Phytochrome and Seed Germination

Phytochrome control of cucumber seed germination is temperature-dependent. A prolonged exposure to radiation from broad spectrum far red sources (Pfr/P = 0.05 to 0.07) prevents germination at temperatures below 20 C. Above 20 C there is no inhibition and it appears as if there is an escape from phytochrome control. However, radiation from a monochromatic, narrow band 730 nanometer source (Pfr/P < 0.02) inhibits germination at temperatures above 20 C. This result supports the idea that, even at high temperatures, Pfr is responsible for the activation of germination. After 4 days of exposure to far red, a short red irradiation is quite effective in promoting germination if temperatures during the dark incubation periods are maintained below 20 C; red becomes effective at temperatures above 20 C. Promotion of germination will take place at a temperature of 25 C or higher without red irradiation. Again, we have an apparent escape from phytochrome control at high temperatures. However, if higher temperatures are used for only short periods, 2 to 6 hours, in combination with short red irradiation, one can demonstrate that activation of germination at high temperatures is still dependent on phytochrome. Phytochrome is probably destroyed during prolonged exposure to far red. Thus, the subsequent short red irradiation establishes levels of Pfr which may not be sufficient to promote germination at low temperatures but are probably adequate at high temperatures.