SOME THEORETICAL ASPECTS OF EFFECTS OF 

 TEMPERATURE ON PLANTS 



F. W. Went, Division of Biology, California Institute 

 of Technology, Pasadena, California 



J.EMPERATURE is an excellent tool in the investigation of physiological 

 processes in general, even though its effects are not specific since i)racti- 

 cally all processes are influenced simultaneously by temperature, and since 

 we cannot supply the temperature only to a particular process, we are al- 

 ways dealing with complicated interreactions. However, in previous work 

 it was found that usually only one of the many processes which are affected 

 by temperature is controlling the behavior of an organism and thus the 

 temperature effect on an organism as a whole becomes analyzable. 



As a first example, the effect of temperature on the growth of tomato 

 plants will be discussed. When tomato plants are grown under a whole 

 range of temperatures, it is found that their growth and fruiting behavior 

 can be correlated very strongly with the nyctotemperature which prevails 

 during the dark period of each day (10). This is understandable since 80- 

 90% of the growth of a tomato plant under natural conditions occurs dur- 

 ing darkness. Thus in curve A of figure 1 this temperature relationship is 

 shown for the intact tomato plant. It will be seen that in a mature plant 

 there is an exponential increase in growth rate as the temperature increases. 

 This increase, however, levels off and an optimal rate of growth is ob- 

 tained at nycto-temperatures of 15°-18°, depending on the variety of 

 tomato. Above the optimal temperature of about 17° the growth rate drops 

 off, although rather slowly, with further increases in temperature. It is 

 obvious that around 17° there is a complete shift of control in the growth 

 of the tomato plant. Below 17° the temperature coefficient suggests con- 

 trol by a chemical process, whereas above 17° a very different process must 

 be taking over. 



In an analysis of the factors limiting tomato growth in different tem- 

 peratures, it can be made likely that below 17° the actual growth process 

 is limiting. This can be seen, for instance, in experiments where plant hor- 

 mones are supplied at different temperatures (9). When emasculated 

 flowers are treated with auxins the fruits will develop parthenocarpically. 

 However, at high nycto-temperatures auxins have no effect on this fruit 

 set. It is only under 17° that strong effects of the applied auxin are ob- 

 tained. 



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