The Chemistry of Fall

1. The Chemistry of Fall Information from the Harvard School of Forestry www. harvardforest.fas.harvard.edu/

2. What causes leaf color change? • It’s all Chemistry – and a little art! Fall leaf color is dependant on three plant pigment types – chlorophyll, anthocyanin, and carotenoids.

3. I. Factors influencing leaf color change • The breakdown of chlorophyll in fall leaves is triggered by shorter days (less sunlight).

4. Leaf intensity influence 1: Adequate summer rain = better fall color • Summer drought conditions stress trees. They may thus lose their leaves prematurely or start color production prematurely. The result is a reduction of color during the peak of the season. Adequate summer rains promote good tree health, leaf retention and, therefore, color production during the autumn.

5. Leaf intensity influence 2: Cool sunny days and cold nights cause better leaf color • The right weather during the autumn can promote more intense color production. The reds (anthocyanins), which require sunlight for production, are enhanced by cold and sunny days.

6. Leaf intensity influence 3: No rain in the fall and low winds Rainy and windy weather during the autumn can knock leaves down prematurely thereby shortening the color display at its peak.

7. Texas doesn't have leaf colors as intense as the north because our fall nights stay warmer, our summers are much drier ,and many species that produce bright colors, such as sugar maple or aspen, do not do well here. One exception to the species generalization is Lost Maples State Park.

8. It’s All Chemistry!

9. II. The Chemistry Behind Leaf Color Change A. In the summer, chlorophyll masks other pigments in the leaves, and all you see is the green of the leaf. As days get shorter, chlorophyll gets broken down and reabsorbed by the plant.

10. Why? The chlorophyll molecule is complex and metabolically expensive for the plant to make. It contains nitrogen, and nitrogen is a limiting nutrient for plants.

12. Chlorophyll is a large and expensive molecule to make, partly because each ring contains four nitrogen atoms. Chlorophyll is normally broken down towards the end of the leaf life span, and much of the nitrogen is reabsorbed by the plant.

13. B. Carotinoids are orange or yellow pigments that are present in the plant chloroplasts. As chlorophyll is recycled, they are visible. Beta carotene is a yellow or orange carotinoid. Carotinoids are produced along with chlorophyll all year. 

15. C. Anthocyanin is a red pigment that is actually produced in the fall, right before the leaves fall from the trees. As a leaf recycles its chlorophyll and becomes a paler green, the chlorophyll that is left is more susceptible to sun damage. Anthocyanin is produced to act as a sun-screen to protect the remaining chlorophyll, and specifically nitrogen from damage.

17. Save the N! Scientific studies have shown that the redder leaves have less nitrogen when they fall from the trees, compared to leaves without anthocyanin. This means that more N was recycled from the leaves, under the protection of anthocyanin, and stored in other parts of the plant for reuse!

18. So Where’s the Art? We’ve discussed the Chemistry of leaf color change… the actual color you see in the leaf is the art! Based on our three pigments (Anthocyanin = red, chlorophyll = green, and Carotinoids = yellow), what combination of pigments causes all the colors we see?

19. Red Anthocyanin

20. Orange left over carotinoids and produced anthocyanin

21. Yellow • Carotinoids

22. Brown? Left over chlorophyll and produced anthocyanin