Influence of microgravity on ultrastructure and storage reserves in seeds of Brassica rapa L.

Successful plant reproduction under spaceflight conditions has been problem atic in the past. During a 122 d opportunity on the Mir space station, full life cycles of Brassica rapa L. were completed in microgravity in a series of three experiments in the Svet greenhouse. Ultrastructural and cytochemi cal analyses of storage reserves in mature dry seeds produced in these expe riments were compared with those of seeds produced during a high-fidelity g round control. Additional analyses were performed on developing Brassica em bryos, 15 d post pollination, which were produced during a separate experim ent on the Shuttle (STS-87). Seeds produced on Mir had less than 20 % of th e cotyledon cell number found in seeds harvested from the ground control. C ytochemical localization of storage reserves in mature cotyledons showed th at starch was retained in the spaceflight material, whereas protein and lip id were the primary storage reserves in ground control seeds. Protein bodie s in mature cotyledons produced in space were 44 % smaller than those in th e ground control seeds. Fifteen days after pollination, cotyledon cells fro m mature embryos formed in space had large numbers of starch grains, and pr otein bodies were absent, while in developing ground control seeds at the s ame stage, protein bodies had already formed and fewer starch grains were e vident. These data suggest that both the late stage of seed development and maturation are changed in Brassica by growth in a microgravity environment . While gravity is not absolutely required for any step in the plant life c ycle, seed quality in Brassica is compromised by development in microgravit y. (C) 2000 Annals of Botany Company.