Several factors contribute to fall color, but the main agent is light, or actually the lack of it. The amount of daylight relates to the timing of the autumnal equinox.
As the autumn days grow shorter, the reduced light triggers chemical changes in deciduous plants causing a corky wall to form between the twig and the leaf stalk.
This "abscission layer" eventually causes the leaf to drop off in the breeze. As the corky cells multiply, they seal off the vessels that supply the leaf with nutrients and water and also block the exit vessels, trapping simple sugars in the leaves. The combination of reduced light, lack of nutrients, and no water add up to the death of the pigment chlorophyll, the "green" in leaves.Once the green is gone, two other pigments show their bright faces. These pigments, carotene (yellow) and anthocyanin (red), exist in the leaf all summer but are masked by the chlorophyll. (The browns in autumn leaves are the result oftannin, a chemical that exists in many leaves, especially oaks.)Sugar trapped in autumn leaves by the abscission layer is largely responsible for the vivid color. Some additional anthocyanins are also manufactured by sunlight acting on the trapped sugar. This is why the foliage is so sparkling after several bright fall days and more pastel during rainy spells. In general, a dry fall produces the most-vibrant color!
The reduced light also triggers chemical changes in me too. I sometimes become blue..... the only downside to fall. So I will try very hard to look at all the positives that accompany this season.(and there are many).. the beauty, the birthdays, the holidays and the harvest! Welcome Fall, I'm happy you're here!
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