TEMPERATURE EFFECT IN STRONG LIGHT 1235 



time, even at temperatures at which the saturation rate is strongly de- 

 pressed. 



Speaking of the temperature independence of photosynthesis in weak 

 Hght, one thinks, of course, of true photosynthesis and not of net gas ex- 

 change. Since respiration is accelerated by heating, while true photo- 

 synthesis in weak light is independent of temperature, the net oxygen 

 hberation in weak light must decline with increasing temperature; the 

 compensation point is thus shifted toward the higher light intensities. 

 This fact already was mentioned in chapter 28 (page 984); Table 31. Ill 

 gives some additional examples. 



2. Temperature Effect in Strong Light 



We now consider another extreme case— that of strong illumination 

 and ample supply of carbon dioxide. The transition from temperature- 

 independent photosynthesis in weak light to temperature-dependent photo- 

 synthesis in strong light is best illustrated by light curve families with tem- 

 perature as parameter (figs. 28.6 and 28.7), and temperature curve families 

 with light intensity as parameter (fig. 31.9). In Table 31. IV are collected 

 the results of a number of investigations dealing with the effect of tem- 

 perature on photosynthesis in light- and carbon dioxide-saturated state. 

 We have omitted measurements in which light and carbon dioxide satura- 

 tion probably was incomplete, for example those of van Amstel (1916), 

 carried out in light of only 2500 lux, and of Lundegardh (1924), Yoshii 

 (1928), Beljakov (1930) and Stalfelt (1939), in which ordinary air was used 

 as the source of carbon dioxide. (We saw in chapter 27 that most plants 

 require considerably more than the 0.03% CO2 in the atmosphere for the 

 saturation of their photosynthetic mechanism.) 



Table 31. IV contains values of the temperature coefficient, Qio, and 

 of the heat of activation, Ea,* as characteristics of the temperature effect. 

 The theoretical significance of these constants will be discussed in section 6. 

 The constancy of Ea over a certain temperature range indicates that the 

 rate in this range follows the Arrhenius function : 



(31.7) logF = A + {B/T) 



where A and B are constants and T is the temperature on the absolute 

 scale. 



Figures 31.10 to 31.14 serve as illustrations to Table 31.IV. They 

 clearly show the difference between the results of van der Paauw on 

 CJilamijdomonas, Craig and Trelease on Chlorella and Noddack and Kopp 



* The similarity of this symbol and of the symbol Ea used throughout this book for 

 the carbon dioxide-fixing catalyst should cause no confusion. 



