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CHANGES IN COTYLEDON CELL ULTRASTRUCTURE DURING Brassica rapa SEED DEVELOPMENT IN MICROGRAVITY.    A. Kuang¹ and M. E. Musgrave².  ¹Department of Biology, University of Texas-Pan American, Edinburg TX  78539; ²Biology Department, University of Massachusetts, Amherst MA 01003.

Prior analysis of mature Brassica rapa seeds produced on the Mir station showed that seed production in microgravity resulted in reduced protein in the storage reserves, and a retention of starch as a storage product. Using immature seeds (8-15 days post pollination) of Brassica rapa L. cv. ‘Astroplants’ produced on the shuttle during the Collaborative Ukrainian Experiment on STS-87, we compared the progress of storage reserve deposition in cotyledon cells during early stages of seed development. Immature seeds were dissected from siliques immediately post-flight or following the ground control. These embryos were fixed in 2.5% glutaraldehyde and 1% formaldehyde, postfixed in 1% osmium, embedded in Spurr’s resin, sectioned and stained for light and electron microscopy. Results showed that starch accumulation began in plastids at an early developmental stage and grain size and number increased with seed development. At the stage of cotyledon elongation, starch reached a maximum in both flight and ground control seeds. In the spaceflight seeds, starch was retained after this stage, while starch grains decreased in size in the ground control. Large and well-developed protein bodies were observed in cotyledon cells of ground control seeds at 13 days post-pollination, but their development was delayed in the spaceflight material. Numerous mitochondria were observed in cotyledon cells of ground control seeds at 8 days post pollination, suggesting the need for large amounts of energy for cell division and protein synthesis and deposition at later developmental stages. Degradation of starch grains may provide the main source of energy for these processes. Restricted energy supplies would retard protein deposition and cell division, resulting in lower cotyledon cell number, smaller sized protein bodies, and persistent starch grains observed in the mature seeds from Mir. 

(Supported by NASA: NAG-100139 and NAG2-1375.)