TRANSIENT PHENOMENA IN LEAVES 405 



creases as long as their rate of formation from triose is greater than 

 their rate of further oxidation to carbon dioxide and water. This 

 pool size reaches a maximum after a dark period of 10 minutes. These 

 effects may ])e seen in Fig. 3. Curve A was recorded after a dark 

 period of 10 minutes. The pool of newly formed triose is built up to its 

 maximum concentration during the previous illumination. This 

 produces the maximum oxygen burst because the pool of half-oxi- 

 dized intermediates is allowed to reach its maximum concentration 

 during this optimum dark period. Curve B, however, which followed 

 curve A by a dark period of only 1 minute, shows a very small oxygen 

 V)urst because, during such a short dark period, very little of the re- 

 ducible material is accumulated. On the other hand, a very small 

 burst is seen in curve C because the 40-minute dark period is suffi- 

 cient for respiration to reduce the pool sizes of the triose and of the 

 half-oxidized intermediates to their very low steady-state dark con- 

 centrations. Curve D, however, shows that triose must have accumu- 

 lated during the 10-minute illumination period of curve C since, 

 following a 10-minute dark period which allowed for the formation 

 of half-oxidized intermediates from the triose, we again obtain a 

 large oxygen burst. 



Figure 4 shows the CO2 exchange which accompanies the O2 ex- 

 change at low temperature and high CO2 pressure. The primary effect 

 on the curves of lowering the temperature has been to reduce the 

 steady-state value of photosynthesis to a very small value without 

 decreasing the transient gas exchange very much. As in the case of 

 room temperature (see Fig. 1) the assimilator}^ quotient during the 

 first moment of illumination has a minimum value of Vs- On darken- 

 ing there is a release of CO2 which is probably related to the initial 

 uptake of CO2. 



The initial anomalous quotient found under a high CO2 pressure 

 is of interest since it indicates that the process of CO2 assimilation 

 during the first minute of illumination is somehow different from 

 that at the steady state, which has a quotient of about unity. Ex- 

 periments with radioactive CO2 are currently in progress to determine 

 what compounds result from the carboxylation occurring during this 

 transient period. These experiments, which are l)eing done at low 

 temperature in order to isolate the phenomenon from normal photo- 

 synthesis, should elucidate the process responsible for the anomalous 

 quotient. 



