elicit a curvature response (Perbal and Driss-Ecole 2002). Future studies in microgravity
can clarify the nature of the duration of stimulus needed to elicit a response in plants.
The lowest acceleration that a plant can detect has been estimated to be approximately
10 –4gfor roots and 10–3gfor shoots (Shen-Miller et al. 1968; Merkys and Laurinavicius
1990). These results were obtained from studies with oat and lettuce (Lactuca sativa)
plants grown on clinostats or in microgravity. Since experimental limitations could not
provide the lower levels of gravitational acceleration, these authors used mathematical
methods to predict the minimum acceleration that could be detected. Microgravity pro-
vided by spaceflight environments may not be low enough to study the minimum accel-
erations that plants can detect, mainly due to other acceleration on the craft from crew ac-
tivity and centrifuges. Therefore, directly identifying the lowest magnitude of
acceleration that plants can detect may be a challenge with today’s technology.
Results from ground-based studies to determine the sensitivity of plants to gravity
using clinostats have been conflicting. For example, the curvature of white clover roots
due to gravity stimulation after previous growth in microgravity, 1g, or on a 2-D clinos-
tat showed that roots only curved ~35 degrees from clinostat-grown plants, ~75 degrees
from 1g-grown plants, and ~60 degrees from microgravity-grown plants after 10 hours
(Smith et al. 1999). The authors suggest that the reduced curving response of roots grown
on the clinostat treatment compared to both plants grown in microgravity and on 1g con-
trols was due to root cap deterioration. Other researchers have found that roots from
microgravity-grown plants are more sensitive to gravity compared to clinostat-grown
plants (Lorenzi and Perbal 1990; Perbal and Driss-Ecole 1994; Volkmann and Tewinkel
1996; Perbal et al. 1997). Therefore, the application of clinostats to study gravity re-
sponses in plants is limited.
8.5 Phototropism
Light is another important stimulus that is involved in determining the direction of plant
growth. Therefore, plants have evolved a variety of photoreceptors to sense the quality, quan-
tity, direction, and intensity of light. In flowering plants, the photoreceptors can be grouped
into the blue/UV-A photoreceptors (cryptochromes and phototropins) and the red/far-red
photoreceptors (phytochromes). Phototropic responses are largely controlled by the pho-
totropins with the other photoreceptors modulating the response (Briggs and Christie 2002).
Hypocotyls and stems typically curve toward blue or white light (positive phototropism),
whereas roots typically curve away from the light source (negative phototropism).
Since all phototropic responses of plants on Earth have to contend with gravity, it is
not surprising that phototropism and gravitropism are constantly interacting to influence
plant form. Recently, a gene downstream of phytochromes was identified in Arabidopsis
that links gravity and light-regulated processes, GIL1(Gravitropic in the Light 1) (Allen
et al. 2006). The hypocotyls of gil1mutants do not show the random gravitropic orienta-
tion found in wild-type plants grown in red or far-red light. Identification of this novel
gene is a significant step toward understanding how light and gravity interact to deter-
mine plant form. The many interacting effects of phototropism and gravitropism are de-
scribed in Chapter 4 and reviewed in Correll and Kiss (2002).