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Posts Tagged ‘plant pigments’

Fall leaves cover the lawn

This past weekend I spent several hours blowing the oak leaves out of our front perennial beds and off the front lawn and into the woods beside the house. The leaf cover in the back yard was much lighter so I used our mulching mower to chop them up and then used the lawn sweeper to collect the chopped leaves. These shredded bits of oak and hickory leaves were added to our compost pile. Good stuff for the garden next summer.

Deciduous magnolia broken in an early fall snowstorm.

Deciduous magnolia broken in an
early fall snowstorm.

This whole process got me thinking about the science behind leaf drop in autumn. It’s a rather fascinating story. Can you imagine what it would be like if the leaves on our deciduous trees didn’t drop off in the fall? Many have first hand knowledge of the destruction that can take place when snow and ice storms occur in the early fall before the leaves have fallen from the trees. Definitely not healthy for the trees!

Leaves are the food factories of plants. The green color that you see in most leaves comes from a plant pigment called chlorophyll which is responsible for absorbing sunlight to initiate the process of photosynthesis; a chemical reaction that converts carbon dioxide and water into simple sugars. These simple sugars are then converted into more complex carbohydrates. Photosynthesis is quite possibly the single most important chemical reaction in nature as it provides the ultimate source of food for most organisms.

The tender young foliage of this tree peony is red from anthocyanin

The tender young foliage of this tree
peony is red from anthocyanin

Most people know about chlorophyll but there are two other groups of pigments found in the leaf; the carotenoids, which include carotene (orange pigment) and xanthophyll (yellows and tans), and the anthocyanins which are primarily reds and purples. While the carotenoids are found in the leaf throughout the growing season, the anthocyanins are mainly present in the leaf during the spring and fall. Anthocyanin is responsible for the brilliant reds and purples in the fall and the reddish tint commonly seen in newly emerging leaves in the spring. It is thought to function similar to a “sunscreen” for these tender young leaves – an interesting tidbit of knowledge!

As the chlorophyll breaks down in this hickory, the yellow xanthophyll shows through.

As chlorophyll breaks down, the yellow
xanthophyll begins to show through.

Throughout the spring and summer, high levels of chlorophyll in the leaves normally mask the colors of the other leaf pigments causing the leaves to appear green. In the fall, as the days get shorter and the intensity of the sunlight decreases, changes begin to occur in the leaves. Photosynthesis slows down and chlorophyll production ceases. As the chlorophyll breaks down, the other pigments begin to show through. This is when we start to see the colorful changes in the leaves.

Oak branch in fall. Note the buds that will sprout new growth in spring.

Oak branch in fall. Note the buds that will sprout new growth in spring.

In actuality, it’s the longer nights of fall not the cooler temperatures which are the main trigger for these physiological changes in the leaf. As the nights become longer, a layer of cells (the abscission layer) begins to develop where the leaf petiole (leaf stem) joins the branch. The growth of these cells begins to restrict the movement of sugars out of the leaf and the flow of water and minerals into the leaf. This is the beginning of the end for the life of the leaf! It is slowly being cut off from the main part of the tree – its usefulness is basically over.

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By autumn, the leaves have fulfilled their task of producing food and supplying it to the buds, branches, stems, and roots. The buds for next year’s leaves have been produced and will survive the winter because they are protected by tough bud scales.

Fallen oak leaves cover the ground.

Fallen oak leaves cover the ground.

The leaves of deciduous trees become a liability in winter. Trees lose a lot of water through their leaves – water that would be hard to replace in the winter when the water in the ground is frozen. If the leaves were retained, the tree would slowly die from dehydration. Besides, these thin, tender leaves can’t withstand freezing temperatures anyway, so – they are shed from the tree. Eventually the abscission layer completely blocks off the leaf, weakening the junction between leaf and branch, and the leaf falls; either under its own weight or when wind or rain knocks it off.

Snow falls through leafless trees

Snow falls through leafless trees

An added benefit of the autumn leaf drop is that, without their canopy of leaves, these trees do not hold heavy amounts of snow after a storm. Snow tends to fall right through the bare branches with very little accumulation. If the leaves were still on the trees … well, you know how that ends up!

But what about evergreens, you ask. Why don’t they lose their leaves? How do they survive the freezing temps?

Interesting questions with equally interesting answers!

Flexible evergreen boughs bend under the weight of snow preventing heavy accumulation

Evergreen boughs bend under the
weight of snow to reduce accumulation

Evergreen leaves/needles survive the winter because they are protected by a thick, waxy cuticle. This heavy coating also helps to reduce water loss from the leaves. In addition, rather than the thin, watery sap that is found in the cells of deciduous leaves, evergreen leaves have a kind of “antifreeze” in their cells that protect them from freezing. Cool, huh!

And as far as retaining heavy amounts of snow, evergreens (at least the needled evergreens) tend to have a conical shape with flexible boughs that quite easily shed heavy accumulations of snow. When the snow melts, the branches spring back up.

Ain’t nature wonderful?

Happy Thanksgiving everyone!

Until next time – Happy Gardening!

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