Explain the phenomenon of leaves changing color in autumn

Recalling from high school biology knowledge, we all know that the green color of leaves is produced by chlorophyll. When the weather changes to autumn, the leaves begin to turn yellow or red. That absolutely does not prove that they are dying but instead is an external expression of a series of very clever processes taking place inside the leaf.

Yellow leaves and red leaves undergo two different metabolic processes. When chlorophyll is no longer active, most types of leaves turn yellow. This is a kind of color that already exists in the leaves but is overwhelmed by green in the growing season of the tree.

But in the past decade or so, researchers have discovered a very different mechanism in red leaves. When chlorophyll stops functioning, the leaves will turn yellow if there is no very rapid production of a colorant called Anthocyanin . It is a leafless substance - a pigment called anthocyanin.

There is a hypothesis that red autumn leaves are the result of 35 million years in the struggle between plants and insects as they search for food and egg laying in the fall. Red leaf color will make it difficult for insects to identify, so they tend to look for plants with yellow leaves. The difference between autumn leaf colors in North America and Europe can be seen as evidence for this hypothesis. In Europe, indigenous tree leaves mostly turn yellow. However, in North America, the number of red leaf trees is much equivalent to the number of yellow leaves.

To test this hypothesis, Dr. Thomas Doring, a lecturer at Imperial College London (UK) and two colleagues, studied the trend of choosing aphids' colors when they reach the tree in the fall to mating and laying eggs. They applied 70 different colors to 140 discs and filled them with water to track the number of aphids per each disc. Two weeks later, the team found that the number of aphids on the blue plates was three times more than the red ones, but the yellow disc attracted four times more bugs than the green disc. Later, Dr. Thomas used the colors of several hundred types of leaves from many plants to apply on the plate and repeat the test.

The results showed that the red leaves did not attract many aphids with green and yellow leaves. From this experiment, the team identified some leaves as red plants to resist aphids. If the insect lands on leaves to lay eggs in large numbers, its growth may slow down and its health will decline.

Another hypothesis is that the difference between the amount of anthocyanin pigments in the leaves of a plant in the same area may be related to the fertility of the soil where the plant grows. They reflect efforts to retain the amount of leaf nutrients that have been synthesized in their life cycle. A preliminary survey of red maple leaf and sweeg gum (red leafy green leaf-shaped tree) of a student in Charlotte, NC showed a correlation between green color in autumn and soil quality. Where lowland is more nutritious, most leaves turn yellow in the fall. In the arid highlands, the leaves turn red.

'The relationship is expressed very clearly,' said Bill Hoch, a plant physiologist at the University of Montana in Bozeman. Moreover, these findings are similar to the results he found about the function of the miraculous Anthocyanin.

'Experimental results are a very clear proof that Anthocyanin helps plants get the maximum amount of nutrients synthesized from leaves before they leave the branches,' he explained. The longer the fall is, the more nutrients will be stored for spring use. So, in many places as barren as the hills of North Carolina (USA), people realize autumn when the leaves turn to bright red. When the Fall comes, Anthocyanin protects the remaining green chloroplasts in the leaves. This is especially meaningful for plants that grow in extreme conditions, where soil is poor in nutrients, as it allows them to produce more organic compounds.

The benefits of red are clear, but why do leaves of some plants still turn yellow when autumn comes?'In theory, plants suffer losses for turning red. If the damage caused by insects is greater than the losses arising from the production of red pigments in the fall, the leaves will tend to turn red. Conversely, if the damage caused by the insect is lower, the tree will retain the yellow color on the leaf ' . Although it is still a hypothesis about origin, at least people understand that, if you want to scare away harmful insects from the field in the fall, we should plant many types of red leaf trees.

Reference: Earthsky