Acceleration of dark reversion of phytochrome in vitro by calcium and magnesium.

Rates of dark reversion of the far red-absorbing form of phytochrome, Pfr, to the red-absorbing form, Pr, have been determined in the presence of several salts. Low concentrations of calcium chloride and magnesium chloride (up to 3 mm) accelerated the rate of dark reversion at all stages of purification of phytochrome from etiolated rye (Secale cereale L. cv. Balbo) seedlings. The complex kinetics of the dark reversion could be resolved into two first-order components. The effect of the added divalent cations was on the relative proportion of the fast and slow reacting components, rather than on the rate constants of the two populations. It was possible to reverse the effects of the cations by adding the chelating agents ethylene-bis-(oxyethylene-nitrilo) tetraacetic acid or ethylenediaminetetraacetate. The effect of the divalent cations is not a nonspecific ionic strength effect. The relative proportion of the two populations was also affected by the degree of purity of the phytochrome samples.

Far-red Sensitive Dark Processes Essential for Light- and Gibberellin-induced Germination of Lettuce Seed.

The action of prolonged far-red on seed germination was studied in Lactuca sativa L. var. Grand Rapids. Exposure of imbibed seeds to 6 hours far-red before the application of gibberellic acid (GA(3)) and thiourea completely prevented germination. Using GA(3), this far-red was effective after the sixth hour of imbibition. At 6, 12, and 18 hours of imbibition equal durations of far-red had equal effects. The kinetics of far-red action was investigated: it was found that although far-red for several hours, irrespective of the energy level, was needed for maximum inhibition, shorter durations (15 and 30 mins) were also appreciably effective provided they were followed by several hours darkness before the supply of GA(3). This is taken to indicate the existence of labile product(s) of the action of a far-red sensitive pigment. Evidence is provided for the existence of promotive dark processes controlled by this pigment, which are essential for germination whether triggered by GA(3), thiourea or red-light. A model for the operation of the pigment system is proposed and its role in the germination mechanism of this seed is discussed.

Immediate phytochrome action in lettuce seeds and its interaction with gibberellins and other germination promoters.

An immediate action of phytochrome (PFR) was found by a technique of adding subthreshold concentrations of gibberellic acid (GA3) after irradiation. It was shown that phytochrome (PFR) is active within 5 mins after its formation. The interaction between PFR, GA3 and six other gibberellins (GA1, GA4, GA5, GA7, GA9 and GA13) was synergistic. This result is interpreted to mean that PFR does not produce any of these gibberellins, at least during the first 30 mins. of its action. This conclusion was confirmed by interaction experiments in darkness using GA3 combined with the other gibberellins. A similar synergism was found between PFR and kinetin, thiourea and chloramphenicol.

Mediation of phytochrome in the inductive action of low temperature on dark germination of lettuce seed at supra-optimal temperature.

The induction of dark germination in light-requiring lettuce (Lactuca sativa) seed at supraoptimal temperatures by cold treatment (in darkness) was partly reversed by a brief far-red irradiation made at time of transfer, and even more so when the irradiation was made at the beginning of the cold pretreatment. When the inhibitory far-red irradiation was followed by additional cold treatment, the promotion was greatly restored. The promotive effects of brief irradiations with red light were further enhanced by a following cold period, before transfer to the supraoptimal temperature. These results are interpreted as indicating that the active (far-red absorbing) form of phytochrome is pre-existing in the dry seed, and interacts with a co-factor which is built-up during imbibition. The rate of build-up of this co-factor, as well as of the dark inactivation of active phytochrome increase with temperature. The products of the interaction pass through a photo-labile thermo-stable phase, before becoming photo-stable as well.