Phytochrome-mediated Phototropism in Adiantum Protonemata II. Participation of Phytochrome Dark Reversion

A microbeam irradiation technique was used to analyze phytochrome-mediated phototropism of the protonema of the fern Adiantum capillus-veneris. One side of the sub-apical zone of a dark-adapted protonema was irradiated with a red-light (R) microbeam to induce phototropic curvature toward the irradiated side. Except the cases where fluence-response relationships were examined, the protonema was stimulated with the microbeam for about 30 s to provide a fluence (5.3–5.5 mmol m^-2) optimal for curvature. When the whole protonema was pretreated with a high fluence of R (about 9 mmol m^-2), no significant curvature could be induced by immediately subsequent one-sided microbeam stimulation. It was found, however, that curvature became inducible progressively as the time of microbeam stimulation was delayed after the R pretreatment. The protonema gained nearly full responsiveness within 40 min after the pretreatment. Moreover, the inductive effect of microbeam R escaped rapidly from the reversing effect of far-red light. These results indicated that most of the far-red-absorbing form of phytochrome (Pfr), which mediates the phototropic response, undergoes relatively fast dark reversion to the red-absorbing form. The dark reversion kinetics was analyzed further by taking into account that the phototropic responsiveness after the R pretreatment was measured in the presence of background Pfr over the protonema. This analysis revealed a rate constant of about 0.002 s^-1 (t_1/2? 6 min) for the dark reversion. It is considered that the Pfr dark reversion plays a role in establishing a lateral Pfr gradient in the unilaterally irradiated protonema.