EFFECT OF LIGHT ON RESPIRATION 569 



It thus seems probable that hght absorbed by the carotenoids has a 

 specific stimulating influence on oxygen consumption, which is, however, 

 not a direct sensitization, since it sets slowly in light and persists for 

 several minutes in the dark. 



Whether the effect of ultraviolet light on the respiration of green cells, 

 observed by Gessner, can be attributed to the same cuase is an open 

 question; the stimulation of respiration in colorless tissues, observed by 

 Montfort and Fockler, seems to point to a different mechanism, since 

 these tissues contain no carotenoids (but may contain water-soluble 

 pigments of the flavonol type). 



We now come to the problem of " photorespiration " proper, that is, 

 a direct photochemical acceleration of normal respiration w^hich disap- 

 pears in the dark as instantaneously as does photosynthesis. The 

 possibility of such an effect is a nightmare oppressing all who are con- 

 cerned with the exact measurement of photosynthesis, and various 

 attempts have been made to bring it to Hght. The problem is: how to 

 determine the true rate of respiration in green plant cells during the 

 illumination. Noddack and Kopp (1940) measured the light curves of 

 photosynthesis of Chlorella at different temperatures and inquired whether 

 the subtraction of "dark respiration" (Rd) from the apparent photo- 

 synthesis at low^ light intensities (Pa) leaves a residue, P = P^ — R^, 

 which is independent of temperature, as this could be expected for true 

 photosynthesis at low light intensities (cf. Vol. II, Chapter 31). They 

 found slight deviations from constancy, but in the direction which 

 indicated a somewhat decreased, rather than stimulated, respiration in 

 light. In similar experiments of Emerson and Lewis (1940), tempera- 

 ture was found to have no effect at all on the calculated quantum yield. 



According to Gaffron (1939), the respiration of cells poisoned with 

 hydroxylamine (which inhibits photosynthesis and does not affect respi- 

 ration) continues in light at the same rate as in the dark. The results 

 obtained with cyanide are more complex because this poison acts on 

 both photosynthesis and respiration {cf. Chapter 12). In most plants — 

 both higher plants and algae, including Chlorella — photosynthesis is 

 more sensitive to cyanide than respiration; in certain species, however, 

 as in some strains of Scenedesmus, the relation is reversed. Thus, 

 hydrocyanic acid should afford an opportunity to study both respiration 

 in Hght with poisoned photosynthesis, and photosynthesis with poisoned 

 respiration. However, experiments of the first kind do not give the same 

 simple results as those with hydroxylamine. In intense light, photo- 

 synthesis is so much stronger than respiration that a small residual 

 capacity for photosynthesis which remains in the cyanide-poisoned cells 

 is sufficient to prevent an exact measurement of respiration. Warburg 

 (1919) observed that photosynthesis cannot be reduced by cyanide 



