The literature indicates that the tip of maize (Zea mays L.) coleoptil
es has the localized functions of producing auxin for growth and perce
iving unilateral light stimuli and translocating auxin laterally for p
hototropism. There is evidence that the auxin-producing function of th
e tip is restored in decapitated coleoptiles. We examined whether the
functions for phototropism are also restored by using blue-light condi
tions that induced a first pulse-induced positive phototropism (fPIPP)
and a time-dependent phototropism (TDP). When the apical 5 mm, in whi
ch photosensing predominantly takes place, was removed, no detectable
fPIPP occurred even if indole-3-acetic acid (lanolin mixture) was appl
ied to the cut end. However, when the blue-light stimulation was delay
ed after decapitation, fPIPP became inducible in the coleoptile stumps
supplied with indole-3-acetic-acid/lanolin (0.01 mg g(-1)), indicatin
g that phototropic responsiveness was restored. This restoration progr
essed 1 to 2 h after decapitation, and the curvature response became c
omparable to that of intact coleoptiles. The results for TDP were qual
itatively similar, but some quantitative differences were observed. It
appeared that the overall TDP was based on a major photosensing mecha
nism specific to the tip and on at least one additional mechanism not
specific to the tip, and that the tip-specific TDP was restored in dec
apitated coleoptiles with kinetics similar to that for fPIPP. It is su
ggested that the photoreceptor system, which accounts for fPIPP and a
substantial part of TDP, is regenerated in decapitated coleoptiles, pe
rhaps together with the mechanism for lateral auxin translocation.