From Seed to Seed:
Plant Science for K-8 Educators

 

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    Other Phenomena Regulated by Auxin

It's remarkable that such diverse growth responses as phototropism, geotropism, and thigmotropism are all controlled, at least in part, by a single chemical, auxin or IAA. But what is even more remarkable is that many other common plant responses are also controlled by this one chemical. Let's look at some other plant growth responses frequently observed in the garden, and the role that auxin plays in each of them.

Apical dominance. Gardeners commonly pinch back young plants to promote bushier growth. But why does tip pruning have this effect?

Many plants exhibit what is known as apical dominance. One shoot becomes the "leader," or tallest stem. Surrounding branches seem to magically defer to this leader, and remain less vigorous. Think of a Christmas tree-the "perfect" tree (unless you're Charlie Brown!) Has a nice pyramid or cone shape. Once again, the plant growth regulator auxin is responsible for this phenomenon.

Auxin is present in the highest concentrations in the apical, or uppermost, bud. The presence of this high concentration of auxin has a growth-inhibiting effect on lower buds and branches. If you remove this apical bud, however, you remove the source of auxin. This allows the axillary buds (located on the stem nodes) to sprout and/or lower branches to grow more vigorously.

On plants with two or more leaves or branches per node, the loss of the apical bud usually results in the "release" of the two (or more) axillary buds. For example, if the leader on a Christmas tree is damaged, you may find two or more competing leaders (shoots arising from axillary buds) growing in its place.


 

Leaf and fruit drop. Leaves don't simply break off at the end of the growing season. If that were the case, there would be a tiny open wound at the site where each leaf petiole broke off, inviting disease and insects. Instead, at the end of the growing season, once the nutrients have been drawn out and the leaves have begun to dry up, the plant actively severs the leaves by destroying the cell walls across a section of the leaf stalk. This area is called the abscission zone, and-you guessed it-auxin is believed to play a role in the process.

Once this layer of cells is destroyed, a corky later begins to form between the base of the leaf stalk and the branch, protecting living tissues. When this layer is complete, the only thing attaching the leaf petiole to the limb is the vascular tissue, and a gentle breeze can easily break these brittle cells. If, during one of their tree-measuring expeditions, students carefully examine a twig, they should be able to see the shield-shaped scars where leaves have dropped. If they look closely, they will also see a line of dots along the bottom of the scar. These are the remnants of the vascular bundles.


Now that we understand the fundamentals of cells and plant growth, we are ready to explore the daily activities of the plant.









































 

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