One more reason to build rotating space stations
Plants that grow in microgravity produce smaller, lower quality seeds. When these seeds germinate their shoots are shorter and weaker. In one experiment using wheat, a food crop, the main observation in the first generation was a complete lack of seeds. Wheat grown in microgravity is 100% sterile. We don't like centrifuges in space, but I think we should seriously consider it.
http://spaceflight.nasa.gov/history/shuttle-mir/science/fb/sc-fb-svet.htm