Vernalization and Photoperlodism 



■156 — 



A Symposium 



optimal nyctotemperature according to light is recorded (for Phalaenopsis 

 15.5°-18.5° during winter and 21° during summer). 



Many more quotations could be made from the published experience 

 of greenhouse growers, but in the botanical literature little more has ap- 

 peared concerning thermoperiodicity. 



mtyola^ 



lan 



sfem growth o^ 

 intact fWut" 



stem ^rovuVW 



0T\\VJ 



QTovutW of" 



excised 



roots 



Fig. 6. — Relations between (night) temperature (abscissa in 

 degree Centigrade) and (1) total growth of intact plants (circles), 

 (2) direct effect on stem elongation (triangles), (3) growth of isolated 

 roots (crosses), and (4) translocation (squares). iirom Went 

 1944a, p. 612). 



The explanation of various phenomena observed in thermoperiodicity 

 can be found in Went (1944a, 1945a). During daytime photosynthesis 

 seems to be the limiting factor for development. This is only true when the 

 nyctotemperature is within the optimal range so that the assimilates can be 

 utilized to best advantage. When the nyctotemperatures are too low, photo- 

 synthesis does not limit development any more (Went 1945). 



In darkness the fairly low optimum temperature in tomatoes is caused by 

 the competition between two individual processes {see figure 6). Most of 

 the stem elongation occurs during night. The growth process has a tempera- 

 ture coefficient Qio>2, its optimum lies around 30°. The rate of food trans- 

 location, however, has a temperature coefficient Qio<l, so that at higher 

 nyctotemperatures, less sugar reaches the growing region, and the food 

 supply becomes limiting. Thus in young small tomato plants, where food 

 translocation takes place over short distances only, the optimal nyctotem- 

 perature coincides with the optimal temperature of the growth process (30° ) . 

 As the tomato plant becomes taller, and the assimilating and growing re- 

 gions become separated by longer distances, the food translocation becomes 

 more and more limiting at the higher temperatures so that the optimal nycto- 

 temperature drops to lower and lower levels (13-18°) in different tomato 

 varieties. This same phenomenon can be seen in Laurie and Kiplinger's 

 data on page 155 for orchids. Seedlings must be kept 4-10° warmer than 

 mature plants. This is also the reason why in spring so many garden crops 

 and flowers are germinated in greenhouses, where they are kept warm. By 

 the time that their temperature requirements have been lowered, the outside 

 temperatures have sufficiently risen to insure good growth of the seedlings 

 when brought into the open. 



