by: Beth Brandt, Environmental Professional – Technical Resource – Customer Service Advocate, Suburban Testing Labs
Fall is such a beautiful time in Pennsylvania. I enjoy the crisp weather, the apple and pumpkin-related treats that are readily available, and the beauty of Mother Nature as she reveals her glorious landscape of colors in the changing trees. And, as a chemist, I’m also fascinated with the chemistry behind the colors of the changing Autumn Leaves.
As most of us know, Chlorophyll is the chemical that gives plant leaves their green color. Plants require warm temperatures and sunlight to produce chlorophyll – in autumn, the amount produced begins to decrease, and the existing chlorophyll is slowly broken down, diminishing the green color of the leaves.
Carotenoids and Flavonoid pigments are always present in leaves, but as chlorophyll is broken down in the autumn, their colors come to the fore. Xanthophylis, a subclass of carotenoids, are responsible for the yellows of autumn leaves. One of the major xanthophylis, lutein, is also the compound that contributes towards the yellow color of egg yolks.
Cartenoids can also contribute orange colors. Beta-carotene is one of the most common carotenoids in plants, and absorbs green and blue light strongly, reflecting red and yellow light and causing its orange appearance. It is also responsible for the orange coloration of carrots. Carotenoids in leaves start degrading at the same time as chlorophyll, but they do so at a much slower rate; beta-carotene is amongst the most stable, and some fallen leaves can still contain measurable amounts.
Unlike the carotenoids, anthocyanin synthesis is kick-started by the onset of autumn – as sugar concentration in the leaves increases, sunlight initiates anthocyanin production to turn leaves a red color. The purpose they serve isn’t clear, but it’s been suggested that they help protect the leaves from excess light, prolonging the amount of time before they fall.