A Semidian Rhythm in the Flowering Response of Pharbitis nil to Far-Red Light

The semidian ( approximately 12 h) periodicity in the effect of far-red (FR) interruptions of the light period preceding inductive darkness on flowering in Pharbitis nil appears to be mediated by phytochrome: (a) promotion by interruptions 2 hours before inductive darkness (-2 hours) and inhibition at -8 hours are greater the higher the proportion of FR/R+FR during the interruption; (b) brief FR exposures followed by darkness are even more effective than FR throughout; (c) the effect of brief FR is reversed by subsequent R; (d) R interruptions of an FR background are most promotive at -8 hours, when FR is most inhibitory. Promotive FR interruptions at -2 or -14 hours shorten the critical dark period whereas inhibitory FR interruptions at -8 hours lengthen it. We conclude that the semidian rhythm is controlled by a ;timing pool' of phytochrome FR absorbing form (Pfr) which disappears rapidly in darkness: four different estimates from our experiments indicate that Pfr was reduced to the level set by FR within 20 to 45 minutes in darkness. However, flowering may also be influenced by a ;metabolic pool' of Pfr with a delayed loss in darkness, the time of which can be advanced or retarded by shifting the semidian rhythm.