occurred in the same root toward both the anode and cathode, but the two responses took
place in two different regions of the root—the central elongation zone (CEZ) and the
DEZ, respectively (Wolverton et al. 2000). Furthermore, both responses are related to the
electric field rather than one being a secondary response to induced gravitropic stimula-
tion, since these oppositely directed responses could be reproduced individually by a lo-
calized electric field application to the region of response. The electrotropic responses of
plant organs described thus far are quite clear, but very little is known about the actual
mechanisms of stimulus reception and directional guidance.
6.4 Chemotropism
In green plants, the ultimate energy source for growth is light rather than a chemical
input. The directional cues from light, supplemented by the orientation afforded by gravi-
tropic growth, help place the aerial part of the plant in the optimal position to intercept
sunlight. Gravity and light are thus two of the main directional signals engaged in the reg-
ulation of vegetative growth. However, chemicals still act as significant vehicles of com-
munication between individuals in their reproductive behavior or, in the case of insectiv-
orous plants, as signals for obtaining nutrients.
Chemotropism is a directional growth response that is driven by a chemical stimulus;
the term chemotaxis is used to depict the locomotory responses of motile organisms or
gametes. Both forms of response can be positive (toward a beneficial or attractive sub-
stance) or negative (away from a harmful or unattractive substance) (Hart 1990).
Responses to chemical substances have been well documented in lower plants, particu-
larly in unicellular organisms and gametes. In multicellular organisms there is usually a
greater homeostasis of the cellular environment, and chemoresponses to external chemi-
cals seem limited to rather specialized situations. A very wide range of chemicals is im-
plicated in these types of responses, indicative of the variety of adaptive advantages that
can be provided in these situations.
In higher plants, considerable chemical interaction does indeed occur. Many bacteria
and fungi release hormones and hormone-like substances into the soil, and these can have
significant effects on root growth (Bilderback 1985). In some older physiological texts,
it was suggested that roots can develop chemotropic responses to soil nutrients. However,
these suggestions were based upon studies in which chemicals were unilaterally applied
to individual roots of several species (Newcombe and Rhodes 1904), and these methods
do not constitute a robust chemotropic directional assay (i.e., re-directional growth in re-
sponse to the repositioning of the stimulus). However, there is a recent report in which
the root cap seems to sense extracellular glutamate to trigger a reduction in the rate of
cell production and/or cell expansion (Filleur et al. 2005), suggesting a specific response
which is likely to involve the action of a specific receptor, but these authors did not re-
port on a directional response to an actual gradient.
Certain plant organs, however, seem to develop directional responses to chemical stim-
uli, such as the trapping organs of insectivorous plants, and pollen tubes during their
growth down the style. Mechanical stimuli usually elicit the initial release of insect traps
(Chapter 5), but in many cases chemical stimuli seem to enable the subsequent tighten-