PHOTOSYNTHESIS AND RESPIRATION 



1935 



to bring into a single picture data obtained with suspensions of widely 

 differing density). At higher light intensities — the greater [O2], the earlier 

 — inhibition was replaced by stimulation. 



The extreme inhibition observed at low [O2] may be at least partially 

 apparent rather than real, because of the possibility (mentioned earlier in 

 this section) of preferential re-utihzation of photosynthetically produced 

 oxygen for respiration. No similar explanation is, however, possible for 



.2-4% 0, 



.8-9% 0, 



1.1-1.3% 0, 



1.5-1.6% 0, 



o 

 o 



200% 



o 



100% 



IE 



o 



\ i 







5 10 5 10 5 10 5 10 



RELATIVE INCIDENT LIGHT INTENSITY (photosynthetic ''*te^espiratory rate) 



Fig. 37D.21. Uptake of isotopic oxygen, 0(16)0(18), by blue-green alga Anabaena 

 in light, in relation to light intensity and oxygen concentration (after Brown and Web- 

 ster 1953). Illumination periods 10-20 min. 



the stimulation of respiration at the higher [O2] values. The latter may, 

 however, be associated with photoxidation (which is more strongly [O2] 

 dependent than respiration, cf. figs. 58 and 59 in vol. I) . However, photoxi- 

 dation has been observed before only under the conditions of inhibited 

 photosynthesis (e. g., in absence of carbon dioxide). Another mechanism 

 of 0(16)0(18) uptake in hght, in which Brown and Webster saw a not un- 

 likely explanation of the apparently enhanced "respiration" of Anabaena 

 in light, is the utilization of molecular oxygen as "Hill oxidant" — a reaction 

 which results in no net chemical change, (H2O + V2O2 ->- V2O2 + H2O), 

 but must lead to accelerated isotopic exchange between O2 and H2O, and 



