Effect of Red Light on Geotropism in Pea Epicotyls

Dose response curves were determined for phytochrome phototransformation and for a phytochrome-controlled decrease in geotropic curvature in epicotyls of dark-grown Pisum sativum L. cv. Alaska. Ten times as much light was required to produce a spectrophotometrically detectable transformation of phytochrome as was required to produce a significant change in the geotropic response. The red light energy required for a 50% phytochrome transformation caused a 90% change in the physiological response.