Pollination and embryo development in Brassica rapa L. in microgravity

Plant reproduction under spaceflight conditions has been problematic in the past. In order to determine what aspect of reproductive development is aff ected by microgravity, we studied pollination and embryo development in Bra ssica rapa L. during 16 d in microgravity on the space shuttle (STS-87). Br assica is self-incompatible and requires mechanical transfer of pollen. Sho rt-duration access to microgravity during parabolic flights on the KC-135A aircraft was used initially to confirm that equal numbers of pollen grains could be collected and transferred in the absence of gravity. Brassica was grown in the Plant Growth Facility flight hardware as follows. Three chambe rs each contained six plants that were 13 d old at launch. As these plants flowered, thin colored tape was used to indicate the date of hand pollinati on, resulting in silique populations aged 8-15 d postpollination at the end of the 16-d mission. The remaining three chambers contained dry seeds that germinated on orbit to produce 14-d-old plants just beginning to flower at the time of landing. Pollen produced by these plants had comparable viabil ity (93%) with that produced in the 2-d-delayed ground control. Matched-age siliques yielded embryos of equivalent developmental stage in the spacefli ght and ground control treatments. Carbohydrate and protein storage reserve s in the embryos, assessed by cytochemical localization, were also comparab le. In the spaceflight material, growth and development by embryos rescued from siliques 15 d after pollination lagged behind the ground controls by 1 2 d; however, in the subsequent generation, no differences between the two treatments were found. The results demonstrate that while no stage of repro ductive development in Brassica is absolutely dependent upon gravity, lower embryo quality may result following development in microgravity.