ropic mutants (Eapen et al. 2003; Olsen et al. 1984). Here, we discuss the potential of a
genetic approach for understanding the molecular mechanisms governing root hydrotro-
pism and their interaction with gravitropism.
6.2.1 Early studies of hydrotoprism
Hydrotropism studies have always been hard to interpret because both thigmotropism and
gravitropism interact with hydrotropism. Mechanical stimuli can generally be avoided
(see Chapter 5), but gravity is ubiquitous on Earth. Consequently, several tools, such as
those involving agravitropic mutants, clinorotation, or microgravity in space have been
utilized to differentiate the hydrotropic from the gravitropic response (Takahashi 1997;
see also Chapter 9). Significantly, experiments with the pea mutant ageotropum, whose
roots were agravitropic but responded positively to hydrotropism, indicated that there are
independent sensing and response pathways for these two tropisms (Jaffe et al. 1985).
Hence,ageotropumwas a model system for the study of hydrotropism for many years. In
particular, roots of ageotropumresponded to a gradient in water potential as small as 0.5
MPa (Takano et al. 1995).
Darwin (1881) studied the location of the sensory system for root hydrotropism. He
covered the apical 1 or 2 mm of roots from different species with a hydrophobic mixture
of olive oil and lampblack and found that they no longer responded to a moisture gradi-
ent. When root caps from ageotropumor corn roots were removed, they failed to curve
124 PLANT TROPISMS
Figure 6.1. Hydrotropism in roots. Von Sachs (1887) demonstrated for the first time with the “hanging bas-
ket” technique that roots developed a hydrotropic curvature in response to moisture gradients against the
gravity vector.