interact with movements induced by other stimuli (Barlow 1995). The relatively strong
gravitropic responses of plants can mask comparatively weaker tropisms, such as hy-
drotropism or oxytropism. Therefore, in order to better characterize tropisms, scientists
have used several methods to reduce the effects of gravity on plant motion (Figure 8.1).
These methods include the use of the following:
- Plant mutants that have reduced responses to gravity;
- Experimental systems that prevent plants from receiving unidirectional gravity accel-
eration (i.e., clinostats or random positioning machines); - Microgravity/free-fall environments.
The extent to which these techniques reduce the effects of gravity on plant movement
has been a topic of much debate, with each method having unique advantages and disad-
vantages (Sievers and Hejnowicz 1992; Kordyum 1997; Correll and Kiss 2002).
In this chapter, experimental results from microgravity experiments are compared with
other ground-based studies on plant movements for experiments performed from ~1990
to the present. Although we will concentrate on discussing ground-based and micrograv-
ity research on tropisms, other movements such as circumnutation will also be briefly
considered.
8.2 The microgravity environment
The gravitational acceleration on Earth is approximately 9.8 m/s^2 , defined as 1g. The
term microgravity has been used to refer to levels that are less than 1g, typically 10–3gto
10 –6g. In this chapter, we consider microgravity to range from approximately 1% of
Earth’s gravitational acceleration (0.01g) to approximately 1 millionth of the Earth’s
gravitational acceleration (10–6g). Other terms that have been used to describe different
levels of gravitational accelerations include hypogravity (accelerations less than 1gbut
greater than 10–3g), weightlessness (net sum of forces acting on a body equaling zero),
zero-g (an object that does not experience any gravitational pull), and hypergravity (ac-
celerations greater than 1g) (Schaefer et al. 1993; Klaus 2001).
Since gravity can influence other nongravity-related movements of plants, scientists
have attempted to reduce gravitational effects below biologically detectable levels to
study certain types of plant movements. One way to mitigate the effects of gravity is to
travel away from the major source of gravitational interference, the Earth. Unfortunately,
to reduce the effects of gravity on plants to 1 millionth of Earth’s gravity, you would have
to travel approximately 6.4 million km away from Earth (Rogers et al. 1997). To put this
in context, the moon orbits the Earth at approximately 380,000 km. Thus, studies on plant
movement at this distance are unlikely in the near future, so other options are necessary
to mitigate the effects of gravity on plants.
The most effective way to reduce gravity effects on objects is through the use of free
fall. An object is in free fall when it is falling under the sole influence of gravity, thus ne-
glecting air friction. Free fall can be described as a condition where no external forces
acting on the body of interest produce stress and that any forces cannot be detected by the