LIGHT PROMOTION OF SEED-GERMINATION IN DATURA-FEROX IS MEDIATED BY A HIGHLY STABLE POOL OF PHYTOCHROME

The duration of the far-red light-absorbing form of phytochrome (Pfr) of the photoreceptor pool involved in the control of seed germination was investigated for Datura ferox seeds. These seeds require both Pfr and alternating temperatures (20/30-degrees-C) to germinate. After 24 h imbibition (25-degrees-C), the seeds received pretreatment-light pulses providing different phytochrome photoequilibria (Pfr/P), followed by a 24 h dark incubation (25-degrees-C), and test-light pulses providing different Pfr/P immediately prior to transfer to alternating temperatures. Germination increased with increasing Pfr/P provided by the test-light pulses, but was unaffected by the pretreatment-light pulses. This suggests that phytochrome synthesis, phytochrome degradation and phytochrome-mediated changes in response to phytochrome were negligible. In other experiments, red light-pretreatment pulses were followed by dark incubations (25-degrees-C) of different duration before transfer to alternating temperatures. The proportion of Pfr remaining after the 25-degrees-C incubation period was estimated by comparing germination rates with those of seeds that received test-light pulses of known calculated Pfr/P immediately prior to the start of the cycles of alternating temperatures. More than 80% of the Pfr established by a Pfr/P = 0.87 light pulse was present and active even after 48 h dark incubation at 25-degrees-C. Suprisingly, when a pretreatment-light pulse providing a Pfr/P = 0.70 was given, the reduction in [Pfr] was significantly faster. Germination of Datura ferox seeds is under the control of a highly stable (type II like) phytochrome pool. Apparently, this pool follows Pfr dark reversion to the red light-absorbing form, the times to reach half the original Pfr pool being > 96 h or < 14 h after light pulses providing Pfr/P = 0.87 or 0.70, respectively.