Leaves turn yellows and oranges in autumn because chlorophyll, the green pigment, breaks down, revealing the pre-existing colours beneath.
Why Leaves Change Colour in Autumn: A Detailed Explanation
As days shorten and temperatures cool in autumn, trees prepare for winter dormancy. This preparation involves several key changes that lead to the vibrant colours we associate with the season.
1. Chlorophyll Breakdown
- Trigger: The primary trigger for colour change is the decrease in sunlight.
- Process: As sunlight decreases, trees produce less chlorophyll. Chlorophyll is the pigment responsible for the green colour of leaves and is essential for photosynthesis (the process by which plants convert sunlight into energy).
- Result: The existing chlorophyll breaks down, and the tree doesn't replenish it. This breakdown allows other pigments already present in the leaf to become visible.
2. Unmasking of Existing Pigments
- Xanthophylls and Carotenoids: Two prominent pigment groups revealed during autumn are xanthophylls (producing yellow hues) and carotenoids (producing orange hues).
- Presence: These pigments are present in leaves throughout the growing season but are masked by the abundance of chlorophyll.
- Visibility: As chlorophyll degrades, the yellows and oranges of xanthophylls and carotenoids become visible.
3. Production of Anthocyanins
- Red and Purple Hues: In some tree species, such as maple and oak, another group of pigments called anthocyanins are produced in the fall.
- Synthesis: Anthocyanins are synthesized from sugars trapped in the leaf as sap flow to the branches decreases.
- Colour Variation: The intensity and hue of red and purple colours produced by anthocyanins depend on factors such as sugar content within the leaf and the acidity of the sap.
4. Other Factors Influencing Colour
- Sunlight: Sunny autumn days promote sugar production and consequently more vibrant red colours in trees that produce anthocyanins.
- Temperature: Cool nights trap sugars in the leaves, also favouring anthocyanin production. Mild temperatures without freezing conditions are optimal for vibrant autumn foliage.
- Water Availability: Drought conditions can stress trees, leading to premature leaf drop or less vibrant colour.
In summary, the beautiful yellows and oranges are always there, but hidden. The reds and purples are made from sugars as the plant prepares to shed its leaves for the winter.
