Question

Describe an experiment to show the effect of gravity on radicals

Answer 1

Answer:

Plants respond directly to Earth’s gravitational attraction, and also to light. Stems grow upward, or away from the center of Earth, and towards light. Roots grow downward, or towards the center of Earth, and away from light. These responses to external stimuli are called tropisms. Plants’ growth response to gravity is known as gravitropism; the growth response to light is phototropism. Both tropisms are controlled by plant growth hormones.

Indoleacetic acid, or auxin, is a plant hormone that, in high concentrations, stimulates growth and elongation of cells in stems, while retarding the growth of root cells. When auxin is distributed uniformly throughout a stem, all sides of the stem grow at the same rate, thereby enabling the plant to grow toward light and away from gravity (see illustration on page 5). If the plant is tipped over on its side, auxin concentrates on the lower side of the stem, causing the cells on the lower side of the stem to elongate. This process turns the stem so that it once again grows upward, presumably toward the light.

Roots also will change direction when a plant is tipped on its side. Auxin concentrates on the lower sides of the roots and inhibits the elongation of root cells. As a result, root cells on the upper side of the root grow longer, turning the roots downward into soil and away from the light. Roots also will change direction when they encounter a dense object, such as a rock. In these cases, auxin concentrates on the lower side of the roots, enabling the roots to change direction and find a way around the rock so that normal growth can resume.

investigate the effects of gravity

To learn the effects gravity has on growing plants, students create a simple germination chamber from a Zip-loc®-type plastic bag and a moistened paper towel.

Note: For in-depth information regarding the role auxins play in plant growth and development, and about Brassica rapa, please download the Plants in Space Teacher's Guide.