REACTIONS OF CARBON DIOXIDE IO5 



(1949) was able to show with the sunflower leaf that during 

 active photosynthesis the accumulated starch will account 

 for nearly the whole of the carbon assimilated. The mature 

 leaf might perhaps be regarded as a tissue with little growth. 

 In Chapter i the over-all response to light was formulated 

 simply in terms of carbohydrate formed. But this simple 

 formulation is never strictly true because proteins as well 

 as carbohydrates are to a greater or lesser extent concerned 

 in the metabolism of leaves. In sunflowers the assimilatory 

 quotient CO2/O2 is very nearly unity, corresponding to the 

 formation of carbohydrate. In wheat leaves, on the other 

 hand, the assimilatory quotient is sensibly less than unity, 

 the difference being accounted for by protein synthesis. The 

 earlier studies of the carbohydrate relations in leaves sug- 

 gested that a soluble carbohydrate, either hexose or sucrose, 

 was the first recognizable product of photosynthesis to 

 accumulate. This again justified the simple formulation of 

 the over-all process as resulting in formation of (CH20),i. 



The photosynthetic studies with micro-organisms have 

 emphasized the need to formulate, as the end product of the 

 process, the cell substance as a whole, the organism itself. 

 Here the nature of the final product is best defined by 

 the value of the photosynthetic quotient and the analyses 

 of total cell material rather than by any precise chemical 

 formula. Thus the chemical nature of what might be termed 

 the first product of photosynthesis may have no relation to 

 the measured photosynthetic quotient; the adjustment to 

 the carbohydrate level or to those corresponding with amino 

 acids might be regarded as secondary processes. These 

 latter would of course be included in the over-all measure- 

 ment of the photosynthetic quotient. 



When organisms are supplied with nitrate as the source 

 of inorganic nitrogen the value of the photosynthetic 

 quotient does not only indicate the extent of the reduction 

 of the CO2. Nitrate has to be reduced as well when proteins 

 are formed, and part of the oxygen measured may correspond 

 to the reduction of nitrate. Such an effect, a lowering of the 

 photosynthetic quotient due to nitrate reduction, was dis- 

 covered bv Warburg (1922) with Chlorella. Although the 

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