Pigments

Day 358: Every year, at least one person asks me why leaves change colour in the fall. Although there's a bit more to it, the basics are easy to understand. During the months of spring and summer, warm temperatures and length of daylight encourage the production of chlorophyll, the chemical substance which allows plants to convert carbon dioxide into starches and sugars. As nights grow cooler and daylight diminishes in autumn, the cells at the juncture of leaf and stem begin to thicken, restricting the uptake of carbohydrates. This process is called "abscission," i.e., "cutting off," and will eventually cause the leaves to fall. With the supply of carbohydrates limited, the chlorophyll begins to break down, allowing other pigments present in the leaf to become dominant visually. Different plants contain various levels of the different pigments, so those which have high concentrations of xanthophyll appear yellow, those with carotene appear orange, and those containing anthocyanins turn various shades of red. Other factors such as drought or an early frost may affect the colours as well.