Directional light perception
The steady change of stem curvature throughout the day that
characterizes solar tracking may be driven by the ongoing stimulation of the mechanisms that generate permanent curvature of
young sunflower hypocotyls as a phototropic response to persistent unilateral light [28]. These mechanisms have received a
significant amount of study in sunflower, and considering their
details is instructive [29–32]. For instance, when one cotyledon is
shaded, directional growth occurs toward the illuminated cotyledon [10,29]. Notably, hypocotyls of de-etiolated seedlings bend
toward direct illumination by blue light, but the same phototropic
response is not observed with red light [33]. Bending of etiolated
seedling hypocotyls is similarly dependent on light quality [9].
Notably, the precision of heliotropic leaf and floral movements
is maintained with fidelity under blue light illumination but not