This daily cycle dampens and eventually stops at anthesis,
after which the sunflower head maintains an easterly orientation. Although shoot apical heliotropism
has long been the subject of physiological studies in sunflower, the underlying developmental, cellular,
and molecular mechanisms that drive the directional growth and curvature of the stem in response
to extrinsic and perhaps intrinsic cues are not known. Furthermore, the ecological functions of solar
tracking and the easterly orientation ofmature heads have been the subject ofsignificant but unresolved
speculation. In this review, we discuss the current state of knowledge about this complex, dynamic
trait. Candidate mechanisms that may contribute to daytime and nighttime movement are highlighted,
including light signaling, hormonal action, and circadian regulation of growth pathways. The merits of
the diverse hypotheses advanced to explain the adaptive significance of heliotropism in sunflower are
also considered