562 PHOTOSYNTHESIS AND RESPIRATION CHAP. 20 



Although the nature of the hnks between the photosynthetic and the 

 respiratory apparatus is as yet quite uncertain, they will undoubtedly 

 be subject to closer study in the future. We have therefore assembled, 

 in the present chapter, some experimental material pertaining to the 

 relation between respiration and photosynthesis, even though a large 

 part of this material may later prove to be irrelevant. For a more 

 complete, but rather indiscriminate, collection of results pertaining to the 

 relation between light and respiration in plants, we refer to the review by 

 Weintraub (1944). 



1. Effect of Respiration on Photosynthesis 



Does enhanced respiration bring about enhanced photosynthesis? 

 Does inhibited respiration cause an inhibition of photosynthesis? The 

 answer to both questions seems to be in the negative, although the 

 conclusion is by no means final. 



Respiration can be enhanced, for example, by an external supply of 

 appropriate substrates; van der Paauw (1932) found that the photo- 

 synthesis of Hormidium also is increased markedly in a 1% glucose 

 solution. Gaffron (1939) stated, to the contrary, that "innumerable 

 experiments" have provided no convincing evidence of a stimulating 

 effect of this kind. It may be worth mentioning, in this connection, 

 that, according to Emerson (1927), the additional respiration of Chlorella, 

 caused by glucose feeding, is much more sensitive to cyanide than ordi- 

 nary respiration. This is perhaps an indication that the mechanism of 

 glucose-supported respiration in Chlorella is different from that of the 

 normal respiration — either in its location (in the outermost layers of 

 the cytoplasm) or in its catalytic mechanism. In either case, the relation 

 of the "extra" respiration to photosynthesis may be different from that 

 of normal respiration. 



The respiration of certain algae (e. g., Scenedesmus D3) can be reduced 

 by 85% by cyanide without measurably affecting the rate of photo- 

 synthesis (c/. page 305). Gaffron (1937) pointed to this result as an 

 argument in favor of essential independence of respiration and photo- 

 synthesis. 



Spoehr and McGee (1923), who observed that leaves kept for a long 

 period in the dark lost their capacity for both photosynthesis and respi- 

 ration, saw in this fact a proof that the two processes are interrelated. 

 However, a direct influence of respiration on photosynthesis can hardly 

 be deduced from the mere fact of their simultaneous disappearance in 

 starved tissues. 



In chapter 13, we described the inhibition of photosynthesis after a 

 period of "anaerobic incubation." This could be quoted as a proof that 

 inhibition of respiration brings about, sooner or later, also an inhibition 



