Roots of Merit variety corn (Zea mays L.) require red light for orthog
ravitropic curvature. Experiments were undertaken to identify the step
in the pathway from gravity perception to asymmetric growth on which
light may act. Red light was effective in inducing gravitropism whethe
r it was supplied concomitant with or as long as 30 minutes after the
gravity stimulus (GS). The presentation time was the same whether the
GS was supplied in red light or in darkness. Red light given before th
e GS slightly enhanced the rate of curvature but had little effect on
the lag time or on the final curvature. This enhancement was expanded
by a delay between the red light pulse and the GS. These results indic
ate that gravity perception and at least the initial transduction step
s proceed in the dark. Light may regulate the final growth (motor) pha
se of gravitropism. The time required for full expression of the light
enhancement of curvature is consistent with its involvement in some l
ight-stimulated biosynthetic event.