D. Volkmann et M. Tewinkel, GRAVISENSITIVITY OF CRESS ROOTS - INVESTIGATIONS OF THRESHOLD VALUES UNDER SPECIFIC CONDITIONS OF SENSOR PHYSIOLOGY IN MICROGRAVITY, Plant, cell and environment, 19(10), 1996, pp. 1195-1202
The minimum dose (dose = stimulus x time), one of three threshold valu
es related to gravity, was determined under microgravity conditions fo
r cress roots. Seedlings were cultivated on a 1g centrifuge in orbit a
nd 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 microgravity roots have a higher sensitivity than 1g r
oots. These results do not confirm the threshold value of 12 gs which
was determined for cress roots using the slow rotating clinostat. Foll
owing application of intermittent stimuli to microgravity-grown roots,
gravitropic responses were observed after two stimuli of 13.5 gs sepa
rated by a stimulus-free interval of 118 s. Generally, this demonstrat
es 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 dem
onstrated a small displacement of statoliths in root statocytes. No si
gnificant correlation was observed between gravitropic curvature and s
tatolith 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 syste
m - probably involving cytoskeletal elements - must have been affected
during microgravity seedling cultivation.