Vernalization and Photoperiodism 



•154 — 



A Symposium 



This consideration shows already that we can expect different optimal tem- 

 peratures during day and night. This is the basis for thermoperiodicity/ 

 Actually in most plants investigated thus far optimal growth and develop- 

 ment occurs when day temperatures are considerably higher than night 

 temperatures. For future discussion it seems advisable to refer to the light 

 period, which usually, but not necessarily, coincides with the daytime, as the 

 photopmod, and to the temperature prevailing during this period as photo- 

 temperature. Since in thermoperiodicity the dark period is equally im- 

 portant as the photoperiod, and has to be referred to often, the term nycto- 

 period is suggested. The temperature during the nyctoperiod is the nycto- 

 temperature. This same term can be used in photoperiodism discussions. 



day temperature 26.S* 



niqhr femperafrurc aa 



indicaVed on abscissa 



Fig. 5. — Relationship between stem growth rate (ordinate, in mm./day) 

 and temperature (abscissa, in degree Centigrade) of tomato plants. The 

 circles represent plants kept both day and night at the indicated tempera- 

 ture. The crosses show growth rates of plants kept during eight day 

 hours at 26.5°C. and during night at the temperatures indicated on 

 abscissa. Squares; plants kept during day at 19-20°, during night at 

 26.5°C. (from Went 1944, p. 140). 



In work designed to find the optimal growing conditions for tomato 

 plants (Went 1944) the optimal nyctotemperature was determined as 17- 

 18°C., whereas the optimal phototemperature was closer to 26°C. than to 

 17° {see figure 5). These temperature relations were worked out in greater 

 detail in later publications (Went 1944fl, 1945a) . The nyctotemperature 

 optimum was high in the seedling stage (about 30°C.), and during the 

 course of development gradually fell to 18° for the San Jose Canner, and to 

 13° for the Illinois-19 tomato. This optimum was also influenced by the 

 light intensity during the photoperiod, being lower for lower light intensities. 

 The previous temperature treatment equally determined the response to the 

 nyctotemperature. The conditions optimal for stem elongation were also 

 optimal for fruit set and fruit growth. 



An analysis of the temperature response of the chili pepper {Capsicum 

 annuum) gave essentially the same results (Borland and Went, 1947). 

 Like the tomato, optimal growth was obtained at a phototemperature of 



^ On consideration of priority the term photoperiodism is now favored over photo- 

 periodicity. On the same basis we should accept thermoperiodicity (Went 1944). Be- 

 sides the word periodism does not occur in Webster so that periodicity is preferable on 

 linguistic grounds. 



