Photophysiology of Kalanchoë Seed Germination

The effects of irradiations with different proportions of red/farred light and of gibberellic acid on the phytochrome-mediated seed germination of Kalanchoë blossfeldiana cv. Feuerblüte, were studied. The seed coat transmits much more red than far-red light, and therefore the energy ratio between 660 nm and 730 nm is given only for the transmitted light. Decreasing this ratio from 65 to 1.0 caused only a very slight inhibition. If this ratio is further lowered to 0.64, a 10 min terminal irradiation after a 3-h white light photoperiod is inhibitory, but a 12-h photoperiod or continuous irradiation is not. If the ratio is decreased to 0.44 or 0.31, a 12-h photoperiod is now also inhibitory, although continuous irradiation and 10 min terminal irradiation are still more inhibitory. These results are discussed in terms of phytochrome phototransformations. Although gibberellic acid is unable to cause any germination in complete darkness, it can result in a very high germination percentage, if combined with treatments which by themselves do not induce any germination such as continuous far-red, terminal far-red after short photoperiods, or very short photoperiods at 25°C. These results point to a strong synergism between gibberellic acid and the so-called stabilized form of phytochrome, P*_FR.