TRANSIENT PHENOMENA IN LEAVES 403 



Even though the steady-state rate in 4% CO2 at 0°C. is very low, 

 the initial induction phenomena may remain sizable. In Fig. 3 ob- 

 tained at 0°C., curve A has had a previous dark period of 10 minutes. 

 When the dark period is shorter, such as 1 minute for curve B, or 

 longer, such as 40 minutes for curve C, the initial O2 burst is smaller. 

 Curve D, however, follows the 10-minute illumination period of 

 curve C by a 10-minute dark period. Here the large effect has been re- 

 gained. This dark period produces the maximum O2 burst. 



Under these conditions of high CO2 pressure and low temperature, 

 there is a CO2 uptake associated with the O2 burst. As shown in Fig. 4, 

 the assimilatory quotient during this transient phenomenon is about 



0' 



J I I 1 I 1 L 



_/, minutes 



// V-O2 + CO2 

 / / 



Fig. 4. Rate of gas exchange vs. time for a hydrangea leaf at 0°C. in an atmosphere 



of 4% CO2 and 20% O2 in He. 



Vs. When the irradiation ceases, there is a release of CO2 but no con- 

 comitant O2 effect. This CO2 release following illumination is also 

 present at high temperatures if the CO2 concentration is high, as can 

 be seen by a closer inspection of Fig. 1. 



If we now try to interpret the data of Figs. 1, 2, and 3 in more 

 specific metabolic terms relating them to the known chemistry of 

 photosynthesis, those acquainted with the literature of transient 

 phenomena will realize that the discussion must of necessity be 

 rather involved. 



Franck and others have previously postulated that half-oxidized 

 intermediates of respiration may be photochemically reduced when 

 the normal photosynthetic intermediates are missing. Thus at the 

 beginning of a period of illumination, following a dark period of suffi- 

 cient duration to reduce the concentration of the CO2 acceptor 

 molecule, ribulose diphosphate, to its low steady-state dark value, 

 PGA from respiration may be photochemically reduced. This will, so 



