Monday, October 13, 2014

Why do leaves change and fall?

Writing the first post, Autumn 1, sent me looking for answers that I thought I knew.  I ended up learning some things, and had a couple of insights along the way.  I am drawing on several web sites (links at the end) for much of the following.

Why do autumn leaves have the colors they do?
Plants use leaves to do a job you and I can't: make food for themselves.*  Inside the cells of every leaf are tiny solar-powered factories, called chloroplasts, that use carbon dioxide and water to produce sugar and starch molecules.  These are both food and raw materials for further growth and reproduction.  These factories use the pigment chlorophyll to absorb the red and blue light that provide their energy.  (Chorophyll is green because that is the color of light "left over" after the red and blue are absorbed.)  Leaves contain other pigments, notably yellow and orange carotenoids (yup--named for carrots in which they are prominent) to help absorb sunlight, but during the growing season these are obscured by the more abundant chorophyll.

The two most abundant carotenoids are responsible for whatever oranges and yellows plants display.
I love the beauty of these structures in the spare short-hand of chemical structural diagrams: straight lines represent covalent bonds between atoms, carbon atoms are at the ends of lines, and hydrogen atoms are assumed wherever a carbon atom has not otherwise "used up" its four bonds.

With the shortening days of autumn, plants begin withdrawing valulable nutrients from leaves.  With this and lessening light, chlorophyll production in leaves declines, so that the carotenoids become more visible.  Meanwhile, many plants also produce another pigment, anthocyanin, that lends leaves shades of red, purple, or blue.  Anthocyanin production is favored by warm sunny fall days and cool nights (resulting in more sugar in leaves), so this kind of fall weather brings out brighter reds in foliage.

The structure of anthocyanin, and how that structure alters
 at low (very acidic) pH to give a red color, and at slightly higher (less acidic) pH to give blue.

  Anthocyanin is interesting stuff.  It is a water-soluble pigment inside plant cell vacuoles whose structure is sensitive to pH: when the sap is a trifle acidic, anthocyanin is red, but if the sap goes a bit alkaline, the molecule alters slightly to give a bluer color.  (Generations of science students have made their own pH indicator from red cabbage.)  Anthocyanin is the pigment responsible for the red and blue colors in all plants and plant parts that have them, including flowers and fruits.   Because it is water-soluble, a rainy period can "wash out" the bright reds by, well, washing them out.  On the other hand, sugar production on warm, sunny fall days favors anthocyanin production, making a brighter fall.  Among the plants that often produce a lot of anthocyanin are sumacs, sassafras, white oak, scarlet oak, and winged euonymus ("burning bush").

Though the role of anthocyanin in coloring flowers and fruits to attract animals to pollinate or disperse seeds seems pretty straightforward, its purpose in leaves is not.  It may serve the plant as an antioxidant, or protect the leaf's photosystems from excess light, or both these things, and perhaps protect leaves from freezing a little longer, or even poison the soil against competitors.

The brown of leaves such as oaks is partly the color of tannins that defend oak leaves from insects--if not always successfully.  If you are a tea drinker, you have probably encountered tannins in tea steeped too long: tannins are responsible for that dry feeling in the back of you throat when you drink it.

The color of a leaf is a combination of all these possibilities: remaining chlorophyll, revealed carotenoids, and anthocyanins and/or tannins if the plant produces them.

Which pigment--chorophyll, carotenoid or anthocyanin--dominate in each of these white ash leaves?
Bonus point: what is the likely pH in the top leaf?

Leaves don't just fall--they're pushed!
It turns out that leaf turning and leaf fall are not directly coordinated.  Plants drop their leaves in response to the daylength--or rather, the length of uninterrupted darkness.  The plant pigment phytochrome times the length of darkness by changing form at a certain rate once darkness falls.   As days shorten in autumn, a particular plant is triggered to begin the process of pulling valuable nitrogen and phosphorus out of the leaf and preparing to cast the leaf loose.  As nutrients from the leaf go into storage in the twigs and branches, cells in the leaf stem (or petiole) begin to generate new cells in layers across the base of the petiole.  A soft parenchyma cell layer bounds the leaf side of the petiole, while a waterproof layer of bark-like cells cuts off the leaf and protects the twig.  Once this "abscission zone" is complete, a little breeze or rain does the rest, and the leaf begins its earthward journey.

You may have noticed that neighboring leaves on the same plant can be different colors; it is common for a single red maple leaf to be different colors in different areas.  Plants have no nervous system, and a rather limited ability to communicate within themselves using plant hormones.  So instead of the coordinated response to the environment typical of animals, each part of a tree responds individually to its own local conditions.  

I got a vivid lesson in this when an undergraduate at URI.  A tree near the student union had a street light nestled among its foliage.  In response to lengthening nights, the tree colored, then dropped its leaves--all except those clustered around the glowing street light.  These remained most of the winter, gradually dying and becoming sadder and more tattered, but never really falling until the spring's new growth.

Okay--that's how they fall, but why?
I once assumed trees dropped their leaves because of the cold.  Leaves would not survive in the cold the way the tougher buds would.  In fact, cold is only part of it.**  Leaves are a tree's "employees"--they work to make the food.  But they are also an expense to maintain, and when they can no longer make enough food to pay the costs of maintaining them, they become a liability.  Leaves that remain on a plant make it more vulnerable to wind, for example.  Like a practical but heartless employer, deciduous tree evolution has resulted in a calculus that "lays off" the leaves when they risk becoming a liability. Plants such as conifers which maintain leaves year-round typically have needle-shaped leaves that present less resistance to wind and snow. 

*In other words, we are among the branches of the tree of life that must steal to live: we eat the plants that create the food, or we eat other animals, fungi, etc. that ate plants.

**I was two years in the Peace Corps in Samoa, a tropical Pacific island nation.  There, it was warm and rained frequently around the ends of the year, but was sunny and dry in the middle of the year.  Even though I never saw a temperature below 60 degrees F, many trees dropped their leaves in the dry season.  

The sources I remembered to save:
How Plants Prepare for Winter
Pigment chemistry, with a little about nutrient withdrawal from leaves.
Wisconsin's site for environmental education for kids
Autumn Changes in Deciduous Trees
Why Leaves Change Color

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