1928 



KINETICS OF PHOTOSYNTHESIS 



CHAP. 37d 



leaf tissues were investigated. The C(12)02 ;^ C(13)02 exchange was found 

 to be too large to be neglected. In contrast to Weigl et at., Van Norman and 

 Brown found the assumption that light does not interfere with respiration 

 to be compatible with observations. Assuming such constant respiration, 



.075 



X 



E 

 E 



a. 

 <n 



V) 

 UJ 



a. 



q: 



< 

 a. 



UJ 

 (9 



>- 



X 



o 



.050 - 



.025 



O • MASS 34 X 1,00 

 • • MASS 32 X 0.47 



20 



100 



40 60 



MINUTES 

 Fig. 37D.14. Respiratory utilization of oxygen isotopes by tobacco leaf tissue. The 

 tissue was permitted to consume oxygen until too little remained for accurate measure- 

 ment (Brown 1953). The two isotopes were used up in proportion to their partial pres- 

 sures. This result could not have been obtained if gaseous oxygen participated in iso- 

 tope exchange with water or if respiration discriminated between the isotopes. 



the isotopic discrimination factors for C(13)02 and C(14)02 (relative to 

 C(12)02 as unity) were calculated to be 0.96 and 0.85, respectively. The 

 second figure agrees with Weigl's result (0.83). It was unexpected to find 

 the rate constant for C(13)02, not half-way between those for C(14)02, and 

 C(12)02, but close to that of C(12)02. Similar results have been reported 

 for other chemical reactions, but the reason is as yet unexplained. 



According to Buchanan, Nakao and Edwards (1953) the C(13)-C(12) 



