Relationships Between Phytochrome State and Photosensitive Growth of Avena Coleoptile Segments

Abstract

Using various photo-stationary state light sources to obtain reproducible phytochrome conversion of from 5 to 88% PFR assayed by 2 wavelength in vivo spectro-photometry, relationships between initial percent PFR and elongation of apical Avena coleoptile segments over the succeeding 20 hours in darkness were studied. With material grown in total darkness, all PFR levels promoted elongation and maximal promotion required roughly 50% PFR. The promotion caused by an initial 5-minute red (88% PFR) treatment at hour 0 was partially reversible at hour 5 by sources forming less than 48% PFR, but totally irreversible at hour 8, though less than 50% of the growth had been accomplished by this time. Direct photometric assays at hour 5 indicated a phytochrome state of roughly 45% PFR, consistent with the reversal data. At hour 8, however, 11-22% of the phytochrome still assayed as PFR, an inconsistency suggesting simply that the elongation process nad proceeded beyond photochemical control. Thus, in contrast to results previously reported for Pisum and Phaseolus, there was no contradiction between photometric and physiological assays of phytochrome state in Avena coleoptile segments. Attempts to expand this study by using segments from seedlings pretreated with red light showed that such pretreatment as little as 1-2 hours before drastically reduced subsequent elongation and pho-toresponse on the medium employed. This decline in growth potential could be halted at any time before its completion by either excision of the segment or far-red treatment of the intact seedling.