Vernalization and Photoperiodism — SO — A Symposium 



plants, that the temperature or the photoperiod or both may merely delay 

 flower development, of which there are numerous instances on record and 

 many others would have been observed if the experimental plants had been 

 grown long enough. In analyzing the photoperiodic and temperature re- 

 actions of soybeans obtained from different regions, Rudorf and Schrock 

 (1941) noted that the higher temperatures given only in the early stages of 

 exposure to short-days hastened the beginning of flowering and that this 

 was irrespective of temperature and photoperiodic conditions obtaining 

 afterwards. Heath (1943) found that both long days and high tempera- 

 ture promoted bulb formation and prevented bolting of onions, but short 

 days inhibited bulb production but allowed bolting, if the temperature was 

 not too high. Day lengths sufficiently long for bulbing at high temperatures 

 will prevent their formation at low temperatures. Planting onions late in 

 the season, when days are sufficiently long, will produce satisfactory bulbs, 

 depending largely on the prevailing temperature. While studying the 

 effects of some environmental factors on floral initiation in Xanthium 

 pennsylvanicum, a short-day plant, Mann (1940) found that the tempera- 

 ture during the photoperiod had a pronounced influence on the resultant 

 time of flowering, but as the photoperiod increased in length, the tempera- 

 ture effects diminished. Temperature seems to have a bearing also on the 

 length of the critical dark period required by this plant (Long, 1939). 



Whether the night or the day temperature is most important has not 

 been determined for all species and varieties (Thompson, 1944). More 

 or less specific night temperatures are now commonly considered as being 

 desirable for raising ornamentals in greenhouses. Cineraries, cyclamens, 

 chrysanthemums, carnations and snapdragons are usually grown at a night 

 temperature of 45-50° F., roses and begonias at 60° F., and gardenias, 

 orchids and bouwardias above 60° F- (Post, 1942). 



In consideration of the accumulated information on the relationship of 

 temperature to photoperiodism, both are now being taken into account in 

 experimental work on the influence of length of day on plant development 

 and either the day or night temperatures, or both, are kept under control 

 (Rudorf and Schrock, 1941; Aberg, 1943; Sivori and Went, 1944; 

 Lewis and Went, 1945 ; Loworn, 1945). 



Of considerable interest in this discussion should be the fact that 

 temperature itself, independently of the photoperiod, may be a potent factor 

 in induction and maintenance of sexual reproduction in plants. Krasan, 

 as early as 1870, had called attention to temperature as a factor in initiation 

 of flowering of many plants and Rlebs (1913) observed that when beet 

 roots were kept during the winter in a warm greenhouse no flowers were 

 produced the following summer, as this biennial plant usually does, while 

 roots stored outdoors flowered and set seed abundantly. 



The extensive investigations by Thompson and associates (1933, 1939) 

 on vegetables, by Post (1937, 1940) on ornamentals and by others would 

 seem to furnish ample testimony that various plants, though responsive to 

 photoperiod, can be induced to bloom by temperature treatment alone, com- 

 monly by "chilling." In some recent studies on the effects of temperature 

 on initiation of flowering in celery, Thompson (1945) found that an ex- 

 posure for as short a period as 2 days at 40-50° F caused floral inception. 



