forces were even used to reversibly abolish graviresponsiveness by disturbing the
polar cytoplasmic organization of statocytes (Wendt and Sievers 1986).
Since Julius Sachs made his first attempts to neutralize the unilateral effect of
gravity by rotating plants around a horizontal axis (Sachs 1882), numerous types of
clinostats have been developed in order to simulate the effects of weightlessness
on Earth (cf. Chap. 2). Classical clinostats rotating with 1–10 rpm, fast-rotating
clinostats rotating with 50–120 rpm as well as three-dimensional clinostats and
random positioning machines rotating around one axis or two axes have shed light
on the gravisensitivity, perception and presentation time of gravitropism in roots and
shoots.
A new quality of research tools for studies on gravitropism became available with
the advent of drop towers, parabolic flights of aircrafts and rockets, NASAs Space
Shuttles, satellites and Space Stations in the low Earth orbit (cf. Chap. 2). With the
completion of the International Space Station ISS with the commissioning of the
Columbus module in 2008, the biggest microgravity lab that ever existed provides an
almost stimulus-free environment of real microgravity for long-term experimentation. The microgravity quality in a range of 10
À3
–10
À6 g is beyond the susceptibility
of most biological sensory systems and is, therefore, regarded as functional weightlessness allowing biologists to address especially molecular and cellular mechanisms
involved in plant gravity sensing and graviorientation. Only after cress seeds had
been germinated on the Space Shuttle (Volkmann et al. 1986) and on Russian Bion
satellites (Laurinavicius et al. 1996), it became evident that the development of
polarly organized statocytes and gravisensing mechanisms are neither induced nor
affected by the absence of gravity. And although a reduced starch content was
reported, microgravity-grown roots responded more strongly to only small acceleration doses in microgravity aboard Space Shuttles (Volkmann and Tewinkel 1996;
Perbal et al. 2004). The presentation time, the time a stimulus needs to be applied
continuously to a sensing system in order to trigger a response, was found to be in the
range of 20–30 s when stimulated with normal 1 g acceleration, whereas the
presentation time of cress and lentil roots grown on a 1 g centrifuges was in a
range of 50–60 s (Perbal and Driss-Ecole 1994; Volkmann and Tewinkel 1996). By
rotating roots on a clinostat and stopping it several times, a perception time of 1 s was
determined for cress roots. The perception time defines the minimum time a stimulus
is registered by sensing systems but must be given repeatedly in order to trigger a
response (Heinowicz et al. 1998). In this short period, statoliths in gravistimulated
roots are displaced only a fraction of a μm, which is good evidence that—taken into
account that actin is not required for graviperception as was demonstrated by the
uninhibited graviresponse of roots with disrupted actin microfilaments (Hou et al.
2003)—graviperception must occur very close to statoliths already sedimented on or
in close contact with a gravisensitive endoplasmic reticulum membrane.
Clinorotating seedling has been used very often to successfully prevent static
gravitropic stimulation of roots by randomizing the gravity vector and, consequently, roots continued to grow straight. However, clinorotation failed in most
cases to eliminate also dynamic stimulation. There are studies reporting an
overloading of the sensory system due to vibrations and shifting statoliths
6.5 Microgravity Research and Modifying Gravitational Acceleration. . .
87
polar cytoplasmic organization of statocytes (Wendt and Sievers 1986).
Since Julius Sachs made his first attempts to neutralize the unilateral effect of
gravity by rotating plants around a horizontal axis (Sachs 1882), numerous types of
clinostats have been developed in order to simulate the effects of weightlessness
on Earth (cf. Chap. 2). Classical clinostats rotating with 1–10 rpm, fast-rotating
clinostats rotating with 50–120 rpm as well as three-dimensional clinostats and
random positioning machines rotating around one axis or two axes have shed light
on the gravisensitivity, perception and presentation time of gravitropism in roots and
shoots.
A new quality of research tools for studies on gravitropism became available with
the advent of drop towers, parabolic flights of aircrafts and rockets, NASAs Space
Shuttles, satellites and Space Stations in the low Earth orbit (cf. Chap. 2). With the
completion of the International Space Station ISS with the commissioning of the
Columbus module in 2008, the biggest microgravity lab that ever existed provides an
almost stimulus-free environment of real microgravity for long-term experimentation. The microgravity quality in a range of 10
À3
–10
À6 g is beyond the susceptibility
of most biological sensory systems and is, therefore, regarded as functional weightlessness allowing biologists to address especially molecular and cellular mechanisms
involved in plant gravity sensing and graviorientation. Only after cress seeds had
been germinated on the Space Shuttle (Volkmann et al. 1986) and on Russian Bion
satellites (Laurinavicius et al. 1996), it became evident that the development of
polarly organized statocytes and gravisensing mechanisms are neither induced nor
affected by the absence of gravity. And although a reduced starch content was
reported, microgravity-grown roots responded more strongly to only small acceleration doses in microgravity aboard Space Shuttles (Volkmann and Tewinkel 1996;
Perbal et al. 2004). The presentation time, the time a stimulus needs to be applied
continuously to a sensing system in order to trigger a response, was found to be in the
range of 20–30 s when stimulated with normal 1 g acceleration, whereas the
presentation time of cress and lentil roots grown on a 1 g centrifuges was in a
range of 50–60 s (Perbal and Driss-Ecole 1994; Volkmann and Tewinkel 1996). By
rotating roots on a clinostat and stopping it several times, a perception time of 1 s was
determined for cress roots. The perception time defines the minimum time a stimulus
is registered by sensing systems but must be given repeatedly in order to trigger a
response (Heinowicz et al. 1998). In this short period, statoliths in gravistimulated
roots are displaced only a fraction of a μm, which is good evidence that—taken into
account that actin is not required for graviperception as was demonstrated by the
uninhibited graviresponse of roots with disrupted actin microfilaments (Hou et al.
2003)—graviperception must occur very close to statoliths already sedimented on or
in close contact with a gravisensitive endoplasmic reticulum membrane.
Clinorotating seedling has been used very often to successfully prevent static
gravitropic stimulation of roots by randomizing the gravity vector and, consequently, roots continued to grow straight. However, clinorotation failed in most
cases to eliminate also dynamic stimulation. There are studies reporting an
overloading of the sensory system due to vibrations and shifting statoliths
6.5 Microgravity Research and Modifying Gravitational Acceleration. . .
87