1936 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



will thus be registered as "oxygen uptake" in Brown's apparatus. We re- 

 call that the capacity of oxygen to serve as Hill oxidant (first suggested on 

 p. 543 in vol. I) was made plausible experimentally by Mehler's experi- 

 ments with chloroplasts (chapter 35, section B4(c)). The mechanism, sug- 

 gested by Mehler on the basis of indirect chemical evidence, was confirmed 

 by a mass-spectroscopic study of Mehler and Brown (p. 1565). Brown and 

 Webster quoted an (as yet unpublished) investigation of Good, which 

 showed this kind of "closed-cycle Hill reaction" to occur also in whole cells. 

 There, is, however, no explanation as to why this process should occur in 

 Anabaena more easily than in Chlorella, and why it should occur at all in 

 carbonate buffers, i. e., with an abundant supply of carbon dioxide. 



Johnson and BroAvn (1954) used the 0(18) method to confirm directly 

 van Niel's surmise (cf. page 110) that light inhibits (rather than compen- 

 sates) the oxygen consumption by (aerobic) Athiorhodaceae. 



(c) Polarography 



Polarographic respiration (oxygen consumption) measurements at 10 

 sec. intervals — i. e., with a much better time resolution than is possible in 

 manometry — have been carried out by Brackett, Olsen and Crickard (1953). 

 Characteristic changes are illustrated by figs. 37D.31 and 37D.32. 



Brackett concluded from these measurements that changes in the rate 

 of oxygen uptake result from the superposition of several phenomena. If 

 intermittent illumination is preceded by a sufficiently long period of dark 

 (aerobic) incubation, respiration starts at a very low rate and increases 

 logarithmically through the whole period of intermittent illumination. 

 (This concept led to the interpolation of respiration during the light periods, 

 shown by dashed line in fig. 37D.32.) During each dark interval, this 

 main trend of increasing respiration is superimposed by a — also logarith- 

 mic — decline, which begins, however, not immediately, but only 2-3 min- 

 utes after the cessation of illumination. In consequence of this delay, the 

 average respiration during the dark periods of intermittent illumination 

 (with dark and light intervals not longer than 2-3 minutes) may be higher — 

 instead of lower — than that during the light periods. 



In addition to these two, relatively slow respiration trends, a sharp 

 "gulp" of oxygen occurs in the first 0.5-1 minute of darkness. 



The two main trends become clear only when the initial dark period had 

 been long enough, and the "hght dose" suflSciently large, as in fig. 37D.32. 

 Without previous dark adaptation, and with smaller light doses (shorter 

 periods, or lower light intensity) the fluctuations of oxygen uptake during the 

 dark intervals, and its change after a light period, begin to appear irregular 

 (although the O2 "gulp" in the first 0.5-1 minute of darkness, and the maxi- 

 mum of respiration about 2 minutes later, remain recognizable in most, if 



