OBJECTIVES:
Phototropism is the characteristic exhibited by plants as they grow towards the main
source of light. Plant leaves near a window will orient themselves toward the Sun.
Geotropism is a plants reaction to the force of gravity. Plant roots react
positively to geotropism and grow down, while the stem exhibits a negative response to
gravity and grows upward in the opposite direction.
This experiment was a combined study that converged from two similar proposals. The
first proposal suggested the study of microgravity effects on the orientation of roots and
stems of germinating seedlings. The second proposal suggested an experiment to determine
the phototropic response under various light intensities to determine the illumination
level required to produce the phototropic response. The combined objectives of this
experiment were (1) to observe and record the differences in root and stem growth and
orientation of rice seeds germinated in the Skylab environment and (2) to assess whether
phototropism can serve as a geotropism substitute for rice seeds germinated and developed
in the Skylab environment.
The germination and growth of seedlings in microgravity raised secondary questions
concerning the role of gravitational forces on seedling development. New plant life begins
when a sperm nucleus from a pollen grain unites with an egg nucleus and the resulting cell
begins developing a seed. The seed develops into an embryo-like stage and begins to grow
or germinate after it is exposed to water. This experiment addressed a fundamental
question of how microgravity affects the process of germination.
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APPROACH:
The growth container for this experiment consisted of eight compartments arranged in two
parallel rows of four. The growth container was similar to cardboard potting cartridges
found at plant nurseries. Each compartment had two windowed surfaces, which allowed
periodic photography of the developing seedlings from both a front and side view. The
study of light intensities on plants was accomplished by using light filters. For this
purpose, five windows were covered with special filters with different degrees of light
transmittance, two windows were blocked to prevent any light from reaching the seeds, and
the remaining window had no filter, allowing 100 percent transmission of light. Three rice
seeds were inserted into each compartment through covered holes. Twenty-four seeds were
inserted into a nutrient agar medium with the aid of an automatic seed planter.
Photographs were taken at regular intervals for 30 days.
RESULTS:
Plant growth was first observed on the 4th day after the seeds were planted, somewhat
slower than expected for Earth-grown rice. Plant growth progressed at a normal rate, but
the direction was extremely irregular and inconsistent. Some plant stems made 180-degree
turns away from the light and many plant tips demonstrated curled patterns. Thus, the
stems exhibited no phototropic effect. Of the 24 seeds planted, only 10 developed. This
ratio was close to the germination ratio of 12 out of 24 observed in the control group
planted on Earth. The longest stems to develop in testing on Earth were approximately 2
inches long. The stem on the plant grown on board Skylab grew to 4.2 inches. The results
indicated that the Skylab rice plants grew as fast or faster than Earth plants after the
seeds had germinated. The percentage of seeds that developed was too small to provide
significant information regarding the threshold light level required for seed development
in zero gravity. Two plants grew in totally dark compartments. The three largest plants
grew in compartments 1, 4, and 6 with filter transmission factors of 100 percent, 3
percent, and 2 percent, respectively. Thus, the illumination level did not appear to be a
contributing factor to the growth rates of the small sample of seeds aboard Skylab. It was
thought that the lack of any phototropic effect demonstrated by the rice seeds was
attributed to alterations in the auxin distribution system of the plants that rely upon
gravity. Without gravity the auxins may have been distributed unevenly, with pockets
collecting somewhat randomly, causing irregular stem and root growth.
Anonymous. MSFC Skylab student project report. NASA Marshall Space Flight Center; Huntsville, AL, United States. August 01, 1974. NASA-TM-X-64866.
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Newkirk RW, Ertel ID, and Brooks CG. Skylab: A Chronology. January 01, 1977. NASA SP-4011.
[NTRS]
Skylab Student Project Summary Description. Huntsville, AL: NASA George C. Marshall Space flight Center; 1973. MSFC SL-73-3.
Summerlin LB, ed. Skylab, Classroom in Space. NASA Marshall Space Flight Center; Huntsville, AL, United States. January 01, 1977. NASA SP-401.
[NTRS]
