Light promotes the net acropetal movement of 14C through 6-mm subapical segments of dark-grown roots of Zea mays supplied at their basal ends with 1 tzM IAA-I-laC in agar blocks. This promotion occurs only when the segments are irradiated during the transport period, and both red and blue light appear to be as effective as white light at the radiant flux densities used in this investigation. The promotion is not found if the segments are pretreated with light and then returned to darkness before the transport of IAA-l-14C is determined. The very slight basipetal movement of 14C through the segments supplied with an apical source of IAA-l-14C is unaffected by light.
Only one radioactive substance is found in the apical receiver blocks. This substance has an Rf virtually identical to those of the stock solution of IA_A incorporated into ~he donor block and of unlabelled IAA. The movement of radioactivity into the receiver blocks through the illuminated segments therefore appears to reflect the movement of IAA. Light thus increases the acropetal movement of IAA through the Zea root segments.
The primary roots of Zea mays var. Giant Horse Tooth seedlings grown in total darkness do not exhibit a positive geotropic response. When the seed is orientated with the embryo uppermost the radicle grows out horizontally. On exposure to light, however, the roots bend down. This reaction appears about 3--9 hours after the onset of illumination, and white, red and blue light appear to be equally effective at the flux densities employed in this study. Green light in the spectral band between 510--530 nm did not appear to induce this positive geotropic responsiveness.