Autumn is the season of yellow, red and orange. More than the green summer and the flowery spring, autumn is when plants impose their colors and give this season its uniqueness. In this period leaves gradually lose the color they maintained throughout the year. While days get shorter and temperature gets lower, they acquire a warmer, yellow-orange coloration. It is precisely the cold and the decrease of daily light hours that trigger the process leading to the change of leaves colors and, eventually, to their fall.
The green is due to chlorophyll, the pigment responsible for capturing sunlight. The energy coming from the sun is later used for the production of sugars, in a process known as photosynthesis. Chlorophyll is essential for life on this planet, because it is one of the few existing molecules that are able to channel energy into biological systems. However, it is as important as fragile. Plants indeed invest a good amount of energy in replacing damaged chlorophyll with fresh chlorophyll molecules in order to keep photosynthesis efficient. During autumn, when light and heat are lacking, plants don’t have enough energy to invest in chlorophyll production. Therefore, when their chlorophyll is damaged, they cannot replace it with fresh one and leaves lose their greeness.
Chlorophyll is not the only pigment in leaves: among others, there are yellow–orange flavonoids and carotenoids carrying out accessory functions in photosynthesis. For instance, they protect leaves from excessive sunlight exposure. Different colors absorb and filter different light wavelengths, which otherwise might damage essential life molecules – DNA, RNA and proteins – inside leaves. This is why plants contain these pigments at any time of the year, even when their color is not visible because is hidden by the greeness of chlorophyll. When, in autumn, chlorophyll abundance decreases, flavonoids and carotenoids become finally visible, and give plants their autumn look. Even if also the production of these pigments stops with cold, the pre-existing ones are more stable than chlorophyll and will remain throughout the season.
And what about red? It is due anthocyanins, pigments that are common in red fruits and are known for their beneficial effects in our diets for their antioxidant properties. Unlike chlorophyll and carotenoids, anthocyanins are not normally present in leaves, but some plant species produce them when leaves start to turn yellow. It is not clear which is the function of these pigments nor why plants invest resources in their production during a season when energy begins to run low. Some speculate that the synthesis of anthocyanins and their storing in leaves might help disposing of toxins along with the leaves when they fall. Others hypothesize that the red color act as a warning signal to discourage herbivorous insects, or as a further protection for a less and less efficient photosynthetic systems.
Triggered by external factors, the leaf senescence that leads to color change and leaf fall is actually finely controlled by plants at the genetic and physiological level. The reason why plants lose their leaves is to replace them with fresh and more efficient ones. Evergreen plants such as conifers (pines and firs, for instance) do it continuously throughout the year, while deciduous plants lose all of them at the same time just before the critical season. In our climates, this corresponds to winter, during which light availability is low and the risk of freeze damage is high. Therefore, leaf maintenance costs outweigh the benefits of photosynthesis, which is limited by the length of nights and small amount of light.
In their survival strategy, plants perceive the lengthening of the daily dark period that occurs in autumn and start dismantling leaf structures in order to recycle their constituents. At the same time, the transport of nutrients towards leaves becomes limited by structural changes occurring at the base of each leaf, between the petiole and the branch to which it is attached. Here lies the abscission zone, a layer of cells that is degraded as autumn proceeds. At the end of the process, the connections between leaves and branches are so weak that some wind is enough to make them fall.
Before winter comes to take away all the leaves, we have a few weeks to enjoy the autumn colors that are just beginning to appear. Lets do like the Japanese do: they might be better known for the cherry blossom contemplation during hanami, but in this season, during the momijigari, they go “hunting for red leaves”.
References and further readings:
– Compound of interest, The Chemicals Behind the Colours of Autumn Leaves
– Feild, T., Lee, D., & Holbrook, N. (2001). Why Leaves Turn Red in Autumn. The Role of Anthocyanins in Senescing Leaves of Red-Osier Dogwood PLANT PHYSIOLOGY, 127 (2), 566-574 DOI: 10.1104/pp.010063
– Schippers, J. (2015). Transcriptional networks in leaf senescence Current Opinion in Plant Biology, 27, 77-83 DOI: 10.1016/j.pbi.2015.06.018
– For US residents: here is when autumn colors will peak in your area this year
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