Floral inhibition in relation to photoperiodism in Biloxi soybean

Abstract

Biloxi soybean is a short day plant with a critical photoperiod of about 14y2 hours. It has been demonstrated that if short days are given in direct succession, the number of nodes which produce flowers on a plant is directly proportional to the number of inductive short days between three and approximately ten. If the short day treatments are not given in direct succession, however, there may be no flowering response. Long (11) conducted experiments with Biloxi soybean in which he gave plants alternating short day and long day treatments, and the plants remained vegetative over long periods of time. He also gave varying numbers of short days in succession, alternating varying numbers of long days. His results show that the inductive effect of one or two short days may be cancelled out when followed by one or more long days. Numerous other workers have studied the inhibitory effect of long days when intervened between the short days of an inductive treatment in various short day plants. Results of such experiments have been reported for Kalanchoe blossfeldiana by Harder and Giummer (10); for Perilla ocymoides, Chenopodium amaranticolor, and Biloxi soybean by Schawabe (14); for Chrysanthemum morifolium by Post (13), and for Chenopodium by Carr (7). The above studies indicate that, in short day plants, partial induction by short day treatment may be reversed by subsequent exposure of the plants to long day conditions. A direct relationship is exhibited between the number of short days used in the partially inductive treatments and the number -of long days required to cause inhibition or reduction of the flowering response. In these previous studies long days known to be non-inductive were interspersed between inductive short days. It was inferred that such long day treatments were capable of destroying the flowering stimulus. No previous studies have been made in which varied lengths of photoperiods during the long days were investigated. In the following experiments, both the length and the intensity of the photoperiod during the long days have been investigated to determine the importance of their role in floral inhibition. Biloxi soybean, with an 8 hour photoperiod in each cycle, produces the same magnitude of flowering on both 24 and 48 hour cycles. One may consider that the plants on such a 48 hour cycle are actually being exposed to one short day followed by a day of complete darkness. Such treatments have also been called bidiurnal cycles (6). We have employed the 48 hour period as a unit of experimental treatment. After normal short day (8 hour photoperiod & 16 hour dark period) during the first 24 hours of the treatment, we have varied the photoperiodic conditions of the second 24 hours. Obviously, if the second 24 hour period were a long day, no flowering would result. On the other hand, if the second period were complete darkness, the amount of flowering would be the same as that exhibited by plants which received 7 consecutive short days. Any 48 hour period in which two photoperiods are given shall be considered as two photoperiodic cycles by us regardless of the length of each light treatment used. Differential flowering due to varying lengths of photoperiod during the second 24 hour period provides information concerning the nature of floral inhibition in Biloxi soybean.