Science Education Standards:

Short Description: PP. 88-91. With the chamber constructed from activity #10, it is easy to investigate the effects of light quantity on bending of seedlings. Note: This is a 112 page PDF.

Activity 11 -- How Little Light Will Bend a Seedling

Introduction

On Earth gravity is present in the quantity of 1 g (unit gravity). The quantity of light on the other hand can vary enormously from very large quantities of irradiance in the order of 8000 µEm^-2s^-1 in sunlight at noon to vanishingly small amounts. With the chamber that you and your students constructed in the phototropism activity, it is easy to
investigate the effects of light quantity on bending of seedlings.

Question

How much light is needed to bend a seedling?

Hypothesis

You provide an amount. Then find out!

Design

• Groups of students will use a set of film can phototropism chambers to vary the quantity (intensity and duration) of unidirectional light reaching seedlings and measure the responses over time. This experiment can be run at home to facilitate observation and data collection.

• Students will record observations and measurements on the Phototropism Data Sheet (page 91).

Time Frame

Construction of the phototropism chamber will require approximately half of one 50 minute class period. The experiment will run 96 hours from the time seeds are placed in the chamber, with observations and measurements made at specific intervals. The time required for the measurements will be approximately 15 minutes at each interval.

Learning Objectives

In participating in this activity students will:

• learn to construct their own experimental equipment from low-cost materials;
• apply simple geometry to the determination of the experimental variables of light quantity impacting a seedling; and
• learn to interpret complex interactions involving three variables, including three-dimensional graphing of data representing two independent variables (time and quantity of light) and one independent variable (angle of plant bending).

Materials (per group of four students)

• four 35 mm black film cans with lids

• four floral foam discs, 28 mm diameter x 2 to 4 mm thick

• four grid strips, 0.5 cm x 4 cm (page 43)

• four wick strips, 1 cm x 4.5 cm, made of soft paper toweling (page 43)

• four brassica or other similar-sized seeds (turnip, lettuce or alfalfa)

• water bottle

• forceps to handle seed

• 2 cm wide clear adhesive tape

• 2 cm wide black vinyl electrical tape

• scissors

• four 1.5 cm squares of aluminum foil

• fine needles or pencil point for making holes in aluminum

• dissection strips (page 23)

• hand lens

• hand-held hole punch

• small plastic protractor or Tropism Response Measuring Card (page 90)

Figure 5: Side view of tropism chamber, and aluminum foil squares with varying apertures.

Procedure

1. On each of four black film cans, use a hand-held hole punch to make a single hole of 6 mm diameter 1.5 cm from the rim and cover it with a clear strip of tape to make a window.

2. Taking a 1.5 cm square of aluminum foil, puncture the center of the foil with a very fine needle to make a circular hole (aperture of 1 mm diameter).

3. On a second square of foil, use the needle to puncture an aperture of about 2 mm diameter. On a third square puncture an aperture of 6 mm (Figure 5).

4. On a fourth foil square, make no aperture.

5. Measure the actual diameter of each aperture with the aid of a hand lens and ruler, estimating to a fraction of a millimeter, enter the diameter on the Phototropism Data Sheet. Calculate the area of each window and enter it on the data sheet.

6. With clear tape, mount each foil square over the window on a different film can phototropism chamber, so that now there are four chambers each with a window of differing aperture diameter (0 aperture, 1 mm, 2 mm and 6 mm).

Figure 6: Film can photoropism chamber, view from above.

7. Cover each of the windows with a 3 cm strip of black vinyl tape that has a 5 mm length folded back on itself to produce a tab so the tape can be easily removed.

8. As in the gravitropism experiment, set up each chamber with a wet floral foam disc in the bottom but this time with a single germination strip with one seed located 2 cm down the strip.

9. Place the germination strip and seed opposite the window (Figure 6).

10. Set the chambers under the light bank with the windows in a position to receive light when the black tape window covers are removed.

11. Let germination proceed in the dark for 36 to 48 hours, then remove the window covers.

12. After removing the window covers, open the lid of each chamber and observe the orientation of the seedlings.

Figure 7: How to measure angle of bending from vertical

• Remove each germ strip with its seedling. Lay each down on a Tropism Response Measuring Card as shown in Figure 7. Draw a line indicating the angle of bending from the vertical, 0.

• Carefully return the germination strip and the seedling to the original position in the chamber and replace the cap.

13. With a protractor on the Tropism Response Measuring Card, determine the angle of bending for each seedling and enter the data on the Phototropism Data Sheet under T1. Note the number of hours that have passed between the time the seeds were placed and the time of measurement.

14. Repeat steps 12 and 13 at each of three additional time intervals: 6, 24 and 48 hours after removing the window covers. Record the high, low and average ambient temperatures during each time period.

Concluding Activities and Questions

In this activity students will have observed the effects of light quantity on the growth and development of seedlings in the presence of gravity. Have students consider the following:

• From the Phototropism Data Sheet, plot a line graph with aperture diameter on the x-axis and degrees of plant response from the vertical (0) on the y-axis, with each line representing a different time of exposure to light. On another graph, plot a second set of four lines with time of light exposure on the x-axis and the degrees of response from the vertical (0) on the y-axis, with each line representing a different quantity of light. How do the two line graphs compare?

• This experiment represents two independent variables (time and quantity of light) and one dependent variable (angle of plant bending). These could be displayed graphically in a threedimensional graph with axes x, y and z. Try plotting this graph; some computer software programs have this capability.

• Discuss the observed phototropic responses from the standpoint of the interaction of light direction and quantity (as aperture) as an opposing horizontal force at an angle of 90degrees to the constant vertical force of gravity.

• What do you think the response data from this experiment would look like if it were carried out in the microgravity of the Space Shuttle?

Click on the above for a printable version.

Mission Information*

Microgravity Effects on Pollination and Fertilization

A particularly sensitive time in the life cycle of a plant growing in microgravity seems to be the transition from the vegetative to the reproductive phase. In previous spaceflight experiments, most plants grown full term in space failed to produce any seed at all, and in one experiment in which seeds were produced, the seed quality was very poor. Dosimetry reading taken in flight have failed to explain this ubiquitous sterility in terms of radiation load, thus some developmental failure during plant reproduction seems to be triggered by the microgravity environment itself. Reproductive events in angiosperms have a number of stages which could potentially be influenced directly by gravity. Microsporogenesis (the production of pollen), megasporogenesis (the production of egg cells), pollination and fertilization are all complex developmental events. Brassica rapa, a compact plant with a short life cycle, is ideal for such studies.

Close comparison of pollination and fertilization processes in microgravity with ground controls has not been possible before this experiment because we have not been able to control when pollination occurs. It is only through the availability of a trained participant for in-flight activities that controlled pollination and in-flight fixation of pollinated flowers will be possible. This will yield important information on pollen germination and maturation in microgravity, pollen-stigma interactions, pollen tube growth, fertilization and early embryo development.

Two plant populations are involved in this study. One population will be launched at the pre-flowering stage of growth. A second population will be seeds at time of launch. Using a pollination kit, the Payload Specialist will perform daily pollinations on the first population, and will mark the flowers pollinated with color-coded wire loops. The pollination wands will be stored with desiccant for subsequent viability assays on pollen. Several pollinated flowers will be fixed in-flight for microtubule studies, but the bulk of the flowers will be returned fresh for extensive processing on the ground. From the first population of plants which will be launched at the pre-flowering stage, siliques will be obtained. For high quality microscopy it will be necessary to dissect out the developing ovules prior to in-flight fixation. A small portion of the siliques will be placed in tissue culture for embryo rescue techniques.

From the second population of plants which were seeds at time of launch, flower buds will be obtained. In vivo tests on these buds will include pollen viability (fluorescein diacetate staining), pollen germination, pollen tube growth through the stigma (aniline blue staining), and staining for stigma esterases. Flower buds will be scored for size prior to dissection and processing for microscopy. In many ways, study of this single event in a plant life cycle integrates the many outstanding questions in gravitational biology.

• adapted from CUE Experiment Requirements Document (ERD), draft version (6/13/96)