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The science of Autumn's beauty

For many people autumn weather brings a welcomed time of the year. Adding to the pleasure of the season are the vibrant colors displayed in nature. Leaf pigment, the physics of light, weather conditions, plant species, and geography all play important roles in the color of autumn.

Thanks to the physics of sunlight striking pigments in leaves, we see various leaf colors throughout the year. Leaf pigments play a crucial role in the colors we see. Chlorophylls, carotenoids, and anthocyanins present in a leaf help determine what color the leaf will display.

The pigment that gives leaves their green color is chlorophyll. Chlorophyll is used in photosynthesis which is the process that uses sunlight to transform carbon dioxide and water into carbohydrates (sugars) that fuel tree growth. During the spring and summer, with more hours of sunlight and warmer temperatures, this photosynthetic process is most active thus leaves are green.

When daylight hours become less and temperatures are cooler, photosynthesis slows down and there is less chlorophyll. This decline reveals a yellow or orange pigment-carotenoid. Carotenoids, the same pigment found in carrots and corn, are usually masked by the chlorophyll.

Unlike chlorophyll and carotenoids which are present in leaf cells throughout the growing season, anthocyanins are produced in autumn. Anthocyanins give color to familiar fruits such as cranberries, red apples, cherries, and plums. These complex water soluble compounds in leaf cells react with excess stored plant sugars and exposure to sunlight creating vivid pink, red, and purple leaves.

A mixture of red anthocyanin pigment and yellow carotene often results in the bright orange color seen in some leaves.

Weather conditions that occur before and during the decline of chlorophyll production can affect the color that leaves may display. Carotenoids are always present so the yellow and gold colors are the least affected by weather.

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