The light promoted germination of the seeds of Chenopodium album L. VI. Pfr requirement during different stages of the germination process

Positive photoblastic seeds of Chenopodium album need the action of the active form (Pfr) of phytochrome (P) during nearly the total length of the overall germination process. The first Pfr dependent processes require a Pfr/P ratio above the level being present in darkness. The different requirement for the length of the red irradiation indicates that these first processes have a different Pfr requirement in the individual seeds of one population. Simultaneous irradiations with narrow wavelength bands of red (643 nm) and far-red (768 nm) light, which established different Pfr/P ratios, supported this conclusion. The Pfr requirement is a function of the thickness of the seed-coat. The germination of half operated seeds (the outer seed-coat layer is removed in the area overlying the radicle) can be induced by the dark level of Pfr, being approximately Pfr/P = 0.07. The required Pfr level for the germination processes after the escape from the antagonistic effect of short far-red irradiations, decreases from 0.07 to below 0.02. Osmotic stress increases the Pfr requirement. Prolonged irradiations with far-red light inhibit the germination processes when they are already in the phase of the embryo elongation. The elongation process is supposed to consist mainly of cellular expansion. The process which induces the secondary photodormancy is only related to the first Pfr dependent processes.