Sep 22, 2008
Today, is the first day of autumn, the fall, and Google is celebrating with a new leafy logo. But, why do leaves turn red in the fall? It’s all down to chemistry. Red pigments known as anthocyanins form in leaves from many plant and tree species at the same time as the green photosynthetic apparatus is dismantled by the plant. During this process nutrients containing nitrogen (N) and phosphorus (P) are re-absorbed by the plant from its leaves for winter storage and the plant’s waste products in the leaves are left behind. If these nutrients are not resorbed next year’s growth is inhibited. As the levels of green compounds in the leaf falls and anthocyanins rise so the leaves of many species change from verdant to rusty with a range of colours in between. In one sense (according to my high school biology teacher, Mrs Bradley [no relation], the trees are “urinating”, or more strictly, excreting waste in the annual fall.
For more information on why leaves turn red in autumn, check out this page from Wisconsin University. Science Made Simple has a nice explanation too as does Dr David Wilkinson from Liverpool John Moore’s University, and the USDA.
As an adjunct to this PNAS has just published a paper that reveals the enzymiccascade that controls abscission, the process that determines how and when plants actually shed their leaves:
S. K. Cho, C. T. Larue, D. Chevalier, H. Wang, T.-L. Jinn, S. Zhang, J. C. Walker (2008). Regulation of floral organ abscission in Arabidopsis thaliana Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0805539105
Autumn Leaves is a jazz classic about the bittersweet symphony that’s life. My singing group Big Mouth cover it in a medley of standards. I’ve created a playlist of the other songs we cover, on Youtube, these are either original versions or, as Jon points out, oddities.
This is an updated post Sciencebase from November 2006. Hope you enjoy my new photos too, top one is a snap I took in the English Lake District one autumn, the second leafy view is of a tree at Anglesey Abbey in Cambridgeshire.