The Reactions of the Photoinductive Dark Period

Abstract

Using the short-day plant, Xanthium. the reactions of the photoinductive dark period were studied by assuming that rate of development of the floral bud, as measured by a system of floral stages, is a measure of the amount of flowering hormone produced. Experiments utilizing red light interruptions of the dark period indicated the following: (1) Light interruption is most effective in floral inhibition after ca. 8-hr, of darkness regardless of night length. (2) Effectiveness of light interruption before 8 hr. may be equivalent to returning plants to full light in the greenhouse although in some experiments a destruction of flower ing hormone takes place during the dark period following the light interruption. (4) A constant amount of light is required to bring about maximum inhibition of flowering for any given time after the first 2 to 3 hr. Application of auxin (NAA) at various times during and following an inductive dark period indicated the following: (1) applied auxin has little effect upon critical night length, although the amount of hormone subsequently produced may be greatly reduced. (2) Auxin is most effective in floral inhibition when applied 2 to 3 hr. after beginning of the dark period, and its effectiveness gradually decreases until it is not effective at the time maximum concentration of flowering hormone has been built up outside the leaf (some hours after end of dark period). (3) There is no sharp change in effectiveness of applied auxin at the end of the dark period. (4) Inhibitions due to red light and applied auxin appear to be additive. It is suggested that 3 reactions take place during the dark period, (1) conversion of photoreceptor pigment from a far-red to a red-receptive form (complete in 2 or 3 hr.); (2) preparatory reaction(s), which together with pigment conversion determines the critical night length; and (3) actual hormone synthesis. Applied or native auxin may bring about a destruction of flowering hormone in the leaf, although the nature auxin is ordinarily present in concentrations too small to be of great importance.