6 Other Tropisms and Their Relationship to Gravitropism
Gladys I. Cassab*1236.1 Introduction
Plants have evolved an elaborate and sophisticated set of growth responses to the envi-
ronment that allow them to survive adverse conditions. The degree to which plants de-
pend on environmental cues to orchestrate growth and development is unmatched in the
animal kingdom. Of all of the environmental signals, gravity—which is a constant factor
on Earth—profoundly impacts the form, structure, and function of plants (Hangarter
1997). However, gravitropic response of plant organs can be affected by the abundance
of incoming signals from the environment, and therefore plants modify their growth ac-
cordingly, taking into account all these variables. Responses of plants to environmental
stimuli such as moisture (hydrotropism), temperature (thermotropism), oxygen gradients
(oxytropism), electric fields (electrotropism), touch (thigmotropism; see Chapter 5), and
wounding (traumatropism) are short-term tropic responses that are commonly accom-
plished within a few hours by differential growth. These tropisms have been mostly ob-
served and analyzed in roots, although there are a few studies on pollen tubes, coleop-
tiles, and shoots. Most of these tropisms were documented more than 100 years ago (Hart
1990); nevertheless, research in this field has so far received little attention despite the
biological significance of these growth behaviors in plant survival.
6.2 Hydrotropism
Even though the lack of sufficient water is the single most crucial factor influencing
world agriculture, interest in hydrotropism has fluctuated over the years. Studies on hy-
drotropism have been scarce since Knight and von Sachs (in 1811 and 1872, respectively)
showed that roots move toward water (Takahashi 1997). In particular, von Sachs (1877)
demonstrated that in seedlings grown in a freely hanging sieve basket, the emergent roots
became diverted from the vertical and grew along the bottom of the basket (wet substrate)
(von Sachs 1887) (Figure 6.1).
Around that time, Darwin, Pfeffer, and Weisner (who introduced the term hydrotro-
pism) were all convinced that moisture gradients affected root orientation (Hart 1990).
Interestingly, the idea that plant roots penetrate the soil in search of water to maintain
their growth was first presented as the explanation for the downward orientation of roots
(Dodart, around 1700, reviewed in Hart 1990). However, in comparison to studies on the
roles of other directional signals (such as gravity and light) on the general orientation of
plant organs, studies on hydrotropism have been surprisingly sparse. In fact, genetic
analysis of hydrotropism lagged 19 years behind the first reports of Arabidopsisagravit-
*Corresponding author