NUCLEOLAR TRANSFORMATION IN PLANTS GROWN ON CLINOSTATS

Cells of carrot calli (Daucus carota L.) grown on clinostats (simulate d weightlessness) exhibit increases in nucleolar number and volume. In clinostat-grown whole barley plants (Hordeum vulgare L. cv. Steptoe), nucleoli in similar to 70% of root meristem and root cortical cells i n the 1 mm root apex exhibit multiple nodulations after one day of gro wth. The nucleolar nodules (1.1 mu m mean diameter) are densely and fi nely fibrous, distinctly different from the nucleolus in which the con tent is so compact that the granular component is masked. Control nucl eoli (from vertically rotated and stationary seedlings) rarely exhibit nodule-like protrusions, are not compact, and contain a well defined granular component. Proteins that are heat soluble, characteristic of many stress responses, rapidly increase in barley grown on clinostats. Barley growth on clinostats is slowly and steadily inhibited. There i s no difference between vertically rotated and stationary controls for any of the parameters measured, indicating that clinostat motion per se does not affect significantly barley development. The evidence take n together suggests that barley plants germinated and grown on clinost ats are stressed, the effects of which are expressed sequentially by a lteration of nucleolar morphology, increased production of heat-solubl e proteins, and decreased plant growth. Similar stress-related changes may be expected to occur in plants subjected to weightlessness during space flight. It is therefore of interest that nucleoli in wheat root s (Triticum aestivum L. cv. Broom) obtained from the space flight IML- 1 mission show irregularity that is not observed in any of the ground controls for the flight experiment.