Light-controlled Stem Elongation in Pea Seedlings Grown under Varied Light Conditions

There appears to be an orderly transition from one photosensitive state to another in light-controlled stem elongation in the garden pea, Pisum sativum L. cv. Alaska. Stem elongation in dark-grown plants is known to be phytochrome-controlled. However, seedlings are insensitive to phytochrome after a red light pretreatment. An action spectrum for inhibition in these seedlings has peaks at 440 and 620 nm. Protochlorophyll is suggested as the photoreceptor. If these red pretreated seedlings are further exposed to white light, sensitivity to 440 to 620 nm light is lost. Promotion by blue-green light is the only photoresponse shown by these seedlings. Inhibition of completely white light-grown seedlings required simultaneous exposure to high intensity blue light and 600 nm light.