992 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



before "carbon dioxide inhibition" or "light inhibition" becomes apparent. 

 We have therefore assumed that, independently of any inhibition, certain 

 intrinsic internal factors (such as limited availability of certain catalysts) 

 impose an "absolute" ceiling {i. e., a ceiling independent of both [CO2] and 

 /) on the maximum rate of photosynthesis. 



Determination of this maximum rate is of interest from the practical 

 point of view (estimation of absolute and relative efhciency of different 

 plants as producers of organic matter) as well as from the point of view 

 of the kinetic mechanism of photosynthesis. However, the two aspects 

 call for different methods of comparison. The practical problem can best 

 be answered by using unit surface as the basis of rate determination (since 

 what one wants to know is how much organic matter can be harvested 

 from a unit area covered with plants of different species) . From the point 

 of view of a theorist, comparison should be based on unit cell volume, or 

 unit chloro'phyll content, rather than on unit area. Willstatter and Stoll 

 (1918) designated the maximum quantity of carbon dioxide that can be 

 reduced in unit time by unit quantity of chlorophyll in a cell or tissue as 

 "assimilation number" {va in Table 28. V), and the shortest time in which 

 one molecule of chlorophyll can reduce one molecule of carbon dioxide 

 {Ta in Table 28. V) as "assimilation time." (These constants will be ana- 

 lyzed in chapter 32.) 



We designated, in chapter 27, the maximum rate of photosj'nthesis, at 

 a given light intensity, reached with saturating concentrations of carbon 

 dioxide, by Pmax.l we can use the symbol p™^'' for the maximum rate 

 reached, at a given carbon dioxide concentration, when light intensity be- 

 comes saturating; and the symbol Pmax! for the "absolute" maximum rate, 

 obtained when both carbon dioxide supply and light intensity are saturat- 

 ing. 



Table 28. V shows that for the leaves of land plants the values of PmTx. 

 generally are of the order of 20 mg. COa/hr. 100 cm.^ of leaf surface, and 

 sometimes reach 80-90 mg. Even aurea leaves, despite their very low 

 content of chlorophyll, constitute no exception. Only some algae and 

 aquatic plants investigated by Kniep (1914), Emerson and Green (1934) 

 and Gessner (1938) fell far short of this production. A yield of 20-80 mg. 

 CO2/IOO cm. 2 hr. — assuming it is reached in light of 40 Idux, 80% of which 

 is absorbed by the leaf — corresponds to the conversion into chemical en- 

 ergy of 4 to 16% of absorbed light energ}^ (in the photosynthetically active 

 region, 400-700 m/x), and thus to a quantum yield between 0.018 iyio) and 

 0.07 (H4)- (This estimate is based on factors given in chapter 25.) The 

 relation of these yields, obtainable in strong light, to the maximum quan- 

 tum yields observed in weak light will be discussed in chapter 29. In the 

 case of aurea leaves, the quantum yield in the light-saturated state appears 



