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Effect of Plant Hormones on Apical Dominance, Fruit Development and Abscission

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Author: Janice VanCleave

Certain plant hormones known as auxins are responsible for many plant responses. Auxins affect, for example, the growth of buds, stems, leaves, and roots.

In this project, you will have the opportunity to demonstrate apical dominance, which is the inhibition of lateral bud development by the growth of a terminal shoot—an effect caused by auxins. You will also study the effects of auxins on plant activities such as the stimulation of fruit development and the shedding of leaves and fruit (abscission).

Getting Started

Purpose: To determine whether the terminal bud on a white potato exhibits apical dominance.

Materials

  • Marking pen
  • 10 white potatoes
  • Baking pan

Procedure

  1. Use the marking pen to number each potato.
  2. Observe and record the appearance of each potato.
  3. Place the potatoes side by side on the baking pan. Try to give them as much space as possible.
  4. Set the pan in a closed cabinet.
  5. Observe and record the appearance of the potatoes weekly until a 6-inch (15 cm) growth of one of the buds is observed. This could take three or more weeks.
  6. When observing the bud growth, be very careful not to break any buds. Note: Save the potatoes for later experiments.

Results

A bud on the end of the potato grows into a long shoot, but the other bud growth is shorter.

Apical Dominance: Growth Inhibitor

Why?

A potato is actually part of the underground stem of a potato plant. The buds on a potato are often called "eyes." The eye at the end of the potato that develops into a long shoot is a terminal (apical) bud; the remaining eyes are referred to as lateral buds because they grow from the side of the stem (see Figure 9.1).

The growth of the terminal bud on the potato and on other plant stems generally inhibits the development of the lateral buds on the stem below. This inhibition of lateral shoot growth because of the presence of a terminal shoot is called apical dominance. Apical dominance is more pronounced in plants that have tall, single stems (such as pine trees), but even short, bushy shrubs can develop a single terminal shoot. Gardeners prune the terminal shoots from shrubs and trees to make the plants' lower stems grow. They know that fuller trees and bushes result if the terminal shoots are cut.

Removal of the terminal shoot affects the distribution of auxin (a hormone that causes the cells in a plant to lengthen) in the stem. Apical dominance is thought to be the result of the production of auxin in the meristem cells of the terminal bud that are then transported downward. Meristem cells are located at the top of each stem, and it is these cells that divide by a process called mitosis. The cell division in the meristem of apical buds is stimulated by the presence of auxin, but the downward transport of auxin produced in the apical buds inhibits cell division in the lateral buds.

Apical Dominance: Growth Inhibitor

Try New Approaches

  1. Can lateral buds develop without the presence of a terminal shoot? Repeat the experiment two times, first using slices from potatoes with developing shoots, and then using slices from potatoes with undeveloped buds. For the first experiment, use several potatoes from the original experiment that have well-developed terminal shoots and partially developing lateral shoots. Cut ten sections from the potatoes, one shoot to a section. Break off any extra shoots so that there is only one shoot per slice. For the second experiment, use several potatoes not from the original experiment that do not have developing shoots. Cut ten sections from the potatoes, one bud to a section (see Figure 9.2).
  2.  
    1. Do lateral buds develop if the terminal bud is removed? Repeat the original experiment. As the buds develop, remove the dominant terminal buds from all but two of the potatoes. These two will be the controls. Science Fair Hint: As the buds develop, measure the shoots on the potatoes and use graphs to represent the growth of shoots with and without the terminal shoots.
    2. When the terminal shoot is removed, do any of the lateral shoots become more prolific and reimpose apical dominance? As the preceding experiment progresses, observe the lengths of the shoots and determine whether one or more shoots on the potatoes begin to outgrow the others and replace the removed terminal shoots (see Figure 9.3).

Apical Dominance: Growth Inhibitor

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