were grown on a clinostat or in plants that were mutated in their response to gravity (Jaffe
et al. 1985; Takahashi and Suge 1991; Takahashi et al. 1996). Hydrotropism was also ev-
ident in lateral roots from cucumber (Cucurbita ovifera) seedlings grown on a clinostat or
in microgravity (Takahashi et al. 1999). The lateral roots from clinorotated and micrograv-
ity seedlings grew at approximately 40- to 50-degree angles from the primary root axis,
and this curvature was toward the water source, whereas roots from ground controls grew
~90 degrees from the primary root axis. Therefore, although roots do respond to moisture
gradients, it appears that this response is largely masked by gravity-induced responses.
The seemingly random orientation of plant roots out of rooting matrices for plants grown
in microgravity may in fact be a result of oxytropism (Porterfield and Musgrave 1998). On
two space shuttle experiments (STS-54 and STS-68) (Porterfield and Musgrave 1998),
roots of Arabidopsisplants grown in microgravity grew out of the rooting matrices in the
direction of the oxygen gradient. The enzymatic activity, localization, and expression of
mRNA of a protein associated with oxygen stress, alcohol dehydrogenase (ADH), were en-
hanced in roots grown in microgravity compared to ground controls (Porterfield et al.
1997). This observation suggested that roots may either be oxygen-deprived in micrograv-
ity or expressing ADHin response to another stress induced by spaceflight.
To test whether oxygen availability was limited to plants during microgravity, a novel
sensor was designed that measured oxygen availability (not concentration) in micrograv-
ity (Liao et al. 2004). Results using this sensor during parabolic flights showed that oxy-
gen availability changes during periods of microgravity and, therefore, roots from plants
grown in microgravity are likely experiencing oxygen deprivation. Experiments on the
ground showed that indeed roots from both normal and agravitropic mutants of pea
curved in response to oxygen gradients (Porterfield and Musgrave 1998). For example,
roots from normal plants curved about half the amount of roots from agravitropic mu-
tants after 48 hours of treatment. These results suggest that roots respond to gradients of
oxygen, but this is largely masked by gravitropic responses.
Autotropism is the straightening of an organ after the gvector is randomized on a cli-
nostat or is reduced, as is found in microgravity (Stankovic et al. 1998). Cress roots that
curved in response to various gravitational accelerations all underwent straightening once
the acceleration was removed (Stankovic et al. 2001). Roots from clinorotated plants also
showed autotropism, suggesting that the straightening process is a process that does not
depend on the prestimulus orientation.
Microgravity offers a unique ability to identify and study tropic responses of plants.
Studies in microgravity can reduce the interacting effects of gravitropism with other tro-
pisms, allowing the characterization of the tropism of interest without the ever-present
gravity responses. It also seems likely that plants curve in response to a variety of as-yet
unidentified stimuli which may only be found when grown in low gconditions, or with
studies that use plants that lack the ability to perceive gravity.
8.7 Studies of other plant movements in microgravity
The microgravity environment is a tool that can also be used to study a range of other
plant movements and growth responses. These other movements of plants have been