How Well Does the Clinostat Mimic the Effect of Microgravity on Plant Cells and Organs

The effect of clinostatting and microgravity on plant cells and organs is considered on the basis of distinguishing two types of gravistimulation: static and dynamic. The former is switched off both by clinostatting and microgravity, the latter is switched off by microgravity but occurs inevitably during clinostatting and may be perceived by cells if the rotation is not fast enough. Effects of clinostatting and microgravity on different examples of static gravistimulation (tonic effects, formation of compression wood, growth of "grass nodes," compensation of epinasty, stabilization of cellular polarity) are considered. The mechanism of the dynamic stimulation is presented; it is related to the displacement of the gravity sensing masses in the cell containing them, and involves disturbance of cytoskeletal tension. The low threshold for gravity perception and short minimal time of dynamic stimulation are emphasized. Only a relatively fast rotating clinostat, on which the radial distance of the cells from the rotational axis is small enough to keep the centrifugal force low, can effectively compensate gravity. However, one must take into account the extreme sensitivity of plants to mechanical stresses that may appear during clinostatting at different levels of plant organization.