Anthocyanin: Sunblock...For Autumn Leaves

by Holly McKelvey

photographs by Atika Laribi

It's autumn, and the trees around us are transforming in a visual cacophony of orange and red. We're caught up in the transience of this phenomenon, even as we shiver at the onset of cold, and grumble at the endless raking of leaves to come.

But why are these stunning changes taking place? What exactly is happening inside those leaves to conduct this colorful concert?

We know that the color of leaves is influenced by the pigments they contain: most well-known is chlorophyll, which provides the characteristic green of summer leaves as it absorbs light energy from the sun; light energy that will ultimately be converted into chemical energy through photosynthesis.

But chlorophyll is not the only pigment present in plant cells. It shares its space with two carotenoid pigments, the sunshine yellow xanthophyll and its carrot-top orange cousin beta-carotene. And when the amount of chlorophyll decreases relative to these other two pigments, their colors begin to come through:

                                                     yellow.

                                                                      red.

                                                           orange.

                                                                          purple.

Traditionally, it was believed that dramatic autumn-color pigments actually reside in leaves year-round, but that their colors are masked by the more powerful green of chlorophyll. When chlorophyll concentrations wane, the yellows and the oranges of carotenoid pigments are at last revealed.

This makes sense: as the leaf reaches the end of its life, it no longer replaces chlorophyll. The relative abundance of green pigment falls, and we see a relative increase in the carotenoid pigments that remain. Green to yellow. End of story.

Except it's not.

A special autumnal pigment, called anthocyanin, is actively produced as fall approaches, seemingly in preparation for the leaves' senescence. This is the pigment that produces the brilliant reds and deep purples of autumn: the boldest player in the color show.

But why should a tree expend energy to produce a brand new pigment right before dropping its leaves? Surely all that effort will just be lost in leaf litter: isn’t making anthocyanin a waste of resources that could be better spent in storing nutrients for winter?

In fact, preparing for winter is precisely what a tree is doing as it concentrates anthocyanin in its aging leaves. This pigment plays a seemingly counterproductive role for a leaf as fall approaches: it acts like sunblock.

Leaves, just like humans, can be damaged by intense sunlight. Sun damage impairs photosynthesis, and inhibits the tree's ability to reabsorb nutrients from its leaves. The effect is exaggerated at lower temperatures: precisely the sunny, cold conditions experienced in autumn.

As trees begin to fatten their stores for the winter, they reabsorb nutrients invested in their leaves. For instance, as light-harvesting systems break down in the leaves, they release amino acids that are stored in the tree's branches, trunks, and roots, and are recycled in leaf production the following spring.

That's right. A tree recycles the nutrients it puts into its leaves from year to year. It draws these nutrients out of its old leaves in the autumn, and pours them into its new buds in spring.

So if a tree loses its ability to reabsorb nutrients, it will find itself lacking in the basic resources to grow new leaves come spring.

Anthocyanin, the passion-red pigment that trees produce in abundance each autumn, is the answer. Concentrated in the epidermal, or outermost, layers of leaves, anthocyanin effectively "shades" chlorophyll by absorbing wavelengths that could be damaging. This allows the tree to undergo its process of reabsorption, uninhibited by damage from icy autumn sunlight.

And when a tree has finally harvested all it can from its summer leaves, they drop. They take with them a wash of brilliant red pigment: a stain as evidence of all the hard work the tree has done to gather back its stockpiles, survive the winter, and thrive again in spring.

Holly McKelvey is a graduate student in Applied Ecology at the Université de Poitiers, France, working on bio-indicators in stream ecology. She can be reached at holly.mc.kelvey@etu.univ-poitiers.fr