Photoperiodic Flowering Response of Biloxi Soybean in 72-Hour Cycles

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Abstract
Photoperiodic flowering responses of Biloxi soybean (Glycine max), a short day plant, were investigated in a 72-hour, or tridiurnal, cycle using high intensity light breaks and differential photoperiods. Four-hour high intensity light breaks given during the 64-hour experimental dark period of a tridiurnal cycle were stimulatory, innocuous, or inhibitory to flowering depending on the time at which the interruption was applied. The application of such light breaks at various times during the cycle resulted in a rhythmic pattern of floral responses with alternated phases of stimulation and inhibition in periods of approximately 24 hours. The results indicate that the 8-hour photoperiod given at the beginning of each 72 hour cycle initiated a fundamental oscillation which persisted through 3 complete 24-hour periods. High intensity light breaks produced oscillations which dampened the fundamental if their frequencies fell out of phase with it. On the other hand, if secondary oscillations fell in phase with the fundamental, the amplitude of the rhythm increased to the extent that flowering resulted. The application of differential light durations both as initial and intervened photoperiods in a tridiurnal cycle shows that the critical photoperiod is much longer than that obtained in 24-hour cycles. Since 72-hour cycles provide long dark periods after each photoperiod, the photo-periodic response cannot be ascribed to a critical dark requirement, but results from an interaction between stimulatory and inhibitory processes. The influence of temperature during the photoperiod indicates that the high intensity light effect has a high temperature coefficient. The results denote that the basic endogenous rhythm has separate components which interact to produce a temperature-compensated clock mechanism which mediates photoperiodic time measurement .