SEPARATION OF PHOTOLABILE-PHYTOCHROME AND PHOTOSTABLE-PHYTOCHROME ACTIONS ON GROWTH AND MICROTUBULE ORIENTATION IN MAIZE COLEOPTILES - A PHYSIOLOGICAL APPROACH

For separating the physiological actions of photolabile (phy-l) and ph otostable phytochromes, we compared the effects of red (R) and far-red (FR) light on elongation growth and microtubule reorientation in segm ents of maize (Zea mays L.) coleoptiles raised either in darkness (phy -l present) or preirradiated with R (phy-l eliminated). In 4.5-d-old d ark-grown seedlings R first promoted growth and induced a transverse m icrotubule orientation. In continuous R the phytochrome action respons ible for these responses was replaced by an opposite phytochrome actio n that produced a stable growth inhibition and longitudinal microtubul e orientation. In R-preirradiated segments only the second type of phy tochrome action could be observed. Reversion experiments with FR light pulses demonstrated that both types of phytochrome action were depend ent on the FR-absorbing form of phytochrome and mirrored the actual ph ytochrome state after 1 h. We conclude from these and related results that growth promotion and transverse microtubule orientation are media ted by phy-l, whereas growth inhibition and longitudinal microtubule o rientation are mediated by photostable phytochrome. The opposite actio ns of the two phytochromes can be separated by preirradiating the seed lings with R. Photoresponsiveness ascribed to phy-l disappeared after 5 d. phy-l appears to play a distinct but transitory role in coleoptil e development.