The sensitivity of plants to light is difficult to study on Earth due to the interacting ef-
fects from gravity. Therefore, microgravity has been explored to determine the sensitiv-
ity of plants to different wavelengths and intensity of light. The first intensive study on
plant phototropic responses in microgravity was performed on wheat (Triticum aestivum)
seedlings (Heathcote et al. 1995a). Wheat coleoptiles showed a significantly enhanced
curvature response to 6 or 9 seconds of photostimulation in microgravity compared to
ground controls, although no significant differences in curvature were found after 3, 501,
or 1,998 seconds of photostimulation (Heathcote et al. 1995a).
It appears that in microgravity the phototropic curvature may be enhanced relative to
plants grown in 1g, but this enhancement may also depend on the total fluence of the il-
lumination provided. An enhanced curvature response to illumination was found from
clinostat-grown plants relative to both space-grown and ground controls regardless of du-
ration of stimulation (Heathcote et al. 1995a). Since enhanced curving responses were
not found for all durations of light stimulation in microgravity, it appears that in this case,
again, the clinostat does not effectively represent the microgravity environment provided
in space. Other studies with maize also showed an enhanced curvature in response to di-
rectional illumination from coleoptiles grown on clinostats relative to 1gcontrols (Nick
and Schäfer 1988).
The phototropism of moss also was studied in microgravity during spaceflight (Kern
and Sack 2001). Apical cells (protonemata) of moss can display both positive and nega-
tive phototropism to red light. The kinetics of alignment in the red light path (~1.5 μmol
m–2s–1) was similar between protonemata from moss grown in 1gand in microgravity.
However, for dark-grown cultures that were exposed to low irradiance of red light (~50
nmol m–2s–1), more of the protonemata (70%) had aligned in the light path (±45 degrees
of the path) in microgravity compared to 1gcontrols. These results suggest that, on Earth,
gravitropism and phototropism both interact at low fluence of red light to orient moss
protonemata. Interestingly, dark-grown protonemata grew in spirals in microgravity and
an experiment on Space Shuttle mission STS-107 was performed to study this response.
However, due to the Space Shuttle Columbiaaccident, there were relatively few results
from this study (Kern et al. 2005).
Other experiments have been performed to study phototropism in plants in the micrograv-
ity environment, although results have been limited. A future experiment is planned for the
ISS using hardware developed by NASA and the European Space Agency (ESA). This exper-
iment, called TROPI for tropisms, will monitor phototropic responses of roots and hypocotyls
ofArabidopsisto various gravitational accelerations (Figure 8.4and Color Section) in both
red and blue light (Correll et al. 2005). In addition, gene profile analyses on seedlings will be
performed to study the interacting effects of light and gravity on gene expression patterns.
8.6 Hydrotropism, autotropism, and oxytropism
All other tropisms directly interact with gravitropism, although only a few of these have
been studied in space. Roots of some species of plants have shown a positive curvature to-
ward water (i.e., hydrotropism; see Chapter 6) on Earth, but this can be masked by the grav-
itropic response. For example, hydrotropism in pea seedlings was evident in plants that