Evidence for a Flowering Inhibitor Produced in Long Days in Kalanchoe blossfeldiana

Experiments with Kalanchoe blossfeldiana are described in whichperiods of short-day treatment were interrupted by intercalatedlong days or light breaks during long dark periods. The effectsof 24-hour dark periods preceding and following such intercalatedlong days were also investigated. The results of these experiments have shown that: Single longdays intercalated between numbers of short days have a positiveinhibitory effect on flower initiation and are not merely ineffective.The inhibitory effect expressed as the number of inductive cyclesannulled is approximately additive, provided the long days areinterspersed with short days, but not if several long days aregiven consecutively. On the average 1 long day is capable ofannulling the flower-promoting effect of about 1 short days.To a first approximation flower numbers in Kalanchoe increaseexponentially with the number of inductive cycles given—upto at least 12 short days; the inhibitory effect of long daysinterspersed with short days also fits an exponential curve;i.e. the inhibition is roughly proportional to the amount ofprevious photo-periodic induction. A light break of as littleas 30 seconds' duration given in the middle of a long dark periodis as inhibitory as a long day. If followed by a long dark periodthe inhibition of an intercalated long day is almost completelyneutralized; a long dark period preceding it has no such effect. These results have been interpreted as due to the interactionof a flowering inhibitor with a reaction leading to flowering.A mechanism involving competitive inhibition of an adaptivelyformed enzyme has been described as a possible example of thekind of reaction which could account for the results presented.