1334 INDUCTION PHENOMENA CHAP. 33 



slightly the shape of the induction curves calculated by the latter (fig. 

 33. 6F). Much more important is, however, the failure of Brown's measure- 

 ments to support in any way Warburg and Burk's hypothesis of a strongly 

 enhanced oxygen consumption in light, at least as far as Chlorella is con- 

 cerned. 



Much stronger changes of respiration in light were observed by Brown 

 and Webster (1953) in the blue-green alga Anabaena (cf. chapter 37D, 

 section 3) ; they ranged (in dependence on Hght intensity and partial pres- 

 sure of oxygen) from complete inhibition of respiration within a minute or 

 two after the beginning of illumination (in 0.2-0.4% O2, and P ^=^ S.5R), 

 to stimulation of respiration by a factor of two or more (in more intense 

 light and higher oxygen concentration). Measurements of transient 

 phenomena in cells of this type obviously could be entirely misleading if a 

 constant (or continuously interpolated) respiration correction were applied 

 to the pressure readings. It may be, however, that such extreme respira- 

 tion changes in light can occur only in blue-green algae, in which the site of 

 photosynthesis is not separated morphologically from the main site of cell 

 respiration. 



Hill and Whittingham (1953) used the hemoglobin conversion to oxy- 

 hemoglobin for rapid spectroscopic oxygen determination in the induction 

 phase of Chlorella: the delay of response was thus reduced to <10 seconds, 

 from about 1 minute in parallel manometric measurements. The course 

 of induction was the same, corresponding to half-reactivation in 0.5 min- 

 ute at 15° C. for dark incubation times of 13, 30 or 45 minutes; even 22 

 hours spent in darkness did not appreciably slow down the induction. Ex- 

 periments in alternating light at different temperatures showed that at 

 7° C. induction losses were negligible after dark periods of 1 minute or 3 min- 

 utes, but noticeable after 10 minutes darkness; at 16° and 25° C. they be- 

 came significant after 3 minutes in the dark. The half-time of reactivation 

 after a given short dark period was shorter at the lower temperatures. 

 No positive induction (oxygen burst) could be noted in any of the runs. 



3. Carbon Dioxide Exchange during the Short Induction Period 



Observations of short induction by carbon dioxide determination were 

 first made by McAhster (1937, 1939, 1940) with wheat plants. He used 

 the infrared spectrophotometric technique described on page 852. At 

 first, the rate was calculated from carbon dioxide concentration measure- 

 ments made at 0.5 minute intervals; later, it was recorded directly by a 

 differential recorder. 



Figure 33.7 gives an example of curves obtained by the earlier, point- 

 to-point method. The downward trend of the curves corresponds to liber- 



