By Lydia Tate
As the weather across New England becomes increasingly brisk, the beautiful colors of autumn begin to develop in different tree species and signal the start of the season. As an avid fan of fall foliage, I look forward to this time each year and the fascinating transformation that trees undergo. However, this annual process that most residents of New England don't bat an eye at is still a mystery to scientists in many ways.
The green coloration of leaves is due to a high concentration of chlorophyll, a green pigment found in plant cells. Autumnal colors such as yellow and orange occur because of a different pigment known as carotenoids, which are usually masked by the color of chlorophyll but become more visible as chlorophyll breaks down in colder weather. Anthocyanins are the pigment responsible for producing the red and purple colors present in leaves of trees and plants alike. Unlike chlorophyll and carotenoids, anthocyanins are not present in plant cells at all times and are instead produced just before the leaves fall.
The main issue at hand is the question of why leaves that are about to fall turn red, and why only select plant species change color. There are many hypotheses about the reason behind changing colors, but Archetti et al. (2009) suggests that photoprotection and coevolution are two adaptive explanations for red pigments.
Photoprotection is generally defined as the protection against light, and the photoprotection hypothesis suggests that plants can have enhanced resorption of nutrients if they are protected from the harmful effects of light. This hypothesis essentially states that anthocyanins act as sunscreen in lower temperatures and relieve photooxidative stress (damage due to the sun), which is caused by too many photons, or light particles, being absorbed in the chloroplast's electron transport chain and creating reactive oxygen species (ROS). ROS are molecules that contain oxygen and are highly reactive, and they have the potential to build up, damage DNA, and cause cell death. The risk of light damage is higher in autumn for several reasons. These include that cold temperatures reduce a plant cell's capacity for carbon fixation, there is increased light from a thinning cover of leaves, and chlorophyll breaks down so they provide less self-shading.
One study supports the photoprotective hypothesis by indicating that red leaves are less light stressed than non-red leaves under photo-inhibitory conditions. Additionally, red leaves share characteristics of shade plants when compared to green leaves of the same species. However, there is a lack of indisputable evidence since the study only measures the protection of photosynthesis reactions in chloroplasts.
The evidence supporting better nutrient resorption as an adaptive function is less clear. One early study found no difference in the absorption of nitrogen between species with anthocyanins and those without, which is inconsistent with the photoprotection hypothesis. On the other hand, different studies have found a negative correlation between anthocyanin and nitrogen in many New England species. Additionally, evidence indicates that phenotypes containing anthocyanins are able to translocate more nitrogen than individuals of the same species without anthocyanin. Taken together, this links anthocyanins with nitrogen translocation, but no mechanism is currently described to show how they would enhance nutrient uptake.
The coevolution hypothesis is a different adaptive explanation for red pigment deposits, which suggests the color red is a warning signal aimed at reducing insect attack. In this hypothesis, red leaves signal a higher level of chemical defense or a lower nutritional quality to aphids, a subgroup of insects. Aphids lay their eggs on trees in the autumn so they can hatch in the spring, meaning it is important that they choose a suitable type of tree. However, they can cause damage to the tree through feeding, viruses, and bacteria. The coevolution hypothesis proposes that in order to avoid aphids laying eggs on them, certain trees have evolved the ability to synthesize red pigment. Aphids have a tendency to choose green leaves over red, which exhibits coevolution of autumn colors and insect preference.
One study conducted by M. Archetti and S.R. Leather (2005) on red autumn colors and aphid interaction showed that the highest amount of Rhopalosiphum padi aphids migrated to one specific species of tree (Prunus padus) during the peak of leaf color variation. Results of the study indicated that the number of aphids had a negative correlation with the amount of red leaves versus green leaves. This study supports the coevolution hypothesis, which suggests that aphids preferred green leaves over red.
The photoprotection hypothesis is still widely controversial, and both hypotheses need to undergo further testing before they are accepted fully, but they provide intriguing standpoints about the evolution of autumnal colors and the adaptive advantages the color red can provide a tree with. While we can all clearly see that the colors of the changing leaves are stunning, understanding the mechanisms behind this natural marvel provides a new layer of appreciation for such a beautifully complex system.
Archetti, M. and Leather, S.R. (2005). A test of the coevolution theory of autumn colours: colour preference of Rhopalosiphum padi on Prunus padus. Oikos, 110, 339-343.
Marco Archetti, Thomas F. Döring, Snorre B. Hagen, Nicole M. Hughes, Simon R. Leather, David W. Lee, Simcha Lev-Yadun, Yiannis Manetas, Helen J. Ougham, Paul G. Schaberg, Howard Thomas. (2009). Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends in Ecology & Evolution, 24(3), 166-173. ISSN 0169-5347. https://doi.org/10.1016/j.tree.2008.10.006.