Murneek — 49 — Research in Photoperiodism 



In more detailed investigations with soybeans, other field and several 

 horticultural crops, emphasis has been placed to an increasing extent on the 

 importance of temperature in its relationship to the photoperiod (Eaton, 

 1924; Gilbert, 1926; Plitt. 1932; Thompson, 1933; Purvis, 1934; Mc- 

 KiNNEY and Sando, 1935; Steinberg and Garner, 1936). Flowering 

 and fruiting of late maturing soybeans such as Biloxi for instance, is 

 favored by a combination of short days and warm temperature, of Rud- 

 beckia bicolor, by long days and warm temperature, and of sugar beets by 

 long days and cool temperature. 



Many winter wheats and other winter cereals are not really typical 

 long-day plants, as regards sexual reproduction, but are short-day -* long- 

 day plants and may be considered also low-temperature — » high-temperature 

 plants. One of these essential conditions, sometimes both, must be obtained 

 during early stages of plant development in order that flowers be initiated. 

 The other, or both, are necessary for successful sexual reproduction. Typi- 

 cal spring cereals probably are long-day high-temperature plants. While the 

 biennial sugar beet apparently requires both a low temperature and long 

 days, the annual beet long days only for sexual reproduction (Owen et al., 

 1940; Stout, 1945). 



By using a great variety of plants, representing several genera and 

 species, Roberts and Struckmeyer (1938, 1939) have demonstrated that 

 the responses of most of them to the length of day are altered, often strik- 

 ingly, by the night temperatures only a little above or below those cus- 

 tomarily used in greenhouse practice. Poinsettia, a commonly considered 

 short-day plant, for example, failed to bloom under this photoperiod when 

 grown at a minimum temperature of 55° F. The long-day Rudbeckia 

 laciniata was similarly influenced by temperature in its response to length 

 of day. At a night temperature of 60-65° F and short-day light exposure 

 it remained in a rosette stage of growth, but with the same photoperiod at 

 55° F. it grew in height and eventually produced flowers.* 



From these and similar observations it is quite apparent that the sensi- 

 tivity of many plants to the duration of light is affected very much by 

 temperature and, contrariwise, the photoperiod influences the responses of 

 plants to temperature. From the results obtained on the effects of tempera- 

 ture and photoperiod on some pea varieties, Kopetz (1943) concludes that 

 both day-length and temperature appear to have a decisive influence on 

 plant development. Under short-day treatment, however, the influence of 

 temperature seemed to be masked by a stronger effect of day length, but 

 under long photoperiods temperature seemed to be the determining factor 

 of development. Responses to the photoperiod and temperature of several 

 native California annuals, studied by Lewis and Went (1945) pointed to 

 the fact that all but two were long-day plants and reactive to the night 

 temperature. 



One should be always aware of the fact, in observing responses of 



* Viola, hirta and V. silvestris, according to P. Chouard (Comptes Rendus 224: 1S23-1525, 

 1947) produce cleistogamous flowers continuously in a long photoperiod (12-14 hours) if the tem- 

 perature permits uninterrupted vegetative development (greenhouse culture). When the day length 

 is 8 hours, however, they form chasmogamous flowers, but only when the plants have been exposed 

 to a period of frost. An annual temperature rhythm, therefore, is necessary for this mode of flower- 

 ing. The reaction of these two species differs from that of V. papiliotiacae which does not seem to 

 be adjusted to thermoperiodicity. 



