138 PHOTOSYNTHESIS 



ture and OMC the rate of photosynthesis at the different temperatures 

 observed after longer periods of time. 



This interpretation of the influence of temperature and time on the 

 rate of photosynthesis can probably not be taken to picture the kinetics 

 of the reaction in every detail. As a matter of fact we know very little 

 about the inactivation of the chloroplasts, either as to the rate thereof 

 or of the causes which bring this about. As will be shown later the 

 activity of the chloroplasts can be diminished not only by high tempera- 

 tures but as well by. high light intensity and by the accumulation of the 

 products of photosynthesis. The importance of this is emphasized by 

 the fact that in order to obtain uniform results on the rate of photosyn- 

 thesis, it is essential that the plants be subjected to the same treatment for 

 some time previous to the experiment. Just what form the curve DB 

 takes is therefore uncertain. The foregoing also illustrates how exceed- 

 ingly difficult it is to obtain constants of biological reactions if such con- 

 stants are to have even remotely the same meaning as our physical chemical 



values. 



The temperature coefficient of photosynthesis has been determnied by 

 Warburg ^■*° over a range of temperatures. He used the unicellular alga 

 Chlorella and his method had the advantage that it avoided the necessity 

 of determining the internal temperature of a leaf, a procedure that is 

 associated with considerable experimental difficulty and some uncertainty. 

 In view of the fact that the temperature coefficient shows decided changes 

 at different temperatures the method of indicating this for 10" C. is not 

 strictly correct. For the sake of comixirison, however, Warburg cal- 

 culated his results on this basis. He found that when the concentration 

 of carbon dioxide and light intensity are high the temperature coefficient 

 is not constant. Thus he found the following temperature coefficients, 

 calculated for Qio as the indicated temperature differences : 



TABLE 23 



Temperature Coefficients of Photosynthesis at Different Temperatures. 



(Warburg.) 



^T^rlTl^^."'.''''':.... 5-10° 16-25° 5.4-10° 10-20° 20-30° 15-25° 25-32° 



Q^^ 4.7 2.0 4.3 2.1 1.6 1.06 1. 



Relative light intensity... 16 16 45 45 45 1.8 1. 



It is apparent from this that the temperature coefficient decreases with 

 increasing temperature. When the light intensity is low the temperature 

 coefficient is about unity. The latter is in agreement with Blackman's 

 results. Osterhout and Haas ^" working with Ulva rigida found that 

 the temperature coefficient of photosynthesis between 17- and 27° is 1.81. 



^^'Wavhurg, Biochcm. Zcit., 100, 2SS {1919} ^^, ,,_._^ . . „. 



"*i Osterhout and Haas, Jour. Gen. Physiol., 1, 295 (1919). van Amstel, Rev. 

 trav. hot. Neerlandais, 13, 1 (1917). 



