Gravisensitivity of cress roots: investigations of threshold values under specific conditions of sensor physiology in microgravity*

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Abstract
The minimum dose (dose = stimulus × time), one of three threshold values related to gravity, was determined under microgravity conditions for cress roots. Seedlings were cultivated on a 1g centrifuge in orbit and under microgravity, respectively. After continous stimulation on a threshold centrifuge, minimum doses of 20–30 gs for microgravity roots and 50–60 gs for roots grown on a 1g centrifuge were estimated, which indicated that micro-gravity roots have a higher sensitivity than 1g roots. These results do not confirm the threshold value of 12gs which was determined for cress roots using the slow rotating clinostat. Following application of intermittent stimuli to microgravity-grown roots, gravitropic responses were observed after two stimuli of 13-5 gs separated by a stimulus-free interval of 118 s. Generally, this demonstrates that higher plants are able to ‘sum up’ stimuli which are below the threshold value. Microscopic investigations of the cellular structure corresponding to stimulations in the range of the threshold value demonstrated a small displacement of statoliths in root statocytes. No significant correlation was observed between gravitropic curvature and statolith displacement. If the statolith theory is accepted, it can be concluded that stimulus transformation must occur in the cytoplasm in the near vicinity of the statoliths and that this transformation system – probably involving cytoskeletal elements – must have been affected during microgravity seedling cultivation.