Murneek — 51 — Research in Photoperiodism 



though the plants were exposed subsequently to temperature too high (60- 

 70° F) for this process. This was true whether the low temperature treat- 

 ment was given during the dark or light phase of the 24 hour cycle. But 

 when the plants were kept at 70-80° F, after a cold exposure even up to i2 

 days, no flowers were developed. This reminds one of so-called devernali- 

 zation of cereals by high temperature (Purvis and Gregory, 1945). 



According to Went (1945), the cultivated tomato, a photoperiodically 

 neutral plant, sets fruit abundantly only when the night temperature is be- 

 tween 15-18° C and the day temperature about 25° C. With lower and 

 higher temperatures at night fruiting is reduced or absent. This diurnal 

 alteration in temperature requirement would seem to be a sort of simulation 

 of photoperiodicity by thermoperiodicity. The conclusion is drawn from 

 these observations that thermoperiodicity in the tomato, and possibly other 

 plants, is due to the predominance of two processes, one during the day and 

 the other at night, of which the one in the dark evidently has a much lower 

 temperature requirement. Temperature probably acts directly on the 

 terminal meristematic regions, instead of indirectly through the foliage as 

 the receptive organ, as in photoperiodism (Curtis and Chang, 1930; 

 Chroboczek, 1934). 



Photoperiodic Induction : — In their studies of the influence of various 

 photoperiods on reproduction of soybeans Garner and Allard (1923) 

 observed that an exposure to 10 short days was all that is required to bring 

 about flower formation, which was continued when the plants thereafter 

 received long-day exposures. Since then numerous investigators have ob- 

 served in a variety of plants that an initial treatment to a light period con- 

 ducive to sexual reproduction will result in flower and fruit development, 

 though often to various extents, irrespective of the length of day to which 

 the plants are subjected afterwards (Cajlachjan, 1933, 1935; Eghis, 

 1928 ; LuBiMENKo and Sceglova, 1927, 1931, 1933; Purvis, 1934; Psarev, 

 1930&; RuDORF, 1935). This seems to be true of both short- and long- 

 day groups. The effect has been named "photoperiodic induction" or 

 "photoperiodic after-effect." Recognition of this phenomenon has been of 

 great value in detailed studies of photoperiodism in many plants, especially 

 in analysis of the nature of the photoperiodic reaction. 



Plants seem to vary as to their sensitivity to photoperiodic induction, 

 depending not only on the genus and species but also variety and even 

 strain. Thus, for instance, Baeria chrysostoma, a relatively short season 

 annual, apparently requires at least 5 photoperiods for flower induction 

 (Severi and Went, 1944), while for the Biloxi soybean and for Xanthium, 

 with much longer life spans, two and one photoperiods, respectively, appar- 

 ently are sufficient to differentiate floral primordia (Borthwick and 

 Parker, 1938o; Naylor, 1941). It is stated, however, that the longer the 

 treatment the sooner the blossoms developed in Biloxi, and that in Xanthium 

 a single photoperiodic cycle required a comparatively long time (64 days) 

 for flowers to mature, whereas 4-8 such cycles led to flower development in 

 a more normal time and under continuous induction, in 13 days. More- 

 over, with increasing number of photoperiodic cycles given there was a 

 proportional increase in number of flowers initiated (Hamner, 1940). 



Not only the photo but also the dark period seems to be essential as re- 



