more than just a pretty leaf

the science of fall color

Million Dollar Bank, Diablo, WA – photo l. fowler

The Sun has passed the celestial equator heading southward and in most of the world temperatures are dropping – Autumn Equinox for the Northern Hemisphere. The Autumnal and Spring equinoxes mark the point in the calendar at which the length of night and the length of day are almost exactly equal, and in the North Cascades this is a time of vivid color and an explosion for the senses. If you are patient and watch as nature unfolds, you will see chemistry happen right in front of you with brilliant greens changing to red, orange, and yellow – then eventually waft to the ground leaving a silhouette of structural beauty to sleep through winter.

why do leaves change color in the fall and what is photoperiodism ?

A main factor in fall leaf color change is photoperiodism or the length of day and night. As the nights get longer during the autumn, the process ‘age’ becomes evident in the change of color and eventually the falling of leaves. This is the mechanism many hardwoods use to regulate their life processes, including preparing for dormancy during the winter months.

Its not like…all the maples on the block decide…. Hey, its time to change color today!

Change, and the timing of change, can be attributed to the amount of light and shade the individual plant receives at their individual location, the health of the plant, or even the cloud cover during the season. It isn’t clear why different species of plants turn different colors with some changing early and some later – some respond more to temperature or rainfall – some standing right together have different shades of red, yello, or orange.

not easy being green….chloroplasts, chlorophyll, and the process

The processes induced by photoperiodism are called “senescence“, the loss of a cell’s power of division and growth, and is part of the larger process by which a plant goes into dormancy and is more involved than the gradual reduction of growth…..chemistry is at work.

The variety of colors unique to the fall season are actually embedded in leaves all year and the reason we don’t see them all the time is because chloroplasts in trees produce so much chlorophyll (green color) through the spring and summer plant growth that green overpowers the other colors. Chloroplasts are organelles in leaf cells that are responsible for photosynthesis, and photosynthesis provides the energy trees need to grow and reproduce. In plants, photosynthesis relies on chlorophyll, which is manufactured in abundance during spring and summer. The chlorophyll absorbs red, orange, and blue light from the sun but very little green and this is reflected back to our eyes.

During fall the chlorophyll undergoes destruction in the process, and leaves that also gradually lose chlorophyll during the growing season, accelerate during autumn before leaf fall. As the photoperiod decreases, the plants ability to synthesize chlorophyll becomes reduced and carotinoids (yellow) and xanthophylls (orange) begin to show. The senescencing cells also produce other chemicals including anthocyanins (red and purple) and tannin (brown), like those found in many of the genus Quercus. Tannins help protect the plant from predators and may have a role in growth.

Leaves change color during the autumn because the amounts of pigments change as the leaves prepare to fall from the tree. Absorption of solar energy sets off a series of redox reactions that convert carbon dioxide and water into glucose and oxygen. The plant stores some of the glucose and uses the rest to grow, and it releases the oxygen into the atmosphere. The redox reactions also break down the chlorophyll, which means the plant must constantly rebuild it to survive. 

calling home to prepare….dormancy

In temperate regions, deciduous hardwoods become dormant decreasing metabolism during what is termed “winter”. To prepare for winter and to minimize or prevent damage from frost and cold, plant cells switch from production of energy-intensive chlorophyll (for growth) to production of sugars and amino acids that basically act as an antifreeze for the plant. Dropping their leaves also allows trees to conserve water that would otherwise evaporate off leaf surfaces.

During this period, water and nutrients are drawn into the stem and then to the roots, away from the leaf in a very efficient process.

The leaf is attached to the stem by the petiole and two types of cells are formed at the base.

As water and nutrients move out of the leaf, eventually only the the veins of the leaf are all that hold on to the tree. When they break , or are torn away by wind – Falling Autumn leaves.

Leaves that turn red are the result of the anthocyanin pigments. This is the most common color of autumn leaves

leaf pigments

Plants make far more variety of pigment molecules than animals and as ‘creatures of light’ they sense light to control their growth as well as use light as their energy source. These pigments “advertise” rewards for animals which pollinate flowers and disperse their seeds.

Carotenoids present in tissues or cell assist chlorophyll in absorbing light and help to protect the chlorophyll from sun damage.

In the sunflower, a common carotenoid, ß-carotene, is produced in the chromoplasts of the ray flowers to produce bright yellow-orange colors. These pigments primarily absorb in the blue wavelengths. In most plants yellows are attributed to a class of carotenoids called xanthophylls. The orange can come from the carotenoid beta-carotene, which strongly absorbs green and blue light and reflects red and yellow. This allows our eyes to perceive the orange color.

Vibrant reds and purples of some leaves come from anthocyanins and unlike the other colorful compounds, anthocyanins are only produced in the fall; the products of complex reactions with the plant’s glucose, phosphate, and other factors. A type of flavoid, these are water soluble, vacuolar group of polyphenolic pigments that, depending on their pH, may appear red, purple, blue or black. Anthocyanins absorb light in the blue-green wavelengths, allowing the red wavelengths to be scattered by the plant tissues to make these organs visible to us as red.

The more glucose a tree has stored, the more anthocyanins it can produce, making for bold bright displays. Soil acidity can also play a role in leaf color.

Anthocyans are responsible for the pink-red colors of most flower petals, of most red fruits and almost all red leaves during the autumn The colors play a role in reproduction, by attracting pollinators and seed dispersers, but also in protection against various abiotic and biotic stresses.

In trees the more exposure to sunlight produces more anthocyanins and it is thought that anthocyanins may also act as photo-inhibitors, “sunscreens”, to protect the plant’s cells from potential damage from the sun’s high-energy ultraviolet light at a time when chlorophyll is in short supply.

Not all trees and shrubs lose their green color in the winter – conifers and tropical plants are evergreen and retain their chlorophyll supply year round. Coniferous plants have acclimated to tougher conditions of cold temperatures, scarcity of water and nutrients. With these conditions, it is tough to store enough glucose to survive the long winter so conifers have several ‘tricks up the sleeve’ that enables their chlorophyll mill to work year-round. The shape of long, thin needles, coated with cutin help prevent water evaporation, so photosynthesis can continue. In addition, many evergreens produce an anti-freeze (glucose) that depresses the freezing point of water, and in conjunction with antifreezing hormones such as jasmonic acid and proteins, the molecules take on a hexagonal shape that occupies more space than water. Because water expands when it freezes and that would burst the cell walls and kill the plant. So, the antifreeze keeps the water available for photosynthesis and the trees’ cell walls in tact.

above Ross Lake, North Cascade national Park – photo l. fowler

With tropical plants that thrive in temperate climates closer to the equator, the sun and unfrozen water are plentiful throughout the year, and plants have no need to shut down chlorophyll production or shed healthy leaves.

glossary

Anthocyanins – colored water-soluble pigments belonging to the phenolic group. The pigments are in glycosylated forms. Anthocyanins responsible for the colors, red, purple, and blue, are in fruits and vegetables. Berries, currants, grapes, and some tropical fruits have high anthocyanins content

Beta-carotene – a reddish-orange pigment that is an an isomer of carotene found chiefly in orange and dark green and yellow vegetables and fruits (such as carrots, sweet potatoes, and spinach) and that is converted to vitamin A in the body

Carotenoids – also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, corn, tomatoes, canaries, flamingos, salmon, lobster, shrimp, and daffodils

Chlorophyll – is any of several related green pigments found in the mesosomes of cyanobacteria and in the chloroplasts of algae and plants. Chlorophyll is essential in photosynthesis, allowing plants to absorb energy from light

Chloroplasts – In biology, a chloroplast refers to the organelle found within the cell of plants and other photosynthetic eukaryotes that is filled with the green pigment called chlorophyll. … Plants are examples of organisms that possess chloroplasts inside their cells.

Cutin – a waxy, water-repellent substance occurring in the cuticle of plants and consisting of highly polymerized esters of fatty acids.

Dormancy – the state of having normal physical functions suspended or slowed down for a period of time; deep sleep

Free radicals — single oxygen atoms that can damage cells by reacting with other molecules

Jasmonic acid –  an organic compound found in several plants including jasmine. The molecule is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway

Organelles –  a subcellular structure that has one or more specific jobs to perform in the cell, much like an organ does in the body. Among the more important cell organelles are the nuclei, which store genetic information; mitochondria, which produce chemical energy; and ribosomes, which assemble proteins.

Photoperiodism – can also be defined as the developmental responses of plants to the relative lengths of light and dark periods.

Redox reaction–  a type of chemical reaction that involves a transfer of electrons between two species

Senescence – the collective process that lead to the aging and death of a plant or plant part, like a leaf.

Quercus – a genus of hardwood often evergreen trees or shrubs (family Fagaceae) that comprise the typical oaks and include sources of nutgall

Vacuolar – A vacuole is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal and bacterial cells

Xanthophylls – a yellow or brown carotenoid plant pigment which causes the autumn colors of leaves


literature cited

Linus Pauling Institute at Oregon State University, α-Carotene, β-Carotene, β-Cryptoxanthin, Lycopene, Lutein, and Zeaxanthin, Jane Higdon, Ph.D.
Linus Pauling Institute 2004

Project Gutenberg’s The Chemistry of Plant Life, Roscoe Wilfred Thatcher, https://www.gutenberg.org/files/33394/33394-h/33394-h.htm

https://www.biologyonline.com/dictionary/chloroplast

http://www.livescience.com

Wikipedia.org

https://www.genome.gov/genetics-glossary/Organelle

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