Photophysiology of Kalanchoë Seed Germination

Germination of Kalanchoé blossfeldiana seeds is absolutely light‐requiring and needs repeated daily light periods. With increasing length of the photoperiod there was a gradual escape from the far‐red inhibition. This escape depended also upon the duration of the far‐red exposure: 10‐second far‐red caused a strong inhibition after a 10‐ to 30‐minute photoperiod and did not inhibit after a 4‐hour day, although the effect of the latter was completely suppressed by 5 minutes far‐red. The action of a 12‐hour photoperiod was not reversed by 10 minutes far‐red but it was by 12 hours far‐red. Light intensity and temperature during the photoperiod were two other important factors influencing the escape from far‐red inhibition. The common features of this escape displayed in very different photomorphological responses are stressed. In order to explain our results in terms of phytochrome action, we distinguish two effects of white light: 1) on the initial photoconversion of the inactive to the active PFR form 2) on the much slower transformation of PFR to a reacted form P*FR; the latter reaction can also proceed in darkness, but is enhanced by light and is dependent upon light intensity and temperature; this reacted phytochrome is not reversible by a brief far‐red illumination.