8 Space-Based Research on Plant Tropisms
Melanie J. Correll and John Z. Kiss*1618.1 Introduction—The Variety of Plant Movements
Although plants are generally considered stationary organisms, they do move in response
to a variety of environmental stimuli. Plants can move their organs and direct their growth
to avoid harmful situations or to find important resources for survival. The movements of
plants can be classified into three main categories: nastic responses, circumnutations, and
tropisms.
Nastic movements, sometimes called turgor movements, include motion of plants to
external environments that do not depend on the direction of the stimulus source.
Examples include motion due to changes in temperature (thermonasty), light (pho-
tonasty), touch (haptonasty), humidity (hydronasty), chemicals (chemonasty), as well as
other stimuli. Nastic movements may allow the plant to protect itself from harmful ele-
ments or improve growth, development, and reproductive opportunities. These move-
ments can be seen in the opening and closing of some flowers due to the light/dark cy-
cles during the day, or the movement of leaves of the “sensitive plant” (Mimosa) when
touched (see Chapter 5). The mechanisms and level of interactions of nastic movements
with other plant movements are unclear.
In contrast, circumnutations are the endogenous oscillatory movements of plants
around a central axis. Circumnutation may be a simple consequence of growth and may
serve as a mechanism for plant movements to adjust plant growth from movements that
overshoot the plumb line of gravity (Barlow et al. 1994; Antonsen et al. 1995; Kiss 2006).
The purpose of circumnutation is unknown and may simply be a consequence of general
growth processes. In Arabidopsis thaliana,circumnutation of organs has been shown to
be modulated by the circadian clock, with greatest movement occurring during dawn
(Niinuma et al. 2005). The role of gravity in circumnutation is still unclear, although re-
cent evidence suggests that gravity and circumnutation are inherently linked (Kitazawa et
al. 2005; Kiss 2006; Yoshihara and Iino 2006).
Tropisms are the directed growth of a plant organ in response to external stimuli.
Unlike nastic movements, tropisms depend on the direction of the stimulus. Curvature of
a plant organ toward the stimulus is termed positive tropism, and curvature away is
termed negative tropism. The best-characterized tropisms are directed growth in response
to gravity (gravitropism; see also Chapters 1 and 2), light (phototropism; Chapter 4),
water (hydrotropism; Chapter 6), touch (thigmotropism; Chapter 5) and oxygen (oxytro-
pism; Chapter 6), although many other tropisms have been reported.
Since several types of plant movements can occur simultaneously, the ability to study
one movement without the interactions from other movements is extremely difficult. For
example, on Earth, gravity is ubiquitous. Thus, movements induced by gravity inevitably
*Corresponding author