winters, but turns bronze when temperatures drop in winter

Not only is this an interesting process to be aware of, it’s also a useful thing to be discussing while British gardens have experienced a rather profound sudden shock of cold. In areas where cold winter temperatures are more common it’s more likely to be appreciated that certain plants change colour for winter, but it can also be rather disconcerting for new gardeners. It’s important to emphasise that these changes are not signs of the plant’s demise, just clever adaptations to the cold. Here in the South West it’s rare to see the bronzing of chromoplasticity even among plants that commonly exhibit it elsewhere, although with so many rhododendrons and camellias around we are a bit more used to thermonasty.

Not only should you not rush out to act but there’s very little you can actually do. A hardy plant is best left to its own devices. If a plant’s survival colouring and leaf curling lasts well into spring then that’s the time to worry.

In milder regions cryptomerias might only turn bronze at their extremities, where it’s coldest and the plant is most exposed, but in cold areas the whole plant changes

The most interesting example of chromoplasticity in action is the charming little Begonia soli-mutata. I must state at this point that Begonia soli-mutata is not a hardy plant. In this example the plant is well-adapted to very low light levels but is also able to protect itself if light levels increase for whatever reason. If you grow Begonia soli-mutata – soli meaning the sun and mutata meaning to change – in deep shade its leaves will be green, with the chloroplasts angled to capture as much sunlight as possible. However if the plant has a little more light at its disposal those chloroplasts change their angle to reduce how much light reaches them.

Again think to the switch turning off photovoltaic cells to prevent a battery from overcharging.

Note how the more hidden leaves of Begonia soli-mutata are green

The big thing with this behaviour in Begonia soli-mutata is that it happens very quickly, within minutes even. Growers of this remarkable little plant can block out light with a thick piece of card and expose the rest of the plant to bright light, and about 10 minutes later half the plant will be green and half the plant will be red. The effect will remain for as long as it takes for the green leaves to react to the light and turn red.

The reaction that some plants have to winter cold is a real bonus. I wrote recently about willows for winter colour; certain willows and stem dogwoods are greatly enhanced by cold conditions in the winter, thanks to the natural processes that protect their thin stems from winter sunshine. Sure their colouring will be presentable during a mild winter, but many of them almost glow when the weather is just that bit colder.

Dogwoods and willows are usually much more brightly coloured during cold winters

The message of this article is simple: plants are much more adaptable than we give them credit for. At this time of the year it’s fashionable among certain types of gardening communicators to recommend all sorts of ‘vital’ things that gardeners ‘must do’. Cold can be incredibly damaging to plants on the edge of their hardiness zones. Some plants are able to take cold conditions but suffer with cold winds, and there are plants that survive in cold regions where they’re routinely buried under snow but would be killed outright by less frost in non-snowy areas. There is also a question that I need to cover at some point by what we mean when we say a plant is ‘cold hardy’; is it OK to call a plant hardy if it simply survives, or is a truly hardy plant able to fully perform after a cold winter?

When it comes to plants that are known to be fully hardy in our areas we must have faith that the plants know what they’re doing, in the knowledge that what can look like damage or the plant in distress might in fact just be an incredibly clever adaptation to conditions.