F. W. WENT 173 



For many chemical reactions, and sometimes complex biological proc- 

 esses as a whole, a plot of the logarithm of the net reaction rate against 

 the reciprocal of the absolute temperature gives a straight line relationship, 

 in accordance with tlu' Arrhenius theory of chemical reaction rates. But 

 when all the plant reactions described in the preceding paragraphs are 

 plotted in this way, the deviations from a linear relationship show that 

 these complex processes are subject to control by a number of reactions 

 which cannot be analyzed as a whole on the basis of the Arrhenius equation 

 ( fig. 9 ) . Certain deviations of complicated biological rates away from the 

 Arrhenius equation have been interpreted quantitatively with some suc- 

 cess. The present paper suggests the manner in which other factors are 

 involved in complicating any simple relationship for the variation of plant 

 processes with temperature. 



SUMMARY 



Effects of temperature on plants are manifold. In some instances tem- 

 perature clearly influences the activation of a chemical process, in which 

 the Qio ranges between 2 and 3, as most clearly expressed in the effects of 

 temperature on respiration. Above 35°C the rate of respiration is decreased 

 with a pronounced time factor, suggesting a superimposed temperature 

 effect on protein denaturation. For many individual physiological processes 

 such exponential relationships have been found. But for an even greater 

 number of processes, especially of intact organisms, different and far more 

 complicated temperature relationships exist. 



The effect of temperature on protoplasmic streaming is apparently 

 simple. When the rate of streaming is plotted against temperature an al- 

 most straight line appears. This means that the Qio steadily decreases from 

 a high value to well below 2. Here an exponential plot obscures the real 

 relationship. The same straight line temperature relationship exists for 

 the effect of temperature on morphological differentiation in the growing 

 point of peas, for instance. 



For stem elongation and dry weight production in intact plants a very 

 different temperature relationship exists. According to the species, variety, 

 age and size of plant a different, but usually low, optimal temperature is 

 found. In the tomato plant the ojitimal temperature in light is 23°C; in 

 darkness it is high (above 25°C) in the seedling stage, and decreases gradu- 

 ally to 15°-18°C in mature plants, at least in full daylight. At lower light 

 intensities this night optimal temperature drops to below 10°C. Greenhouse 

 tomato varieties have lower optimal temperatures than outdoor varieties. 

 Thus far, breeding has not extended the temperature range of tomato 

 varieties, and they still require a relatively warm climate. 



Again other processes have a temperature coefficient only slightly above 



