In the autumn, the decrease in sunlight and cold temperatures causes the vessels that transport sap to the leaves to close. This results in a decrease in chlorophyll, revealing other pigments such as carotenoids (yellows and oranges) and anthocyanins (reds and purples) which create the beautiful colors of autumn.
Leaves contain different types of pigments, mainly chlorophyll, carotenoids, and anthocyanins. Chlorophyll gives leaves their green color and is responsible for capturing light for photosynthesis. Carotenoids, on the other hand, are usually yellow, orange, or brown and are present all year round, but are hidden by the dominant green of chlorophyll. Anthocyanins, red or purple pigments, mainly appear when the leaf ages or undergoes stress, such as in autumn. As chlorophyll gradually disappears in autumn, these other pigments become visible, giving the famous yellow-orange or bright red hues typical of this season.
In autumn, the days shorten and there is less sunlight available for trees to continue optimal photosynthesis. Photosynthesis is simply how plants convert sunlight into energy using a green pigment called chlorophyll. With less sunlight, less chlorophyll is produced. The leaves then become less efficient at transforming carbon dioxide into sugar, a substance necessary for the plant to feed and grow. Gradually, trees progressively shut down the "energy factory" present in their leaves to enter energy-saving mode in preparation for the upcoming winter.
The color change of leaves depends on several weather factors. For example, cool nights combined with sunny days accelerate the breakdown of chlorophyll. Without it, secondary pigments like carotenoids become prominent and color the leaves yellow or orange. In contrast, a gray and rainy autumn often limits color intensity, giving the leaves a duller hue. Dry cold generally promotes bright reds, especially if temperatures stay above freezing. Prolonged drought, on the other hand, often results in dull coloration and leaves that fall earlier in the season.
During the summer, chlorophyll is continuously renewed, allowing the leaves to maintain their bright green color. But as the days shorten and sunlight becomes less intense in the autumn, the tree slows down and then stops the production of chlorophyll. The chlorophyll still present in the leaves then begins to degrade gradually, losing its predominance over time. As it disappears, this green pigment reveals the colors that have been hidden there all along. This is exactly the phenomenon that explains why, little by little, trees transition from green to orange, yellow, or vibrant red during the autumn season.
As chlorophyll gradually disappears, it makes way for other secondary pigments already present in the leaves but previously masked. Among them are carotenoids, responsible for the bright yellow and orange colors often seen in autumn. These colorful pigments exist all year round but remain hidden behind the green dominance of chlorophyll. Another group called anthocyanins sometimes appears in autumn, giving the leaves particularly vivid red to purple colors depending on the tree species. These pigments form due to light and sugar accumulated in the leaves as temperatures decrease. This is why autumn colors are so varied, ranging from bright gold to brilliant red, delighting the eyes each year.
The red leaves of certain species, such as the maple, derive their vibrant color from a pigment called anthocyanin, which protects the leaves from excess light and cold.
A cold but sunny autumn often enhances the autumn colors, as the cool nights combined with sunny days accelerate the breakdown of chlorophyll while stimulating the production of anthocyanins.
The yellow and orange leaves appear in autumn because the carotenoid pigments, usually masked by green chlorophyll during the summer, become visible when it degrades.
The change of color in autumn is beneficial to trees, as it allows them to recover certain precious nutrients found in the leaves before they fall.
The intensity of autumn colors varies according to several climatic factors. An autumn with sunny days, cool but not freezing nights, and moderate rainfall generally promotes a more vivid and spectacular change in coloration.
Yes, temperature has a strong influence on the color change of leaves. Cool nights cause the rapid degradation of chlorophyll while stimulating the production of red pigments (anthocyanins), resulting in vibrant red and purple colors in the leaves.
Predicting the exact intensity and precise timing of autumn colors remains complex; however, by observing the weather conditions in the weeks leading up to it (temperatures, humidity, sunlight), one can obtain an approximation.
The color itself does not directly influence health, but it is often a visible indicator of the tree's seasonal adaptations. Pigments like anthocyanins can also play a protective role against certain environmental stresses, including excessive light or cool temperatures.
No, only the so-called 'deciduous' species change the color of their leaves in autumn before shedding them. 'Evergreen' species, like conifers, generally retain their green needles throughout the year due to their specific adaptation capabilities.
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