Developmental Physiology Apparent Dark Decay of Phytochrome P fr in Fern Spores

Germination of spores of Dryopteris fllix‐mas has been induced by two pulses of saturating red light, separated by a dark period of about 8 to 24 h. By chosing different wavelengths, different P fr /P tot levels could be established. Thus, by a “null method” the second pulse could be used as a “test pulse”, determining the actual P fr level remaining from the “start pulse”, and thus providing information about an apparent Pf r decay. It cannot be decided yet whether this apparent P fr decay results from dark destruction or dark reversion. The apparent P fr decay depends, as expected, on the temperature, being accelerated with increasing temperatures. Moreover, the later after sowing that the decay is tested, the faster it proceeds; a tentative interpretion is that newly synthesized P r undergoes faster decay after phototransformation than that phytochrome pool present in the resting spores. A third factor that influences the apparent P fr decay is the Pf r /P tot level established by the first pulse (start pulse). The lower this level, the slower the decay kinetics. This could be due to phytochrome biosynthesis partly compensating for P fr destruction, and the relative contribution of this biosynthesis to the total effect increases with lower P fr levels. Spores of D. paleacea yield virtually the same results. Whatever the real basis of the observed P fr decay, i.e. destruction, reversion, or a combination of these reactions with biosynthesis, it can be concluded that modification of this P fr decay by various factors is the basis of the effect of those factors on light‐induced germination.