by Fiona Bayrock
It's easy for a leaf to turn yellow in the fall. Orange, too. It's not a complicated process. No flurry of yellow or orange pigment production required. No mystery.
Xanthophylls (yellow pigments) and carotenes (orange pigments) are already hidden in the leaf, just masked by the green chlorophyll that leaves use to make food during photosynthesis. When deciduous trees stop producing food in preparation for winter, the chlorophyll becomes unnecessary, breaks down, and disappears, revealing yellow and orange pigments that were there all along. Turning yellow is as simple as removing the green.
Turning red, however, is a more complicated process, involving increased anthocyanin (red pigment) production in the fall. Mention red autumn leaves to a botanist, and the questions come fast and furious. What does red pigment do? Why does a tree bother to spend energy making anthocyanins when time is short to salvage nutrients from leaves before they fall off? Why do some leaves on the same tree turn red while others don't?
Several groups of scientists have begun to unravel the anthocyanin mystery. Recent research indicates that it's all about protection.
“Light that is too bright can inhibit photosynthesis any time of the year,” says horticulturalist William Hoch of the University of Wisconsin in Madison. Photoinhibition—when light-gathering chlorophyll slows down due to an overload of light—is a greater risk in the fall, because the chlorophyll is breaking down while the tree is trying to reabsorb nitrogen from leaves before they drop.
Hoch and his colleagues think that the anthocyanins act like sunscreen during this unstable time. “The [anthocyanin] pigments shade photosynthetic tissue,” he says, to prevent photoinhibition and maintain the energy production essential to the reabsorption process.
But the role of anthocyanins doesn't end there. Recent and ongoing studies are looking at how anthocyanins protect plants from:
- Free radicals (substances that can damage DNA, proteins, and membranes)—Anthocyanins are great antioxidants, sopping up free radicals four times as well as Vitamins C and E.
- Water-related stresses (e.g., drought, salt buildup)—Unlike other pigments, anthocyanins dissolve in water, which may mean that they contribute to regulating water movement by slowing its tendency to flow away.
- Frost—Water loaded with dissolved substances (e.g., anthocyanins) has a lower freezing temperature.
- Overheating—Some trees hang on to red leaves year-round, despite minus 40 degrees Celsius temperatures.
- Fungi—Fungus-farming ants avoid taking red leaves to feed their garden, which could mean that anthocyanins discourage some types of fungus growth.
As with all good mysteries, a few questions remain unanswered, but we now have a good idea why trees go to so much trouble for that final flash of red each year.
- anthocyanin: A violet, blue, or red pigment found in plant parts.
- botanist: A scientist who studies plants.
- chlorophyll: A green material in plants that traps energy from sunlight, and gives leaves their green color.
- horticulturalist: A scientist who studies the growth of plants.
- photoinhibition: A condition created when light-gathering chlorophyll slows down due to an overload of light.
- photosynthesis: The process plants use to make food.
- Why do some leaves turn red in the fall?
- Remember what you read about in Lesson 2. After all of the leaves fall off of a tree, what can the tree no longer do?
- After its leaves fall off, how do you think a tree lives through the winter?