1938 



KINETICS OF PHOTOSYNTHESIS 



CHAP. 37d 



with changes in carbon dioxide liberation. Fig. 37D.22 shows a typical 

 recording. Its most striking feature is the practically instantaneous re- 

 turn of cells into the state of compensation after switching the light on; 

 after this, photosynthesis sets in suddenly with almost the maximum rate. 

 When the light is switched off, the cells again instantaneously return — 

 for up to twenty seconds— into the compensation state. This is followed by 

 an outburst of respiration, during which the CO2 production rate may be 

 several times higher than the steady rate the respiration approaches 

 later. This "COa-burst" coincides with, but seems to be considerably 

 more extensive than, the "O2 gulp" observed by Brackett et al. (cf. figs. 

 37D.31and32). 



Fig. 37D.23. Carbon dioxide exchange (measured by pH change) of hydrogen- 

 incubated Scenedesmus, exposed to alternating light and dark periods (after Gaffron 

 1953). Photosynthesis begins at once at full rate. Respiration gets under way after 

 the fourth cycle, and characteristic induction effects in light and darkness follow its 

 appearance. The lower curve is the continuation of the upper one. Ordinate is pH. 



Fig. 37D.23 represents the sequence of events following the incubation 

 of a Scenedesmus culture in hydrogen. Upon switching on the (very strong 

 and thus immediately de-adapting) light, carbon dioxide uptake starts at 

 full rate without any induction loss, showing that the presence of oxygen is 

 not needed at all to initiate photosynthesis {cj. section 2(6) above). After 

 four one-minute light periods, separated by dark periods during which no 

 CO2 exchange occurs at all, respiration gradually gets under way (at the cost 

 of oxygen accumulated in the four light minutes) . As soon as respiration has 

 developed, characteristic "wiggles" begin to appear after each illumina- 

 tion interval. These include, in addition to momentary cessation of CO2 



