Interplay among three pigments is responsible for the changing colors seen in plant leaves each fall.
- Chlorophyll — this is the key green compound that enables the plant to convert sunlight and CO2 to sugar throughout the summer.
- Carotenoid — a yellow/orange compound that is present in the leaf throughout the year. When green chlorophyll breaks down in the fall leaves can take on the color of carotenoid.
- Anthocyanin — a red compound that is usually produced in fall when excess sugar is trapped in the leaf.
In response to longer fall nights, deciduous plants begin withdrawing sugars from their leaves and stop providing leaves with nutrients needed to maintain chlorophyll. The chlorophyll breaks down, which may turn the leaves the yellow color of carotenoid. Alternatively, a leaf may turn red because of anthocyanin created from trapped sugar.
Maple leaves with yellow, red, and green colors reflect different proportions of carotenoids, anthocyanin, and chlorophyll.
Hazelnut bushes in the two photographs above are directly across an east-west road from each other. The side of the road with the southern, sunny exposure has hazelnut bushes that have turned reddish. The other side with less sun, has bushes that have turned yellow.
The likely reason for the color difference is that leaves with more sun exposure contained more sugar, which transformed to red anthocyanin. Yellow carotenoid has colored the bushes that have had less sun exposure.
Color change is not restricted to trees. The image above shows a plant in the buttercup family, Early Meadow Rue, with fall colors. Its leaves with green centers and purple edges likely reflect the distribution of remaining chlorophyll and anthocyanin.