The phytochrome-dependent germination of spores was studied in the fern Pteris vittata. Brief irradiations with red light given at 0 and 25°C resulted in very similar germination rates. Irradiation with far-red light cancelled this promotive effect, irrespective of the temperature at which tested. The maximum rate of germination was induced by red light of ca. 70Jm−2and half of the rate was induced by ca. 15Jm−2When spores imbibed in the dark were kept for 1 h at 0 or 25°C under irradiations with monochromatic lights from 660 to 730 nm at 10 nm intervals, spore germination was induced depending upon the established photostationary states of phytochrome at both temperatures tested. The percent of PFRestimated in spores that had been irradiated briefly with red light was consistent with that resulted from photostationary states under different monochromatic lights in terms of the percent of germination of a spore population. The threshold of thePFRrequired for the germination of each spore ranged widely from a few percent to 80of the PFR. This diversity may vary the timing of germination in n
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