The role of light in suppressing hypocotyl elongation in lettuce and Petunia

Summary Hypocotyl elongation in two varieties of Petunia and in Grand Rapids lettuce is shown to be affected by a high-energy reaction and by phytochrome action. These two photoreactions interact in such a way that, on the one hand, shortening of the hypocotyls due to the high-energy reaction can be entirely masked by brief terminal far-red light treatment, while on the other hand, there is no evidence of phytochrome action unless brief exposures to red light are preceded by relatively long exposure of high-intensity.The action spectra for the high-energy reaction show peak effectiveness at wavelengths of 430–450 m, with a minor peak at 660 m in Comanche Petunia, at 700 m in Pink Cascade Petunia, and at 720 m in Grand Rapids lettuce.Prior treatment with DCMU did not reduce the effect of high-intensity light on hypocotyl lengths in lettuce.The nature of the high-energy reaction, and the relation between it and phytochrome action are discussed. Besides these two photoreactions there appears to be a direct effect of light on elongation, blue light preventing, and far-red light accelerating, elongation during actual exposure.With 9 Figures in the Text