PHYTOCHROME-CONTROLLED FERN-SPORE GERMINATION: PHASE-SPECIFIC MODULATION BY ELEVATED TEMPERATURES*

The phytochrome-mediated spore germination in Dryopteris is inhibited by raising the temperature from 22 to 27 or 32°C. As reported earlier (W. Haupt (1990) Photochem. Photobiol. 52,57–59), the elevated temperature is effective during the “coupling phase”, i.e. when P_fr, the active form of phytochrome, starts the processes leading to germination. In addition, a strong effect has now been found in the “preinduction phase”, i.e. when the spore develops its competence to react to P_fr. This latter effect is overcome, if in the subsequent coupling phase a high level of P_fr is allowed to act for a long time. Moreover, the inhibiting effect in a2–3 day preinduction phase is most obvious in or even restricted to its terminal 24 h. It is proposed that the temperature acts by reversibly abolishing the cell's competence to react to P_fr, and that this effect can extend into the coupling phase. Not yet understood is an “inversion” effect in D. paleacea: if the temperature is raised in the earlier parts of the preinduction phase, the effect of P_fr in the subsequent coupling phase is enhanced rather than inhibited.