species using seedlings planted in moist sawdust in a metal box heated on one side by a
gas burner and cooled on the other side by water.
Others followed Wortmann’s work, but a consensus on the existence of thermotropism
was not reached until recently. Fortin and Poff (1990) showed that primary roots of maize
reoriented from their original vertical direction when exposed to a 4.2oC cm–1thermal
gradient applied perpendicular to the gravity vector, indicating that a thermal gradient can
be sensed by roots. This positive root thermotropic curvature might represent the inte-
grated sum of thermotropism and gravitropism (Fortin and Poff 1991). However, when
roots were placed horizontally under 1 gwith a vertical thermal gradient, the thermal stim-
CHAPTER 6 OTHER TROPISMS AND THEIR RELATIONSHIP TO GRAVITROPISM 133
Figure 6.4 (also see Color Section). Chemotropism in pollen tubes. A concentration gradient of GABA af-
fects the final stages of directional guidance of pollen tubes to the ovule. A. In the wild-type ovule, the inner
integument possess a much higher level of GABA than do other tissues of the ovule or the septum, and the
pollen tube grows toward the micropyle. In the pop2mutant ovule, GABA concentrations in the entire ovule,
including the funiculus and outer integument, are very high, and that in the inner integument is even higher.
The pollen tube of pop2mutants fails to direct its growth toward the micropyle, and grows randomly in the
ovary (after Ma 2003). B. Lily pollen tube showing three consecutive reorientation responses, which were in-
duced by moving the NO source to the locations marked with arrows. The growth axis of the pollen tube al-
ways developed right angles after each challenge by the NO source facing the pollen tube-tip.
(From Feijó JA, Costa SS, Prado AM, Becker JD, Certal AC. 2004. Signaling by tips. Current Opinion in
Plant Biology7:589-598, with permission.)