ulus at 15 oC was stronger than the gravity stimulus and the root curved toward the top of
the dish (Fortin and Poff 1991), providing additional evidence that thermal gradient sens-
ing is accountable for the thermotropic response. The thermoreceptors in plants remain to
be discovered but, in animals, temperature-sensitive channels can relay thermal informa-
tion. Thermotropism could be of adaptive value for root optimal development since hori-
zontal temperature gradients are common in agroecosystems (Fortin and Poff 1990).
Molisch (1884) was the first to demonstrate an aerotropic response of plant roots, with
Pfeffer (1906) reviewing research on this topic and proposing the term oxytropism. As
with many of the tropisms discussed in this chapter, research in oxytropism was ignored
for 60 years until several reports of altered root system growth resulting from limiting O 2
availability appeared (listed in Porterfield and Musgrave, 1998). Recently, oxytropism
has been reexamined in a microrhizotron capable of producing and maintaining an O 2
gradient of 0.8 mmol mol–1 mm–1using two cultivars of pea: Weibul’s Apolo and
ageotropum(Porterfield and Musgrave 1998). Oxytropic curvature was seen all along the
O 2 gradient in both cultivars of pea, with growth toward the higher O 2 concentrations re-
gardless of the starting position within the O 2 gradient. Roots of ageotropumshowed a
curvature of 90 degrees into the O 2 gradient, in contrast to the gravity-sensing cultivar,
which only curved 45 degrees. The curvature of the Weibul’s Apolo cultivar indicates that
roots normally integrate both the gravity and oxygen signal, resulting in the diageotropic
and plagiotropic growth seen in response to soil flooding (Huck 1970).
Oxytropism may allow roots to evade O 2 -deprived soil strata and may also be the basis
of an auto-avoidance mechanism, diminishing the competition between roots for water
and nutrients as well as oxygen (Porterfield and Musgrave 1998). In addition, it has been
shown that pollen tube guidance is influenced not only by chemical or water gradients,
but also by oxygen gradients. Pollen tubes of several species showed a clear tropic re-
sponse to oxygen gradients in an in vitrosystem (Blasiak et al. 2001). The biological sig-
nificance of this phenomenon in vivohas not been analyzed yet, but may be critical for
orienting the pollen tubes toward the stigma, or in maintaining basipetal growth in the
style.
6.6 Traumatropism
Wounding is a mechanical process that harms cells in a localized region, but which also
typically results in alterations in the activities of cells in other regions (Imaseki 1985). A
less well-known effect of wounding is the induction of differential growth in the wounded
organ within the first hour or so after damage. Pfeffer introduced the term traumatropism
in 1893 to describe such wound-induced, directional growth responses. Along with his
experiments on thigmotropism, Darwin (1881) also analyzed the responses of roots to
wounding. The root response to injury was found to be similar to touch, that is, if the in-
jury was close to the root tip, the root curved away from the side that was wounded, oc-
casionally even to the extent of forming a 90-degree curvature; but if the wound was just
beyond the tip, the root bent toward the wounded side.
Aerial organs, overall, seem to be much less sensitive than roots to injury stimuli. Most
of this work has been performed on coleoptiles, in which slicing or abrasion can induce