Studies of Long-day Inhibition in Short-day Plants

Abstract

Previous experiments which the short-day plant Kalanchoe blossfeldiana have shown that long days interspersed in a period of short-day induction exert an active inhibition on flowering, each long-day annulling the effect of approximately two short-day. The present experiments establish that three other short-day species-Biloxi soybean, Perilla ocymoides, and Chenopodium amaranticolor-are similarly inhibited. It is also shown that the inhibition appears to act by making the short-days immediately succeeding the long-day ineffective, rather than by destroying previous induction. The other experiments with Kalanchoelarge temperature effects in light or darkness on the promotive and inhibitory reactions are demonstrated. A repetition of Harder and GÜmmer's experiment was successful and confirmed that as little as one second of light per day is sufficient for flowering in Kalanchoe; no flowering occurs in complete darkness. Explaining earlier results in Kalanchoe a hypothetical scheme was postulated according to which induction of short-day plants leads to the formation of an adaptive enzyme, which is subject to inhibition by a substance formed in long-day conditions. From this scheme a change of critical daylength could be predicted as the result of partial induction. Two experiments set up to test this point indicated that there is in fact such a shift, thus supporting the hypothesis.