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.
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.
|Mission||Launch/Start Date||Landing/End Date||Duration|
|Skylab 3||07/28/1973||09/25/1973||59.5 days|