LIGHT REGULATION OF THE GROWTH-RESPONSE IN CORN ROOT GRAVITROPISM

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.