Vernalization and Photoperiodism — 74 — A Symposium 



(8). Thus the wave length region of light that is most effective in pre- 

 venting floral initiation of short-day plants when applied near the middle 

 of the dark period is most effective in causing floral initiation of long-day 

 plants. Radiant energies at a particular wave length for effecting control 

 of soybean (short-day) and barley (long-day) are moreover of the same 

 order of magnitude. 



Plants that are closely related often exhibit different photoperiodic re- 

 sponses. Genera of plants, such as Nicotiana, contain both short-day and 

 indeterminate species. Certain species, such as soybean (Soja max), con- 

 tain varieties that flower only with short photoperiods and others that 

 flower over a wide range of photoperiods, and some even with continuous 

 light. These last varieties are sometimes erroneously spoken of as in- 

 determinate but they are more accurately to be described as short-day types 

 because they flower much sooner with short photoperiods than with long. 

 Although data are not available it seems probable that within long-day 

 plants there may also be considerable variability in the range of photo- 

 periods on which the various types will flower. 



In the case of a grass, Bouteloua curtipendula, Olmsted (14, 15) was 

 able to isolate clones from different latitudes that varied from short-day 

 to long-day type of response. Such observations suggest a close relation- 

 ship between the reactions of long-day and short-day plants. 



Experiments indicate that effects taking place in the dark period are 

 primarily responsible for control of flowering in both long-day and short- 

 day plants. The significance of the dark period in regulation of flowering 

 is demonstrated by the effectiveness with which irradiation for short times 

 near the middle of dark periods prevents flowering of short-day plants or 

 causes flowering of long-day plants. It is also illustrated by the fact that 

 flowering cannot be induced in long-day plants that received photoperiodic 

 cycles of 24 hours or less if the dark periods of those cycles exceed a cer- 

 tain critical length, nor in short-day plants on cycles of any length unless 

 a critical length of dark period is exceeded. In both cases the length of 

 the uninterrupted period of darkness is critical. 



An hypothesis that is useful to guide design of experiment, that recog- 

 nizes established facts, and that is in harmony with fundamental similarity 

 in the behaviour of long-day and short-day plants is the following: 



Floral initiation is controlled by an active substance produced 

 in the leaves of the plant. This substance or an intermediate in 

 the course of its production is subject to photosensitized destruc- 

 tion in the leaf. The active substance must reach an adequate 

 concentration to cause floral initiation but is inhibitory at rela- 

 tively higher concentrations. 



The hypothesis is independent of the specific type of pigment responsible 

 for the action spectrum and is not concerned with the nature of the active 

 substance or the reactions involved in determining the type of structures 

 differentiated from the meristematic tissues. 



Observed types of photoperiodic response in plants are corollary to this 

 hypothesis. Thus it would follow that short-day plants are those in which 

 the concentration of the active substance derived from illuminated leaves 

 is insuiffcient to cause floral initiation but exceeds the critical value after a 

 dark period of sufficient duration. In long-day plants enough active sub- 



