CARBON DIOXIDE EXCHANGE DURING THE SHORT INDUCTION PERIOD 1351 



In these experiments, the CO2 gush at the beginning of the dark period 

 could be observed only with heat-inhibited plant material; later (c/. van 

 der Veen, 1949) it was found that, in a hydrogen atmosphere (+3% CO2), 

 the gush becomes clearly observable also in non-preheated leaves. 



The separation of the gulp from steady photosynthesis can be achieved 

 also by cooling to 0° C. At this temperature photosynthesis is suppressed, 

 but the gulp is practically unchanged (fig. 33.13E). It is not reversed by a 

 gulp in the dark (or rather, the gulp is smeared out over 10-15 minutes). 

 In experiments with preheating, the gush appeared to be independent of 

 temperature (6-22° C.) ; since it must be due to a dark reaction, this result 



time 



Fig. 33.13E. CO2 uptake by Sciadopitys needles at 0° C. (pho- 

 tosynthesis suspended) (after van der Veen 1949^). Uptake normal, 

 release in dark absent (or delayed). Diaferometer record. 



is puzzling. The slowing-down of the gulp at 0° C, on the other hand, 

 seems natural. 



The gulp observed at 0° C. is strongly dependent on the length of the 

 preceding dark period, reaching "saturation" after about 15 minutes. 

 (With preheated leaves, even the shortest dark period used — 1 minute- 

 was sufficient to prepare the gulp, i. e., to release the carbon dioxide taken 

 up in the preceding light period.) 



Exposing Sciadopitys needles at 0° C, after dark periods of up to 15 

 minutes, to light flashes of varying duration, indicated that 20-30 seconds 

 are needed to produce a gulp of maximum volume. This is the order of 

 magnitude of the "assimilation time" (chapter 32) — the shortest time in 

 which a chlorophyll molecule can reduce a molecule of carbon dioxide in 

 saturating light. 



In a second paper (1949-) van der Veen observed more closely the 

 secondary inhibition waves, using the grass Holcus lanatus as object. He 

 found that they occur only in saturating (or almost saturating) light. For 

 example (fig. 33.13F), only a faint indication of the second wave appeared 

 at 10 klux (in 3% CO2), while a strong second, and a clear third, wave were 



