Photobiology of diagravitropic maize roots.

Light-induced modification of gravitropism in etiolated roots of Zea mays cv Bear x W38 is a low fluence response mediated by phytochrome. This cultivar has a threshold of 10(-6) mol m(-2) and becomes saturated with 10(-2) mol m(-2) of red light. The maximum light-mediated response of 32 degrees downward from horizontal occurs in roots 10 to 30 millimeters in length, 120 to 165 minutes after irradiation. Reciprocity is valid from 2 to at least 9,000 seconds and the response can be about 90% reversed by far red light. Photoreversibility is lost (;escape' occurs) about 20 minutes after red irradiation but appears to be regained 60 to 80 minutes later. A red light-induced (or synchronized) nutation in the apparent curvature rather than unusual escape characteristics may explain these results.