Correlation of phytochrome transformations with photocontrol of Avena coleoptile segment elongation

Abstract

Loss of photoreversibility in phytochrome-dependent Avena apical segment elongation has been reexamined. The effect of intervening darkness between red and far-red irradiations has been corrected for elongation achieved during that period. Maximum reversibility is maintained for the first 7 h, followed by a rapid decline in reversibility from 7 to 9 h. Loss of spectrophotometrically detectable Pfr is about 90% during the first 7 h and approaches 100% by 9 h. The absence of correlation between level of reversibility by far-red light and loss of Pfr from the system is similar to observations previously reported for Phaseolus hook opening.Increase in the irreversible increment of elongation occurs at about the same rate as the increase in the ratio of Pr to total phytochrome. These results suggest that a role for Pr in controlling Avena coleoptile elongation should be considered.