- What is the minimum gravitational acceleration that can be detected in plants to in-
duce a response (g-threshold)? - How do the presentation time and acceleration dose interact to induce a response
(threshold dose; gravitational acceleration time)?
Various doses of acceleration have been provided to plants using clinostats, cen-
trifuges, and microgravity to study the sensitivity of plants to gravitational acceleration
(Brown et al. 1995; Volkmann and Tewinkel 1996; Hejnowicz et al. 1998; Perbal et al.
2004). Originally, it was hypothesized that the reciprocity rule for tropism governed
gravity-induced curvature in plants. The reciprocity rule suggests that if g(gravitation ac-
celeration) and t(time of stimulation) are varied reciprocally to maintain a constant prod-
uct, then the curvature or response should be the same (Johnsson et al. 1995). For oat
(Avena sativa) coleoptiles, reciprocity appeared to hold true for lower accelerations (0.2
to 1g) and stimulation times of 2 to 65 minutes for plants grown in microgravity and on
clinostats (Johnsson 1965; Shen-Miller 1970; Johnsson et al. 1995). However, the sensi-
tivity of oat to gravity also depended on the stage of the plant (i.e, height; Shen-Miller
1970; Johnsson 1965; Johnsson et al. 1995). For example, the threshold dose for taller
plants showing detectable curvature was about 55 g·s, whereas for shorter plants it was
approximately 120 g·s.Therefore, the developmental age or size of the plant may have an
influence on the threshold dose that can be sensed.
For other plant species, the reciprocity rule did not represent the response. For exam-
ple, the curvature of cress roots in microgravity after a dose of 60 g·s for treatments of
0.1gfor 600 seconds and 1gfor 60 seconds should result in similar magnitudes if reci-
procity were valid. However, results indicated that the 0.1g600 seconds treatment had
about half (14 degrees) of the curvature compared to roots from the 1g60 seconds
treatment (32 degrees; Volkmann and Tewinkel 1996, 1998).
Other factors may affect the sensitivity of a plant to gravity, such as the growth his-
tory of the plant. For example, roots from cress grown in microgravity were more sen-
sitive to the stimulation time than were roots from plants grown in a 1gcentrifuge, with
threshold doses of 30 g·s and 60 g·s, respectively (Volkmann and Tewinkel 1996).
Enhanced curvature was also found for lentil roots previously grown in microgravity
compared to plants that were grown on a 1gcentrifuge (Perbal et al. 2004). Surprisingly,
roots from transgenic and wild-type rapeseed (Brassica napus) during microgravity had
no detectable curvature after 1 hour of centrifugation at 1g, whereas roots from ground
controls curved significantly after reorientation (Iversen et al. 1996). These results were
not attributed to altered growth rates between microgravity-grown plants and ground
controls or the additional accelerations imposed by uncontrolled vibrations in the space
craft, but may have been due to other technical difficulties imposed by the spaceflight
environment.
The threshold duration (i.e., the shortest duration of time capable of eliciting a de-
tectable gravitropic response) in microgravity also appears to be variable. For oat, the re-
sponse was less than 1 minute at 1g(Brown et al. 1995). These results are consistent with
Arabidopsisroots, where less than 1 minute at 1gwas also found to cause curvature (Kiss
et al. 1989). However, some researchers propose, based on mathematical extrapolation,
that the threshold value is 1 g·s, indicating that only 1 second at 1g(Earth) is required to