The ability of clinostats to simulate microgravity was evaluated by comparing lentil (Lens culinnrias L. cv. Verte du Puy) seedlings grown in space (Spacelab D1 Mission) with seedlings grown on a slowly rotating elinostat. Seeds were germinated and incubated for 25.5 h at 22°C (1) in microgravity, (2) on a 1g‐centrifuge in space. (3) on a slowly rotating elinostat and (4) on the ground. Morphological (root length and orientation) and ultrastructural (distribution of amyloplasts, location of the nucleus in statocytes) parameters were studied. For clinostat experiments, two different configurations were employed: the longitudinal axis of the root was parallel (horizontal elinorotation) or perpendicular (vertical elinorotation) to the axis of rotation. the same configurations were used for the lg‐controls. Root length and orientation were similar for roots grown on the clinostat and in microgravity. The amyloplasts were identically distributed in statocytes of horizontally clinorolated roots and in statocytes differentiated in microgravity. However, the location of the nucleus was similar in vertically rotated roots and microgravity samples. Since the involvement of the nucleus in graviperception is not known, it can be concluded that horizontal clinorotation simulates microgravity better than vertical elinorotation.
The location of the nucleus in statocytes or lentil roots grown: 1), at 1 g on the ground, 2), on a 1 g centrifuge in space, 3), in simulated microgravity on a slowly rotating clinostat (0.9 rmp) 4), in microgravity in space was investigated and statistically evaluated. In cells differentiated at 1 g on the ground, the nuclear membrane was almost in contact with the plasmalemma lining the proximal cell wall, whereas in statocytes of roots crown on the clinostat there was a distance of 0.47 micrometers (horizontal clinorotation) and or 0.76 micrometers (vertical clinorotation) between these membranes. However, in microgravity the nucleus was the most displaced, 0.87 micrometers from the proximal cell wall. Centrifugation of vertically grown roots in the root-tip direction showed that the threshold of centrifugal force to detach all nuclei from the proximal cell wall was about 40 g. In statocytes developed in the presence of cytochalasin B at 1 g the nuclei were sedimented on the amyloplasts at the distal cell pole, demonstrating that the location of the nucleus depends on actin filaments. The results obtained are in agreement with the hypothesis that gravity causes a tension of actin filaments and that this part of the cytoskeleton undergoes a relaxation in microgravity.
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