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Education

GRAVITROPISM - Chapter 8

Written by Tom Sinclair and published by the National Association of Biology Teachers (NABT).

BACKGROUND

Educational Opportunities for Students

A. Introduce students to active regulation of tissue development in living organisms.
B. Investigate the importance of gravity in plant development.
C. Introduce use of a protractor.

Biology Background

Gravitropism (sometimes called geotropism in older texts) is a key regulatory process in plants to insure that roots grow down and shoots grow up in developing seedlings. As the student should find, the orientation of seeds in the soil is not important because as the radicle and shoot emerge from the seed their growth is regulated through the action of hormones to cause each organ to grow in the proper direction.

The basic mechanism that results in gravitropism is still not resolved. Sufficiently large particles in cells apparently migrate due to gravity in key cells in the growing tips of roots and stems. The distribution of these particles triggers a number of physiological events mediated by hormones that influence cell expansion. The overall effect on the "gravity side" of the tissue is inhibition of cell elongation in the roots and a stimulation of cell elongation in the shoots. The nonsymmetrical distribution of cell elongation in each of these tissues causes the growing root to bend towards gravity and the stem to bend away from gravity.

An interesting question to pose to students is the problem of growing plants on a space station. This is an issue of considerable current research. Obviously, the directional development of roots and shoots is confused in the absence of gravity. Seeds will apparently need to be germinated under some type of gravity to establish the initial growth of the seedling and then transplanted to growth conditions where other plant controls will regulate the growth pattern.

Materials and Experimental Environment

  • Corn seed
  • Rectangular containers 7-cm or more deep (Plastic containers in which supermarkets sell fruit work well.)
  • Potting soil
  • Metric rulers
  • Protractors
  • Shelf area to store germination containers

Procedures

1. Discuss the experiment, bringing in the question of growing plants in a space station. Assign each lab group one of the three seed orientations (see below). All seeds of each lab group will be sown in the single, assigned orientation.

2. The three orientations of sowing are with respect to the embryo end, or pointed end, of the corn seed.

3. Each lab group selects 18 corn seeds and plants them in three rows, six seeds to a row, according to the assigned orientation. Proper marking of each row (marker, grease pencil) is extremely important, as students will be digging each seed row up on different days. It is easy to forget the direction the rows extend. Take the time to agree on a common marking system. Care should be taken in watering as a heavy stream of water could dislodge seeds.

4. Explain and demonstrate the use of the protractor for measuring angles. This was a difficult concept for our students and may be aided with additional exercises on the use of a protractor. It is easiest to measure the angle of root and shoot growth with respect to horizontal. Therefore, a root growing vertically will have a measured angle of -90½.

5. After about five days, students will dig up one "outside row." Two or three days later, the second outside row of seeds will be unearthed. The last row will be dug up in two to three more days. Students will observe germination and measure root lengths and the angles formed with respect to horizontal during germination. These results should be noted in the lab book along with the results from other groups (i.e., other orientations).

6. Students will organize data and draw conclusions. This goes in the lab book and can serve as the basis for a windup discussion.

NOTEBOOKS

Blackboard Information

Gravitropism DATE: _____________________

OBJECTIVE: To test the hypothesis that gravity influences the direction of root (radicle) and shoot growth.

PROCEDURES:

1. Select 18 corn seeds.

2. Fill the potting container 1/2 full with soil.

3. Carefully sow seeds in the orientation assigned to your group, three rows of six seeds. Mark the rows clearly and label the box.

4. Cover the seeds with 3 cm of soil and moisten the soil gently. Do not dislodge the seeds.

5. On (designated day), carefully dig up one "outside" row of seeds and measure root and shoot angles with respect to horizontal. Share your results with groups who used the other orientations. Record results in your lab book.

6. On (designated day), dig up the second "outside" row of seeds and do as before.

7. On (designated day) dig up the last row and do as before.

Results

The data from each group need to be put in a table (see next page) on the blackboard and then copied into each student's notebook.

Root Length and
Orientation
Variable Date 1 Date 2 Date 3 Comments
Down:          
  • Shoot Length
  • ________ ______ ______ ______ _________
  • Shoot Orientation
  • ________ ______ ______ ______ _________
  • Root Length
  • ________ ______ ______ ______ _________
  • Root Orientation
  • ________ ______ ______ ______ _________
    Upside Down:          
  • Shoot Length
  • ________ ______ ______ ______ _________
  • Shoot Orientation
  • ________ ______ ______ ______ _________
  • Root Length
  • ________ ______ ______ ______ _________
  • Root Orientation
  • ________ ______ ______ ______ _________
    Horizontal:          
  • Shoot Length
  • ________ ______ ______ ______ _________
  • Shoot Orientation
  • ________ ______ ______ ______ _________
  • Root Length
  • ________ ______ ______ ______ _________
  • Root Orientation
  • ________ ______ ______ ______ _________

    Discussion Ideas

    1. What did we learn?

    2. Regulation is a major life function. How is it illustrated in this experiment?

    3. What role would hormones play in the gravitropism response?

    4. Should seed orientation be a major concern to a farmer when he or she is planting a crop?

    5. What might happen to root and shoot growth in weightless conditions?

    6. What is the current explanation for the mechanism that makes the shoot grow up and the radicle grow down?

    7. How might a space station create conditions of gravity for germinating seeds?

    Further Activities

    1. Germinate seeds in an alternate gravity field. The turntable of an old phonograph could be used to establish an additional gravity field in which to germinate seeds.

    2. Investigate participation in some of the projects on microgravity sponsored by NASA on various shuttle missions.


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