Light regulation of the growth response in corn root gravitropism.

Roots of Merit variety corn (Zea mays L.) require red light for orthogravitropic 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 whether 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 the 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 indicate that gravity perception and at least the initial transduction steps proceed in the dark. Light may regulate the final growth (motor) phase of gravitropism. The time required for full expression of the light enhancement of curvature is consistent with its involvement in some light-stimulated biosynthetic event.