164 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



It was also found in other plants that plant hormones are ineffective in 

 increasing growth at higher temperatures (for instance, Bonner, ref. 1, with 

 the application of thiamine to Cosmos). 



When isolated tomato cells or tomato roots are cultivated at different 

 temperatures, they will increase their growth rate with increasing tem- 

 perature to well above 25°, which means that the low temperature optimum 

 of the intact plant is in some way related to its being a complete organism 

 (curve B, fig. 1). The difference between an excised root which receives its 

 food supply from all directions in a nutrient solution and an intact root 

 is largely that the latter has to be supplied through its conducting tissues 

 by the top of the plant. This suggests that the low temperature optimum 

 is in some way connected with this supply of sugar and other nutrients 

 from the leaves where these are formed through i^hotosynthesis. In many 

 series of experiments it was actually found (10, 13) that the translocation 



Fig. 1. Effects of nyctotem- 

 perature on growth of mature 

 intact tomato plants (curve 

 A), isolated tomato roots grown 

 in vitro (curve B) and sugar 

 translocation in tomato stems 

 (curve C) From Went 1944a. 



of sugar from leaves to other parts of the plant has a Qio of less than 1 (see 

 curve C, fig. 1). Therefore, as the nycto-temperature for a growing tomato 

 plant is raised higher and higher, first the growth process is accelerated 

 until a point is reached where the supply of sugar and other nutrients 

 cannot keep up with the other phases of the growth process and from that 

 point on the temperature curve of growth will follow the temperature ef- 

 fect on translocation. There are many facts which can be explained with 

 this two-factor scheme. When a tomato plant has been grown for a week or 

 longer at nycto-temperatures below 17°, the supply of sugars to the grow- 

 ing points has been in excess of what is being used. When afterwards for 

 one or two nights the plants are exposed to a night temperature of 26° or 

 30° their growth rate is 50-100% greater than it could ever be in the steady 

 state, but in the third or fourth night returns again to the normal rate at 

 the new temperature. Another fact which can be explained this way is that 

 the optimal nycto-temperature is very high for seedlings and gradually de- 

 creases to an optimum of 17°. This decrease is not a function of the age of 



