Vernalization and Photoperiodism — 54 — A Symposium 



and Cajlachjan and subsequently by other investigators. Cajlachjan 

 (1937) has tentatively named this substance "florigen." It is formed in 

 the leaves and thence transported to other parts of the plant, primarily to 

 certain terminal meristematic regions of the stem where flowers are ini- 

 tiated. This concept is but a resurgence of the idea conceived by Sachs 

 (1863-1866) that flower producing substances ("Bltihstoffe") are formed 

 in the leaves and translocated to terminal meristematic regions where flower 

 formation occurs. 



Though perhaps other portions of the plant may react to some extent 

 to the photoperiodic stimulus, apparently the fully developed new leaves 

 are the chief loci of perception. The young developing and the old ones 

 may even inhibit the photoperiodic reaction of the mature leaves, either by 

 diluting or destroying the substance produced (Hamner and Bonner, 

 1938; Naylor 1941). But Cajlachjan (1937) and Stout (1945) state 

 that the vegetative shoots do not seem to produce a substance that is 

 antagonistic to sexual reproduction. The hormone seems to be stored to 

 some extent in the leaves. When taken from photoperiodically induced 

 plants and grafted on vegetative ones, they continue to supply the hormone 

 (Long, 1939), although there is evidence to the contrary (Moshkov, 

 1941). Light intensity and temperature, as was pointed out previously, 

 have an important bearing on hormone formation in the leaves. 



The production of the photoperiodic impulse in the leaves probably 

 stands in no direct relationship to photosynthesis (Potapenko, 1944) since 

 (o) it can take place in very weak light which does not permit much photo- 

 synthesis and during which respiration certainly is in excess of carbon as- 

 similation and (b) reduction of the light period in short-day plants, which 

 would seem to curtail the amount of carbon assimilation, leads to initiation 

 of reproduction. Moreover, Cajlachjan (1941) has shown the photo- 

 periodism takes place whether the plants are green or chlorotic. On the 

 other hand there is evidence more or less to the contrary. By controlling 

 photosynthesis, either through limitation of CO2 supply or duration of 

 light intensity, initiation of floral primordia was limited in the Biloxi soy- 

 bean (Parker and Borthwick, 1941; Harder and Witsch, 1941). 

 Suggestive of a connection between photosynthesis and photoperiodism 

 are also the results obtained in suppression of floral initiation by interrup- 

 tion of the dark period with spectral light. It showed two regions of maxi- 

 mal efficiency (suppression), one in the red, the other in blue light, thus 

 indicating a possible association, negative though it be, with chloroplast 

 pigments and photosynthetic utilization of carbon dioxide (Parker ef al., 

 1945). 



Withrow and Withrow (1944) have concluded, from their work on 

 intermittent irradiation of several kinds of plants, that the kinetics of the 

 photoperiodic reaction is based on two relationships which appear to limit 

 the photochemical reaction : a) "The relatively slow rate of the non-photo- 

 chemical reaction which forms the substance to be photoactivated and b) 

 the relatively low equilibrium concentration which this substance attains 

 during long periods of darkness." This suggests some possible limitations 

 in two hypothetical reactions but does not indicate their essential character. 



In order to account for the response of Biloxi soybean and Xanthium 

 plants to both the photo- and the dark-periods of the alternating cycles 



