1266 THE PIGMENT FACTOR CHAP. 32 



In some aurea leaves, characterized by extreme chlorophyll deficiency, the rate of 

 photosynthesis in light of 45,000 lux was found to be much lower than in the moderately 

 chlorophyll-deficient yellow leaves, to which figure 32.2 refers. For example, a leaf of 

 Sanibucus containing less than 0.1 mg. chlorophyll/ 10 g. fresh weight assimilated, in 

 strong light, only 12 mg. CO2/hr./10 g. (as against 90 mg. taken up by a leaf containing 

 0.75 mg. chlorophyll, and 150 mg. taken up by a fully green leaf with 23.5 mg. chloro- 

 phyll in 10 g.). These figures show that the assimilation number of the leaf with 0.5% 

 of the normal quantity of chlorophyll was about the same (~120) as that of the leaf 

 with 3% of the normal chlorophyll content, both being about eighteen times larger than 

 the assimilation number of a normal leaf. However, by analogy with the curves in 

 figure 32.2, it appears possible that the leaf with extreme chlorophyll deficiency was far 

 from Hght-saturated at 45,000 lux. If the light curve of this leaf, followed to much 

 higher intensities, would also approach the normal saturation value, this would mean an 

 assimilation number of over 1000 {i. e., an assimilation time of less than 0.15 second)! 

 It would be interesting to find out whether such extremely high assimilation numbers 

 actually occur. 



It was mentioned above that Gabrielsen (1948) found the rate of photosynthesis of 

 aurea leaves in weak light to be even lower than expected from their light absorption, 

 and attributed this deficiency to "inactive" light absorption in cell walls, plasma and 

 vacuoles. No such deficiency is apparent in Willstatter and StoU's curves (fig. 32.2); 

 these curves, however, cannot be evaluated quantitatively because of the absence of 

 correlated absorption data. Gabrielsen's absorption estimates were based not on his 

 own measurements, but on Seybold and Weissweiler's data (chapter 22); consequently, 

 they, too, are not very reliable. Correct estimation of scattering losses is very important 

 in the determination of true absorption by leaves, particularly when the latter are poor 

 in pigments; and figures given in the hterature for "inactive absorption" of visible light 

 by "colorless" leaf constituents (p. 684) may be much too high because of unsatisfactory 

 methods of integrating scattering losses. 



Another experiment which also indicated the essential independence 

 from chlorophyll of the catalyst that limits the rate of photosynthesis in 

 strong light was described by Emerson (1935). He found that, when a 

 suspension of Chlorella was illuminated by strong light for 16 hours, the 

 volume of the cells was almost trebled, without appreciable change in the 

 total quantity of chlorophyll. Thus, the average concentration of chloro- 

 phyll in the suspension was unchanged, but its concentration within the 

 cells was smaller than before, by a factor of three. Photosynthesis was 

 found to be about twice as large as before {v^ about 4 before the treatment, 

 and about 8 afterward) . Only a small part of this improvement can be at- 

 tributed to a more efficient absorption of light by the preilluminated sus- 

 pension (caused by more uniform distribution of chlorophyll and conse- 

 quent dechne in the "sieve effect"); the result can therefore be taken as 

 indication that, after multiplication, the cells contained about twice as 

 much of the rate-limiting catalyst as was present in the original suspension, 

 while their total content of chlorophyll was unchanged. 



A striking demonstration of the existence of an enzymatic component 

 that can limit the rate of photosynthesis, and is independent of chlorophyll 



