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

Abstract

Experiments with Kalanchoe blossfeldiana are described in which periods of short-day treatment were interrupted by intercalated long days or light breaks during long dark periods. The effects of 24-hour dark periods preceding and following such intercalated long days were also investigated. The results of these experiments have shown that: Single long days intercalated between numbers of short days have a positive inhibitory effect on flower initiation and are not merely ineffective. The inhibitory effect expressed as the number of inductive cycles annulled is approximately additive, provided the long days are interspersed with short days, but not if several long days are given consecutively. On the average 1 long day is capable of annulling the flower-promoting effect of about 1⅓ short days. To a first approximation flower numbers in Kalanchoe increase exponentially with the number of inductive cycles given—up to at least 12 short days; the inhibitory effect of long days interspersed with short days also fits an exponential curve; i.e. the inhibition is roughly proportional to the amount of previous photo-periodic induction. A light break of as little as 30 seconds' duration given in the middle of a long dark period is as inhibitory as a long day. If followed by a long dark period the inhibition of an intercalated long day is almost completely neutralized; a long dark period preceding it has no such effect. These results have been interpreted as due to the interaction of a flowering inhibitor with a reaction leading to flowering. A mechanism involving competitive inhibition of an adaptively formed enzyme has been described as a possible example of the kind of reaction which could account for the results presented.