Quantitative Assay of Photoperiodic Floral Inhibition and Stimulation in Biloxi Soybean

Abstract

Evidence has accumulated to suggest the involvement of an endogenous rhythm in the photoperiodic flowering response of long and short day plants. Recent investigations with Biloxi soybean have shown that light breaks applied during the 64-hour experimental dark period of a 72-hour, or tridiurnal, cycle are stimulatory, inhibitory or innocuous depending upon their time of occurrence during the cycle (2). High intensity light applied in the range between the twenty-second to thirty-fifth hour and the fortyseventh to fifty-sixth hour in a tridiurnal cycle was shown to be stimulatory to flowering, whereas light applied in areas surrounding the 16-, 40- and 64-hour points was inhibitory. It was concluded, therefore, that the 8-hour light period which initiated each 72hour cycle established an endogenous rhythm which completed an oscillation each 24 hours and which had alternate 12-hour phases of sensitivity to light. Light is stimulatory if applied during the first 12 hours, or photophil phase, of the oscillation, but is inhibitory if it occurs during the last 12 hours, or photophobe phase. Even a very brief exposure to light during certain of the photophobe phases may completely inhibit flowering, and light during the photophil phases may stimulate flowering to such an extent that, in some cases, nearly every bud on the plant responds. In the following experiments it was presumed that floral repression due to light applied in photophobe phases might be counterbalanced by application of additional light during photophil phases. In other words, by balancing floral inhibition and stimulation, it might be possible to obtain a more accurate quantitative measure of the inhibitory or stimulatory effects of light applied during the different phases. The degree of floral repression due to an inhibitory light treatment, therefore, would be reflected by the differential between the floral response of a balanced treatment and that for the stimulatory treatment alone. Furthermore, various levels of response can be obtained by controlling the amount of floral stimulation. The usefulness of this type of analytical approach, however, is highly dependent upon the occurrence of a functional relationship between the stimulatory treatment and the resulting number of flowers.