PRINCIPAL INVESTIGATOR(S ) : Joel G. Wordekeraper, and Donald W. Schlack 



EXPERIMENT TITLE/NUMBER : Plant Growth - Plant Phototropism, ED61/62 



PROGRAM/MISSION : Skylab 3 



CLASSIFICATION : Plant - Rice seedlings 



DISCIPLINE(S) : Behavioral science, Cell biology 



OBJECTIVES : To compare root and stem growth and orientation in null gravity 

 with growth and orientation on Earth and to determine if visible light (photo- 

 tropism) in null gravity can substitute for geotropism (gravitational effect). 



PROTOCOL : Twenty-four seeds were grown in a container with eight cells, each 

 having two windows. One window was covered with a neutral density filter, and 

 one with a removable opaque cover. Removal of the opaque covers permitted 

 observation and photography of plant growth. Each of the eight cells had a 

 different filter ranging from a transmittance of 1 (clear) to (opaque). 

 Each cell was filled with a sterile nutrient agar. A mechanical seeder was 

 built to implant three seeds in each cell while in orbit with a minimum 

 removal of agar. A special camera bracket held the camera at the proper 

 distance from the plants during photography of seed development. 



EQUIPMENT : Experiment package, camera. 



RESULTS : The direction of growth was extremely irregular with stems making 

 180-degree turns away from the light and many plant tips demonstrated curled 

 patterns. The stems seemed to exhibit no phototropic effect. Some grew 

 toward the light, some away from it. The illumination level did not appear to 

 be a contributing factor to the growth rates of the small sample of seeds. 



CONCLUSIONS : The proposed explanation for the lack of any phototropic effect 

 by the rice seeds is that the auxin distribution system of the plant relies 

 upon gravity. Auxins are plant growth hormones that cause cells to elongate 

 or grow. They are produced in the tips of both the stems and the roots and 

 are distributed away from the tip into the "area of elongation." Without 

 gravity, the auxins may have been distributed unevenly, with pockets 

 collecting somewhat randomly, causing irregular stem and root growth. It is 

 also possible that the light sensing mechanism in the stem tip that triggers 

 the auxin distribution reacted differently in zero-gravity but this mechanism 

 is not understood well enough to hypothesize its reaction to zero-gravity. 



PUBLICATIONS : 197, 377, 500 



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