Changes in Photosensitive Stem Growth in Intact Peas Following Irradiation

Etiolated pea seedlings given a short red-light pretreatment followed by 30 hr of darkness no longer showed a typical red-light inhibition of internode elongation. The induction of phytochrome-insensitive growth was itself mediated by phytochrome, since far-red light reversed the effect of the short red-light pretreatment. Peas grown in white light showed a similar insensitivity to red light. However, in this instance the phytochrome system exerted some control over internode elongation since far-red light promoted growth slightly, and this effect was red-reversible.The loss of sensitivity to red light was correlated with a decrease in the amount of spectrophotometrically assayable phytochrome. However, the loss of phytochrome occurred in a relatively short time compared to the period necessary to attain maximal insensitive growth (2 hr versus about 30 hr). Also, after the red-light and dark pretreatment, although 40% of the original amount of phytochrome remained, red light had no effect on elongation. Neither loss of phytochrome nor loss of red-light sensitive growth was observed at 0 to 1 degrees .