Recalling the biology knowledge from school, we all know that the green color of leaves is due to chlorophyll. As the weather turns to autumn, leaves begin to change to yellow or red. This does not necessarily mean they are dying; rather, it is an outward expression of a series of intelligent processes occurring within the leaf.
Yellow and red leaves undergo two different metabolic processes. When chlorophyll ceases to function, most leaves turn yellow. This is a color that already exists in the leaves but is overshadowed by the green during the plant’s growing seasons.
However, over the last decade, researchers have discovered a very different mechanism in red leaves. When chlorophyll stops functioning, leaves will also turn yellow unless there is a rapid production of a pigment called Anthocyanin. This pigment is not pre-existing in the leaves—it is a type of pigment known as anthocyanin.
One hypothesis suggests that the red color of autumn leaves is the result of 35 million years of struggle between trees and insects seeking food and laying eggs in the fall. The red leaf color makes it harder for insects to identify them, leading them to prefer trees with yellow leaves. The difference between leaf colors in autumn in North America and Europe can be seen as evidence for this hypothesis. In Europe, most native trees turn yellow. However, in North America, the number of trees with red leaves is roughly equal to those with yellow leaves.
To test this hypothesis, Dr. Thomas Doring, a lecturer at Imperial College London, along with two colleagues, conducted a study on the color preference of aphids when they approach trees in the autumn to mate and lay eggs. They painted 70 different colors on 140 plates and filled them with water to observe the number of aphids landing on each plate. Two weeks later, the researchers found that the number of aphids on blue plates was three times higher than on red plates, but the yellow plates attracted four times more aphids than the blue ones. Dr. Thomas then used the colors of several hundred types of leaves from various tree species to paint plates and repeated the experiment.
The results showed that red leaves did not attract as many aphids as green and yellow leaves. From this experiment, the team concluded that some tree species turn red to fend off aphids. If these insects land on the leaves in large numbers to lay eggs, the growth of the trees may slow down and their health may decline.
Another hypothesis suggests that the difference in the amount of anthocyanin pigment in trees growing in the same area may be related to the fertility of the soil where the trees grow. These pigments reflect the effort to retain the nutrients that the leaves have synthesized throughout their lifetimes. Preliminary surveys of red maple trees and sweet gum trees (which have leaves resembling red maple but are green) conducted by a student in Charlotte, N.C., showed a correlation between the leaf color in autumn and soil quality. In nutrient-rich lowland areas, most leaves turn yellow in autumn. In contrast, in dry upland areas, leaves tend to turn red.
“The correlation is clear,” said plant physiologist Bill Hoch from the University of Montana in Bozeman. Furthermore, these discoveries align with his findings on the remarkable functions of anthocyanin.
“The experimental results provide clear evidence that anthocyanin helps plants maximize the nutrients synthesized from leaves before they fall off,” he explained that the longer the photosynthesis process occurs in the leaves during autumn, the more nutrients are stored for use in spring. Therefore, in many barren areas such as the hills of North Carolina, people recognize autumn when they see the leaves turning a brilliant red. As autumn arrives, anthocyanin protects the remaining green chloroplasts in the leaves. This is particularly significant for plants growing in harsh conditions where the soil is nutrient-poor, as it allows them to produce more of the essential organic compounds.
The benefits of the red color are clear, but why do the leaves of some tree species still turn yellow when autumn arrives? “Theoretically, plants must incur a loss for changing to red. If the damage caused by insects is greater than the loss incurred from producing red pigment in autumn, the leaves will tend to turn red. Conversely, if the damage caused by insects is lower, the plants will retain the yellow color on their leaves,” Although it remains a hypothesis regarding the origin, at least people now understand that if they want to deter harmful insects from their fields in autumn, they should plant more trees with red leaves.
References: Earthsky
