Plant Tropisms

object of interest (i.e., the plant). All of the studies with plants in microgravity discussed
in this chapter are actually with plants in free fall.
Some mechanisms to create free fall or a microgravity environment include the use of
drop towers, parabolic flights of airplanes, rockets, and orbiting satellites such as the
Space Shuttle and space stations (Figure 8.1 and Color Section). The quality and duration
of microgravity created in these environments are variable. For example, the longest drop
tower (located in a mine shaft in Japan) is 460 m in length and only provides approxi-
mately 10 sec of free fall (Tokyo 1993). Parabolic flights are when a plane or rocket
climbs rapidly at a ~45-degree angle then descends at a ~45-degree angle (Figure 8.1C).
The parabolic flight between these two maneuvers can reduce gravitational effects to
~10–2g for about 20 sec for airplanes (Pletser 1995) and ~10–5g for up to several minutes
for rockets (European Space Agency 2006). However, since many tropistic responses of
plants can take up to several minutes before detection, the short duration of microgravity
provided by drop towers and parabolic flights limits the types of studies that can be per-
formed with these methods.
Studies with drop towers and parabolic flights have been successfully used with pro-
tists to study the gravitaxis (orientation with respect to the gravity vector) (Häder and
Hemmersbach 1997; see Chapter 7) or to study elements in the gravity-induced-signal
cascade, such as movement of statoliths in plant roots (Volkmann et al. 1991). Parabolic
flights are useful, relatively inexpensive laboratories to study the very early phases of
gravity responses in plants.
However, the best methods for studying tropisms and other plant responses in micro-
gravity are through the use of biosatellites and orbiting spacecraft. Biosatellites (un-
manned missions into space) can provide long-term microgravity, but typically missions
are of about two weeks’ duration. Orbiting space stations such as Skylab, Mir, or the
International Space Station (ISS) have offered the potential to study plant tropisms in a
microgravity environment indefinitely.
All of the methods used to create a reduced-gravity environment have limitations in
their effectiveness as a microgravity laboratory. For example, the quality of acceleration
on orbiting craft depends on the orbital motion of the spacecraft, the position of the item
on the spacecraft, and the aerodynamic drag on the craft. Luckily, these accelerations are
only about 10–6gin magnitude, which is relatively small and possibly below the sensory
threshold for plant responses to gravity (Shen-Miller et al. 1968; Sobick and Sievers
1979; Merkys and Laurinavicius 1990; and see below). However, other accelerations can
be created by vibrations in orbiting craft from fans, pumps, centrifuges, and crew activ-
ity. For the Space Shuttle, these accelerations were found to be about 10–4g, which may
affect some experimental results since the minimum threshold values for root and shoot
curvature may range in the values of 10–4gand 10–3g, respectively (Shen-Miller et al.
1968; Sobick and Sievers 1979; Merkys et al. 1986; Merkys and Laurinavicius 1990).
Therefore, scientists using orbiting spacecraft for microgravity studies should consider
these additional accelerations in their analyses.
Other issues that influence the quality of experiments on plant tropisms performed in
space, aside from gravitational effects, include artifacts from poor gas composition and
exchange, lack of sufficient lighting, radiation effects, unscheduled/scheduled power out-

164 PLANT TROPISMS