Autumn Craze: the biochemistry behind the Autumn colors

Having lived my entire life in Guatemala (except for the last 3y), you can imagine there was not a lot of Autumn craze in my life. I mean, unfortunately, we have a lot of US-influence when it comes to the absurd amount of crap they try to sell to us in the capital city, so Halloween-related-Autumn, orange-and-black, pumpkin-spice, trick-or-treating was always around. But the trees did not turn orange synchronously in October and the days did not get shorter. Not by more than an hour anyway. Of course, we do learn about the higher latitude parts of the world and their four seasons, amongst other things. As a Biology bachelor I eventually learned more about the “why” behind Autumn colors. However, some other questions arise when you see Autumn happening before your eyes.

Back to basics first: are trees really changing color?

Maybe changing is not the correct word, although for sure change can be more romantic than “decolorating”. You might recall the good old photosynthesis: plants taking light energy and CO2 to create their own food with the help of the chlorophyll pigment. This pigment is what gives most plants their green color. However, other pigments are also always there, masked by the center-of-attention that is chlorophyll. These other pigments (mostly different types of carotenoids – yes, like carrots!) play different roles, like giving plants some sunscreen. Yes, plants get sunburn too! In Autumn, as conditions begin to change, plants begin to dismantle themselves and as a first step chlorophyll production decreases. With less chlorophyll, now the other pigments can be better appreciated. Different plant species have various proportions of all these types of pigments, and it is their unique combinations that create all the shades of Autumn.

A common misunderstanding is that the cooler temperatures are what incites this change in chlorophyll production. Truth is, the key trigger is the shorter period of sunlight per day (a.k.a. photoperiod). Plants sense these fewer hours of sun and know it’s time to change. As chlorophyll production decreases and the green fades away, the other pigments show, and even a new type of pigment is formed: anthocyanins. These are responsible for the red-pink-purple tones.

But why do anthocyanins form only at this time of the year?

It’s not just to give us a nice crimson scenery, they have a purpose of their own. But to get to that we need to briefly talk about why plants fade and lose their leaves before Winter. The same way our bodies create heat in cold temperatures to try to keep our main organs warm, trees would also need to spend a lot of energy to keep their leaves fit and green during the winter months. As this is very expensive (energy-wise), several temperate species cope with this by getting rid of them and only re-grow new leaves in Spring.

As the senescence period beings, not only does chlorophyll production decrease but the plants also begin to break down the existing chlorophyll and other molecules. This brings out some nutrients that the plant can store for future use by reabsorbing them. By reabsorbing I mean transporting them away from the leaves and back into the trunk and roots.

To have the most time as possible to re-absorb all they can from the leaves, anthocyanins come into play. Anthocyanins develop as a protective agent that can give the plant more time for the resorption of nutrients even under photoinhibition periods. It’s believed that they help maintain the phloem connectivity (the plants “veins”). This improves and prolongs this exchange during the stressful changing conditions. For plants that don’t produce anthocyanins, the leaves appear to fall sooner (and also they never turn red, of course).

How are all the leaves falling anyway?

Since the beginning of Autumn, another process has been going on at the petiole (the little stem that attaches the leaves to the branches). In Spring, an abscission layer was formed there: a layer of tightly connected cells that are particularly sensitive to a plant hormone called auxin. With Autumn, the auxin flow decreases and this signals the cells in this layer to start elongating. As they change shape, the tight connections are lost. This makes the bond between the leaves and branches weak, so they can now fall by gravity, wind or rain.

Now, all this does not mean the photoperiod is everything. Even though it is the change-inducing spark for most species, temperature and precipitation also matter. They play a role in the duration and intensity of the Autumn shades. Plant biologists can actually “predict” how vibrant Fall will be depending on the temperature and precipitation of late summer and early autumn days.

Enter Climate Change

We can define the temperate seasons both by a meteorological definition and an astronomical one. I guess that’s pretty self-explanatory, even if to an extent they are related. By now you probably already guessed that the astronomical definition, the one that defines the photoperiod, is not going to change the same way as the meteorological one with climate change. Climate change will likely have an impact on the duration of the golden Autumn scenery. As a matter of fact, in some parts of the temperate regions, the summers are now around 14 days longer and winters around 20 days shorter for quite some years already (and the “tropics” limit has been expanding, but I’ll leave that for another time).  

As fascinating as the theory is, it has been really amazing to see it happening in front of my eyes for the fourth time now and I hope you all get a chance to see this at least once! But I still think things are simply cooler when you understand what’s going on “behind the scenes” (thanks, science!).

If you ask me, Autumn is just a warning sign that the freezing unbearable Winter is coming, and with it the point where I question my life choices on a daily basis (particularly moving away from the tropics). But hey, it does look beautiful!

References

Schaberg, P.G. et al. 2017. Experimental Branch Cooling Increases Foliar Sugar and Anthocyanin Concentrations in Sugar  Maple at the End of the Growing  Season. Canadian Journal of Forest Research 47(5):696-701.

Johnson, M.P. 2016. Photosythesis. Essays in Biochemistry 60(2016):255-273

Michelson, I.H. et al. 2017. Autumn Senescence in Aspen is not triggered by day length. Physiologia Plantarum 1(162)

Edlung, E. 2016. Regulatory Control of Autumn Senescence in  Populus tremula. Thesis.