ages, and lack of biological replicates. Several experiments have noted poor gas compo-
sition or exchange rates which resulted in unusual growth responses of plants in micro-
gravity (Porterfield et al. 1997; Kiss et al. 1998, 1999; Perbal et al. 1996). In addition,
many experiments performed in space have not been replicated, which limits the scien-
tific interpretation of results (Cogoli 1996). The major impedances to studying plant tro-
pisms in space are the limited opportunities and the associated costs. Recent delays in
Space Shuttle flights to the ISS have further limited the amount of science that can be
performed in the future. The future outlook for opportunities to study plant movements
in space is also discussed in Chapter 9 of this book.
8.3 Ground-based studies: mitigating the effects of gravity
The problems associated with studying biological responses to gravity on space missions
have encouraged scientists to develop alternative ground-based methods to simulate mi-
crogravity. Devices such as clinostats and random positioning machines (also known as
three-dimensional clinostats) have been used to reduce gravity effects on plants for tro-
pism studies (Figure 8.1). A clinostat generally rotates the specimen around one axis
whereas a random positioning machine offers three-dimensional rotation (Salisbury
1993; Hoson et al. 1997). The extent to which clinostats simulate microgravity has been
debated, since results from clinostats do not always correspond to those from true micro-
gravity experiments (Brown et al. 1976; Sievers and Hejnowicz 1992; Heathcote et al.
1995a; Brown et al. 1996; Kordyum 1997; Klaus 2001). A comparison of experimental
results of plant and other organisms in microgravity with results from clinostats has been
reviewed (Brown et al. 1976; Albrecht-Buehler 1992; Kordyum 1997) and is discussed in
more detail in the subsequent sections of this chapter.
Another ground-based attempt to reduce gravity effects on plant movements is through
the use of mutants in gravity perception. For example, mutants of Arabidopsisthat have
reduced activity in the phosphoglucomutase gene (PGM) have reduced responses to grav-
ity (Kiss et al. 1989). The pgmplants have reduced amounts of starch-filled amyloplasts
in the cells involved in the perception of gravity. An otherwise cryptic positive, red light-
induced phototropism in roots was discovered with studies using these plants (Ruppel et
al. 2001). Interestingly, hypergravity experiments performed with pgmmutants have
shown that gravitropic orientation of organs of the pgmplants could be restored with ac-
celerations of 5gfor roots and 10gfor hypocotyls (Fitzelle and Kiss 2001). Other starch-
deficient mutants (e.g., agravitropic pea, Pisum sativum) have been used to study other
tropisms such as hydrotropism or oxytropism that may be masked by gravitropic response
(Jaffe et al. 1985; Porterfield and Musgrave 1998).
AnotherArabidopsismutant in the gravity-perception phase lacks its endodermal layer
(scr, SCARECROWmutants), the location of statoliths in shoots (Fukaki et al. 1996;
Fukaki et al. 1998). These mutants have been used to study gravitropism and other plant
movements such as circumnutation (Kitazawa et al. 2005). Currently, there are only a few
mutants that have been identified as being specific to gravity perception in plants, so
studies like these are limited.