Phytochrome-Controlled Fern-Spore Germination, Kinetics of Pfr Action

Abstract

Summary Spore germination is induced by well-defined periods of Pfr, established and terminated by saturating red and far-red pulses, respectively. Pf, has to be present at least 5 to 6 hours in order to induce the most sensitive (or responsive) spores, but for saturation Pfr has to act between 12 and 24 h. As derived from these «Pfr period-response curves-, Pf, is most effective between 24 and 72 h after sowing. A subthreshold Pfr pretreatment (i.e., 4 h) increases the effect of a subsequent Pf, period in a purely additive way, the two Pfr periods together have almost the same effect as an uninterrupted Pfr period of equal length, even if they are separated by an interval of more than 40 h. Thus, the internal signal produced by the action of a subthreshold Pf, period is stored and is remarkably stable. The subthreshold Pf, period was allowed to act at various times after sowing and its effect on a later Pfr period was tested. From the results it is concluded that competence of the spore to Pf, develops between 15 and 20 h after sowing, i.e., the «preirradiation effect,, of red light is no longer fully reversible by far-red. This is much earlier than the -apparent coupling point. as derived from loss of reversibility, measuring the terminal response only.